The Objective Caml system release 3.06 Documentation and user's manual Xavier Leroy (with Damien Doligez, Jacques Garrigue, Didier Rémy and Jérôme Vouillon) August 19, 2002 Copyright © 2002 Institut National de Recherche en Informatique et en Automatique Table of Contents ***************** - Part: I An introduction to Objective Caml - Chapter 1 The core language - 1.1 Basics - 1.2 Data types - 1.3 Functions as values - 1.4 Records and variants - 1.5 Imperative features - 1.6 Exceptions - 1.7 Symbolic processing of expressions - 1.8 Pretty-printing and parsing - 1.9 Standalone Caml programs - Chapter 2 The module system - 2.1 Structures - 2.2 Signatures - 2.3 Functors - 2.4 Functors and type abstraction - 2.5 Modules and separate compilation - Chapter 3 Objects in Caml - 3.1 Classes and objects - 3.2 Reference to self - 3.3 Initializers - 3.4 Virtual methods - 3.5 Private methods - 3.6 Class interfaces - 3.7 Inheritance - 3.8 Multiple inheritance - 3.9 Parameterized classes - 3.10 Polymorphic methods - 3.11 Using coercions - 3.12 Functional objects - 3.13 Cloning objects - 3.14 Recursive classes - 3.15 Binary methods - 3.16 Friends - Chapter 4 Labels and variants - 4.1 Labels - 4.1.1 Optional arguments - 4.1.2 Labels and type inference - 4.1.3 Suggestions for labeling - 4.2 Polymorphic variants - 4.2.1 Weaknesses of polymorphic variants - Chapter 5 Advanced examples with classes and modules - 5.1 Extended example: bank accounts - 5.2 Simple modules as classes - 5.2.1 Strings - 5.2.2 Hashtbl - 5.2.3 Sets - 5.3 The subject/observer pattern - Part: II The Objective Caml language - Chapter 6 The Objective Caml language - 6.1 Lexical conventions - 6.2 Values - 6.2.1 Base values - 6.2.2 Tuples - 6.2.3 Records - 6.2.4 Arrays - 6.2.5 Variant values - 6.2.6 Polymorphic variants - 6.2.7 Functions - 6.2.8 Objects - 6.3 Names - 6.4 Type expressions - 6.5 Constants - 6.6 Patterns - 6.7 Expressions - 6.7.1 Basic expressions - 6.7.2 Control structures - 6.7.3 Operations on data structures - 6.7.4 Operators - 6.7.5 Objects - 6.8 Type and exception definitions - 6.8.1 Type definitions - 6.8.2 Exception definitions - 6.9 Classes - 6.9.1 Class types - 6.9.2 Class expressions - 6.9.3 Class definitions - 6.9.4 Class specification - 6.9.5 Class type definitions - 6.10 Module types (module specifications) - 6.10.1 Simple module types - 6.10.2 Signatures - 6.10.3 Functor types - 6.10.4 The with operator - 6.11 Module expressions (module implementations) - 6.11.1 Simple module expressions - 6.11.2 Structures - 6.11.3 Functors - 6.12 Compilation units - Chapter 7 Language extensions - 7.1 Streams and stream parsers - 7.2 Range patterns - 7.3 Assertion checking - 7.4 Deferred computations - 7.5 Local modules - 7.6 Grouping in integer and floating-point literals - Part: III The Objective Caml tools - Chapter 8 Batch compilation (ocamlc) - 8.1 Overview of the compiler - 8.2 Options - 8.3 Modules and the file system - 8.4 Common errors - Chapter 9 The toplevel system (ocaml) - 9.1 Options - 9.2 Toplevel directives - 9.3 The toplevel and the module system - 9.4 Common errors - 9.5 Building custom toplevel systems: ocamlmktop - 9.6 Options - Chapter 10 The runtime system (ocamlrun) - 10.1 Overview - 10.2 Options - 10.3 Dynamic loading of shared libraries - 10.4 Common errors - Chapter 11 Native-code compilation (ocamlopt) - 11.1 Overview of the compiler - 11.2 Options - 11.3 Common errors - 11.4 Compatibility with the bytecode compiler - Chapter 12 Lexer and parser generators (ocamllex, ocamlyacc) - 12.1 Overview of ocamllex - 12.2 Syntax of lexer definitions - 12.2.1 Header and trailer - 12.2.2 Naming regular expressions - 12.2.3 Entry points - 12.2.4 Regular expressions - 12.2.5 Actions - 12.2.6 Reserved identifiers - 12.3 Overview of ocamlyacc - 12.4 Syntax of grammar definitions - 12.4.1 Header and trailer - 12.4.2 Declarations - 12.4.3 Rules - 12.4.4 Error handling - 12.5 Options - 12.6 A complete example - 12.7 Common errors - Chapter 13 Dependency generator (ocamldep) - 13.1 Options - 13.2 A typical Makefile - Chapter 14 The browser/editor (ocamlbrowser) - 14.1 Invocation - 14.2 Viewer - 14.3 Module browsing - 14.4 File editor - 14.5 Shell - Chapter 15 The documentation generator (ocamldoc) - 15.1 Usage - 15.1.1 Invocation - 15.1.2 Merging of module information - 15.1.3 Coding rules - 15.2 Syntax of documentation comments - 15.2.1 Placement of documentation comments - 15.2.2 The Stop special comment - 15.2.3 Syntax of documentation comments - 15.2.4 Text formatting - 15.2.5 Documentation tags (@-tags) - 15.3 Custom generators - 15.3.1 The generator class - 15.3.2 Handling custom tags - 15.4 Adding command line options - 15.4.1 Compilation and usage - Chapter 16 The debugger (ocamldebug) - 16.1 Compiling for debugging - 16.2 Invocation - 16.2.1 Starting the debugger - 16.2.2 Exiting the debugger - 16.3 Commands - 16.3.1 Getting help - 16.3.2 Accessing the debugger state - 16.4 Executing a program - 16.4.1 Events - 16.4.2 Starting the debugged program - 16.4.3 Running the program - 16.4.4 Time travel - 16.4.5 Killing the program - 16.5 Breakpoints - 16.6 The call stack - 16.7 Examining variable values - 16.8 Controlling the debugger - 16.8.1 Setting the program name and arguments - 16.8.2 How programs are loaded - 16.8.3 Search path for files - 16.8.4 Working directory - 16.8.5 Turning reverse execution on and off - 16.8.6 Communication between the debugger and the program - 16.8.7 Fine-tuning the debugger - 16.8.8 User-defined printers - 16.9 Miscellaneous commands - 16.10 Running the debugger under Emacs - Chapter 17 Profiling (ocamlprof) - 17.1 Compiling for profiling - 17.2 Profiling an execution - 17.3 Printing profiling information - 17.4 Time profiling - Chapter 18 Interfacing C with Objective Caml - 18.1 Overview and compilation information - 18.1.1 Declaring primitives - 18.1.2 Implementing primitives - 18.1.3 Statically linking C code with Caml code - 18.1.4 Dynamically linking C code with Caml code - 18.1.5 Choosing between static linking and dynamic linking - 18.1.6 Building standalone custom runtime systems - 18.2 The value type - 18.2.1 Integer values - 18.2.2 Blocks - 18.2.3 Pointers outside the heap - 18.3 Representation of Caml data types - 18.3.1 Atomic types - 18.3.2 Tuples and records - 18.3.3 Arrays - 18.3.4 Concrete types - 18.3.5 Objects - 18.3.6 Variants - 18.4 Operations on values - 18.4.1 Kind tests - 18.4.2 Operations on integers - 18.4.3 Accessing blocks - 18.4.4 Allocating blocks - 18.4.5 Raising exceptions - 18.5 Living in harmony with the garbage collector - 18.5.1 Simple interface - 18.5.2 Low-level interface - 18.6 A complete example - 18.7 Advanced topic: callbacks from C to Caml - 18.7.1 Applying Caml closures from C - 18.7.2 Registering Caml closures for use in C functions - 18.7.3 Registering Caml exceptions for use in C functions - 18.7.4 Main program in C - 18.7.5 Embedding the Caml code in the C code - 18.8 Advanced example with callbacks - 18.9 Advanced topic: custom blocks - 18.9.1 The struct custom_operations - 18.9.2 Allocating custom blocks - 18.9.3 Accessing custom blocks - 18.9.4 Writing custom serialization and deserialization functions - 18.9.5 Choosing identifiers - 18.9.6 Finalized blocks - 18.10 Building mixed C/Caml libraries: ocamlmklib - Part: IV The Objective Caml library - Chapter 19 The core library - 19.1 Built-in types and predefined exceptions - 19.2 Module Pervasives : The initially opened module. - 19.2.1 Module Pervasives.LargeFile : Operations on large files. - Chapter 20 The standard library - 20.1 Module Arg : Parsing of command line arguments. - 20.2 Module Array : Array operations. - 20.3 Module Buffer : Extensible string buffers. - 20.4 Module Callback : Registering Caml values with the C runtime. - 20.5 Module Char : Character operations. - 20.6 Module Complex : Complex numbers. - 20.7 Module Digest : MD5 message digest. - 20.8 Module Filename : Operations on file names. - 20.9 Module Format : Pretty printing. - 20.10 Module Gc : Memory management control and statistics; finalised values. - 20.11 Module Genlex : A generic lexical analyzer. - 20.12 Module Hashtbl : Hash tables and hash functions. - 20.12.1 Module type Hashtbl.HashedType : The input signature of the functor Hashtbl.Make[20.12.3]. - 20.12.2 Module type Hashtbl.S : The output signature of the functor Hashtbl.Make[20.12.3]. - 20.12.3 Module Hashtbl.Make : Functor building an implementation of the hashtable structure. - 20.13 Module Int32 : 32-bit integers. - 20.14 Module Int64 : 64-bit integers. - 20.15 Module Lazy : Deferred computations. - 20.16 Module Lexing : The run-time library for lexers generated by ocamllex. - 20.17 Module List : List operations. - 20.18 Module Map : Association tables over ordered types. - 20.18.1 Module type Map.OrderedType : Input signature of the functor Map.Make[20.18.3]. - 20.18.2 Module type Map.S : Output signature of the functor Map.Make[20.18.3]. - 20.18.3 Module Map.Make : Functor building an implementation of the map structure given a totally ordered type. - 20.19 Module Marshal : Marshaling of data structures. - 20.20 Module Nativeint : Processor-native integers. - 20.21 Module Oo : Operations on objects - 20.22 Module Parsing : The run-time library for parsers generated by ocamlyacc. - 20.23 Module Printexc : Facilities for printing exceptions. - 20.24 Module Printf : Formatted output functions. - 20.25 Module Queue : First-in first-out queues. - 20.26 Module Random : Pseudo-random number generator (PRNG). - 20.27 Module Scanf : Formatted input functions. - 20.27.1 Module Scanf.Scanning : Scanning buffers. - 20.28 Module Set : Sets over ordered types. - 20.28.1 Module type Set.OrderedType : Input signature of the functor Set.Make[20.28.3]. - 20.28.2 Module type Set.S : Output signature of the functor Set.Make[20.28.3]. - 20.28.3 Module Set.Make : Functor building an implementation of the set structure given a totally ordered type. - 20.29 Module Sort : Sorting and merging lists. - 20.30 Module Stack : Last-in first-out stacks. - 20.31 Module StdLabels : Standard labeled libraries. - 20.31.1 Module StdLabels.Array - 20.31.2 Module StdLabels.List - 20.31.3 Module StdLabels.String - 20.32 Module Stream : Streams and parsers. - 20.33 Module String : String operations. - 20.34 Module Sys : System interface. - 20.35 Module Weak : Arrays of weak pointers and hash tables of weak pointers. - 20.35.1 Module type Weak.S : The output signature of the functor Weak.Make[20.35.2]. - 20.35.2 Module Weak.Make : Functor building an implementation of the weak hash table structure. - Chapter 21 The unix library: Unix system calls - 21.1 Module Unix : Interface to the Unix system - 21.1.1 Module Unix.LargeFile : File operations on large files. - 21.2 Module UnixLabels: labelized version of the interface - Chapter 22 The num library: arbitrary-precision rational arithmetic - 22.1 Module Num : Operation on arbitrary-precision numbers. - 22.2 Module Big_int : Operations on arbitrary-precision integers. - 22.3 Module Arith_status : Flags that control rational arithmetic. - Chapter 23 The str library: regular expressions and string processing - 23.1 Module Str : Regular expressions and high-level string processing - Chapter 24 The threads library - 24.1 Module Thread : Lightweight threads for Posix 1003.1c and Win32. - 24.2 Module Mutex : Locks for mutual exclusion. - 24.3 Module Condition : Condition variables to synchronize between threads. - 24.4 Module Event : First-class synchronous communication. - 24.5 Module ThreadUnix : Thread-compatible system calls. - Chapter 25 The graphics library - 25.1 Module Graphics : Machine-independent graphics primitives. - Chapter 26 The dbm library: access to NDBM databases - 26.1 Module Dbm : Interface to the NDBM database. - Chapter 27 The dynlink library: dynamic loading and linking of object files - 27.1 Module Dynlink : Dynamic loading of bytecode object files. - Chapter 28 The LablTk library: Tcl/Tk GUI interface - 28.1 Module Tk : Basic functions and types for LablTk - Chapter 29 The bigarray library - 29.1 Module Bigarray : Large, multi-dimensional, numerical arrays. - 29.1.1 Module Bigarray.Genarray - 29.1.2 Module Bigarray.Array1 : One-dimensional arrays. - 29.1.3 Module Bigarray.Array2 : Two-dimensional arrays. - 29.1.4 Module Bigarray.Array3 : Three-dimensional arrays. - 29.2 Big arrays in the Caml-C interface - 29.2.1 Include file - 29.2.2 Accessing a Caml bigarray from C or Fortran - 29.2.3 Wrapping a C or Fortran array as a Caml big array - Part: V Appendix - Chapter 30 Index to the library - Chapter 31 Index of keywords --------------------------------------------------------------- This manual is also available in PDF (1). Postscript (2), DVI (3), plain text (4), as a bundle of HTML files (5), and as a bundle of Emacs Info files (6). --------------------------------------------------------------- --------------------------------------- (1) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.pdf (2) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.ps.gz (3) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.dvi.gz (4) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.txt (5) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.html.tar.gz (6) ftp://ftp.inria.fr/lang/caml-light/ocaml-3.06-refman.info.tar.gz Foreword ******** This manual documents the release 3.06 of the Objective Caml system. It is organized as follows. - Part I, ``An introduction to Objective Caml'', gives an overview of the language. - Part II, ``The Objective Caml language'', is the reference description of the language. - Part III, ``The Objective Caml tools'', documents the compilers, toplevel system, and programming utilities. - Part IV, ``The Objective Caml library'', describes the modules provided in the standard library. Conventions *=*=*=*=*=* Objective Caml runs on several operating systems. The parts of this manual that are specific to one operating system are presented as shown below: MacOS: This is material specific to MacOS 7, 8, 9. (For MacOS X, see ``Unix''.) Unix: This is material specific to the Unix family of operating systems, including Linux and MacOS X. Windows: This is material specific to Microsoft Windows (95, 98, ME, NT, 2000). License *=*=*=* The Objective Caml system is copyright © 1996, 1997, 1998, 1999, 2000, 2001, 2002 Institut National de Recherche en Informatique et en Automatique (INRIA). INRIA holds all ownership rights to the Objective Caml system. The Objective Caml system is open source and can be freely redistributed. See the file LICENSE in the distribution for licensing information. The present documentation is copyright © 2002 Institut National de Recherche en Informatique et en Automatique (INRIA). The Objective Caml documentation and user's manual may be reproduced and distributed in whole or in part, subject to the following conditions: - The copyright notice above and this permission notice must be preserved complete on all complete or partial copies. - Any translation or derivative work of the Objective Caml documentation and user's manual must be approved by the authors in writing before distribution. - If you distribute the Objective Caml documentation and user's manual in part, instructions for obtaining the complete version of this manual must be included, and a means for obtaining a complete version provided. - Small portions may be reproduced as illustrations for reviews or quotes in other works without this permission notice if proper citation is given. Availability *=*=*=*=*=*= The complete Objective Caml distribution can be accessed via the http://caml.inria.fr/Caml Web site. The http://caml.inria.fr/Caml Web site contains a lot of additional information on Objective Caml. Part: I ******* An introduction to Objective Caml ********************************* Chapter 1 The core language ****************************** This part of the manual is a tutorial introduction to the Objective Caml language. A good familiarity with programming in a conventional languages (say, Pascal or C) is assumed, but no prior exposure to functional languages is required. The present chapter introduces the core language. Chapter 3 deals with the object-oriented features, and chapter 2 with the module system. 1.1 Basics *=*=*=*=*=* For this overview of Caml, we use the interactive system, which is started by running ocaml from the Unix shell, or by launching the OCamlwin.exe application under Windows. This tutorial is presented as the transcript of a session with the interactive system: lines starting with # represent user input; the system responses are printed below, without a leading #. Under the interactive system, the user types Caml phrases, terminated by ;;, in response to the # prompt, and the system compiles them on the fly, executes them, and prints the outcome of evaluation. Phrases are either simple expressions, or let definitions of identifiers (either values or functions). <<#1+2*3;; - : int = 7 #let pi = 4.0 *. atan 1.0;; val pi : float = 3.14159265359 #let square x = x *. x;; val square : float -> float = #square(sin pi) +. square(cos pi);; - : float = 1. >> The Caml system computes both the value and the type for each phrase. Even function parameters need no explicit type declaration: the system infers their types from their usage in the function. Notice also that integers and floating-point numbers are distinct types, with distinct operators: + and * operate on integers, but +. and *. operate on floats. <<#1.0 * 2;; This expression has type float but is here used with type int >> Recursive functions are defined with the let rec binding: <<#let rec fib n = # if n < 2 then 1 else fib(n-1) + fib(n-2);; val fib : int -> int = #fib 10;; - : int = 89 >> 1.2 Data types *=*=*=*=*=*=*=* In addition to integers and floating-point numbers, Caml offers the usual basic data types: booleans, characters, and character strings. <<#(1 < 2) = false;; - : bool = false #'a';; - : char = 'a' #"Hello world";; - : string = "Hello world" >> Predefined data structures include tuples, arrays, and lists. General mechanisms for defining your own data structures are also provided. They will be covered in more details later; for now, we concentrate on lists. Lists are either given in extension as a bracketed list of semicolon-separated elements, or built from the empty list [] (pronounce ``nil'') by adding elements in front using the :: (``cons'') operator. <<#let l = ["is"; "a"; "tale"; "told"; "etc."];; val l : string list = ["is"; "a"; "tale"; "told"; "etc."] #"Life" :: l;; - : string list = ["Life"; "is"; "a"; "tale"; "told"; "etc."] >> As with all other Caml data structures, lists do not need to be explicitly allocated and deallocated from memory: all memory management is entirely automatic in Caml. Similarly, there is no explicit handling of pointers: the Caml compiler silently introduces pointers where necessary. As with most Caml data structures, inspecting and destructuring lists is performed by pattern-matching. List patterns have the exact same shape as list expressions, with identifier representing unspecified parts of the list. As an example, here is insertion sort on a list: <<#let rec sort lst = # match lst with # [] -> [] # | head :: tail -> insert head (sort tail) #and insert elt lst = # match lst with # [] -> [elt] # | head :: tail -> if elt <= head then elt :: lst else head :: insert elt tail #;; val sort : 'a list -> 'a list = val insert : 'a -> 'a list -> 'a list = #sort l;; - : string list = ["a"; "etc."; "is"; "tale"; "told"] >> The type inferred for sort, 'a list -> 'a list, means that sort can actually apply to lists of any type, and returns a list of the same type. The type 'a is a type variable, and stands for any given type. The reason why sort can apply to lists of any type is that the comparisons (=, <=, etc.) are polymorphic in Caml: they operate between any two values of the same type. This makes sort itself polymorphic over all list types. <<#sort [6;2;5;3];; - : int list = [2; 3; 5; 6] #sort [3.14; 2.718];; - : float list = [2.718; 3.14] >> The sort function above does not modify its input list: it builds and returns a new list containing the same elements as the input list, in ascending order. There is actually no way in Caml to modify in-place a list once it is built: we say that lists are immutable data structures. Most Caml data structures are immutable, but a few (most notably arrays) are mutable, meaning that they can be modified in-place at any time. 1.3 Functions as values *=*=*=*=*=*=*=*=*=*=*=*= Caml is a functional language: functions in the full mathematical sense are supported and can be passed around freely just as any other piece of data. For instance, here is a deriv function that takes any float function as argument and returns an approximation of its derivative function: <<#let deriv f dx = function x -> (f(x +. dx) -. f(x)) /. dx;; val deriv : (float -> float) -> float -> float -> float = #let sin' = deriv sin 1e-6;; val sin' : float -> float = #sin' pi;; - : float = -1.00000000014 >> Even function composition is definable: <<#let compose f g = function x -> f(g(x));; val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = #let cos2 = compose square cos;; val cos2 : float -> float = >> Functions that take other functions as arguments are called ``functionals'', or ``higher-order functions''. Functionals are especially useful to provide iterators or similar generic operations over a data structure. For instance, the standard Caml library provides a List.map functional that applies a given function to each element of a list, and returns the list of the results: <<#List.map (function n -> n * 2 + 1) [0;1;2;3;4];; - : int list = [1; 3; 5; 7; 9] >> This functional, along with a number of other list and array functionals, is predefined because it is often useful, but there is nothing magic with it: it can easily be defined as follows. <<#let rec map f l = # match l with # [] -> [] # | hd :: tl -> f hd :: map f tl;; val map : ('a -> 'b) -> 'a list -> 'b list = >> 1.4 Records and variants *=*=*=*=*=*=*=*=*=*=*=*=* User-defined data structures include records and variants. Both are defined with the type declaration. Here, we declare a record type to represent rational numbers. <<#type ratio = {num: int; denum: int};; type ratio = { num : int; denum : int; } #let add_ratio r1 r2 = # {num = r1.num * r2.denum + r2.num * r1.denum; # denum = r1.denum * r2.denum};; val add_ratio : ratio -> ratio -> ratio = #add_ratio {num=1; denum=3} {num=2; denum=5};; - : ratio = {num = 11; denum = 15} >> The declaration of a variant type lists all possible shapes for values of that type. Each case is identified by a name, called a constructor, which serves both for constructing values of the variant type and inspecting them by pattern-matching. Constructor names are capitalized to distinguish them from variable names (which must start with a lowercase letter). For instance, here is a variant type for doing mixed arithmetic (integers and floats): <<#type number = Int of int | Float of float | Error;; type number = Int of int | Float of float | Error >> This declaration expresses that a value of type number is either an integer, a floating-point number, or the constant Error representing the result of an invalid operation (e.g. a division by zero). Enumerated types are a special case of variant types, where all alternatives are constants: <<#type sign = Positive | Negative;; type sign = Positive | Negative #let sign_int n = if n >= 0 then Positive else Negative;; val sign_int : int -> sign = >> To define arithmetic operations for the number type, we use pattern-matching on the two numbers involved: <<#let add_num n1 n2 = # match (n1, n2) with # (Int i1, Int i2) -> # (* Check for overflow of integer addition *) # if sign_int i1 = sign_int i2 && sign_int(i1 + i2) <> sign_int i1 # then Float(float i1 +. float i2) # else Int(i1 + i2) # | (Int i1, Float f2) -> Float(float i1 +. f2) # | (Float f1, Int i2) -> Float(f1 +. float i2) # | (Float f1, Float f2) -> Float(f1 +. f2) # | (Error, _) -> Error # | (_, Error) -> Error;; val add_num : number -> number -> number = #add_num (Int 123) (Float 3.14159);; - : number = Float 126.14159 >> The most common usage of variant types is to describe recursive data structures. Consider for example the type of binary trees: <<#type 'a btree = Empty | Node of 'a * 'a btree * 'a btree;; type 'a btree = Empty | Node of 'a * 'a btree * 'a btree >> This definition reads as follow: a binary tree containing values of type 'a (an arbitrary type) is either empty, or is a node containing one value of type 'a and two subtrees containing also values of type 'a, that is, two 'a btree. Operations on binary trees are naturally expressed as recursive functions following the same structure as the type definition itself. For instance, here are functions performing lookup and insertion in ordered binary trees (elements increase from left to right): <<#let rec member x btree = # match btree with # Empty -> false # | Node(y, left, right) -> # if x = y then true else # if x < y then member x left else member x right;; val member : 'a -> 'a btree -> bool = #let rec insert x btree = # match btree with # Empty -> Node(x, Empty, Empty) # | Node(y, left, right) -> # if x <= y then Node(y, insert x left, right) # else Node(y, left, insert x right);; val insert : 'a -> 'a btree -> 'a btree = >> 1.5 Imperative features *=*=*=*=*=*=*=*=*=*=*=*= Though all examples so far were written in purely applicative style, Caml is also equipped with full imperative features. This includes the usual while and for loops, as well as mutable data structures such as arrays. Arrays are either given in extension between [| and |] brackets, or allocated and initialized with the Array.create function, then filled up later by assignments. For instance, the function below sums two vectors (represented as float arrays) componentwise. <<#let add_vect v1 v2 = # let len = min (Array.length v1) (Array.length v2) in # let res = Array.create len 0.0 in # for i = 0 to len - 1 do # res.(i) <- v1.(i) +. v2.(i) # done; # res;; val add_vect : float array -> float array -> float array = #add_vect [| 1.0; 2.0 |] [| 3.0; 4.0 |];; - : float array = [|4.; 6.|] >> Record fields can also be modified by assignment, provided they are declared mutable in the definition of the record type: <<#type mutable_point = { mutable x: float; mutable y: float };; type mutable_point = { mutable x : float; mutable y : float; } #let translate p dx dy = # p.x <- p.x +. dx; p.y <- p.y +. dy;; val translate : mutable_point -> float -> float -> unit = #let mypoint = { x = 0.0; y = 0.0 };; val mypoint : mutable_point = {x = 0.; y = 0.} #translate mypoint 1.0 2.0;; - : unit = () #mypoint;; - : mutable_point = {x = 1.; y = 2.} >> Caml has no built-in notion of variable -- identifiers whose current value can be changed by assignment. (The let binding is not an assignment, it introduces a new identifier with a new scope.) However, the standard library provides references, which are mutable indirection cells (or one-element arrays), with operators ! to fetch the current contents of the reference and := to assign the contents. Variables can then be emulated by let-binding a reference. For instance, here is an in-place insertion sort over arrays: <<#let insertion_sort a = # for i = 1 to Array.length a - 1 do # let val_i = a.(i) in # let j = ref i in # while !j > 0 && val_i < a.(!j - 1) do # a.(!j) <- a.(!j - 1); # j := !j - 1 # done; # a.(!j) <- val_i # done;; val insertion_sort : 'a array -> unit = >> References are also useful to write functions that maintain a current state between two calls to the function. For instance, the following pseudo-random number generator keeps the last returned number in a reference: <<#let current_rand = ref 0;; val current_rand : int ref = {contents = 0} #let random () = # current_rand := !current_rand * 25713 + 1345; # !current_rand;; val random : unit -> int = >> Again, there is nothing magic with references: they are implemented as a one-field mutable record, as follows. <<#type 'a ref = { mutable contents: 'a };; type 'a ref = { mutable contents : 'a; } #let (!) r = r.contents;; val ( ! ) : 'a ref -> 'a = #let (:=) r newval = r.contents <- newval;; val ( := ) : 'a ref -> 'a -> unit = >> In some special cases, you may need to store a polymorphic function in a data structure, keeping its polymorphism. Without user-provided type annotations, this is not allowed, as polymorphism is only introduced on a global level. However, you can give explicitly polymorphic types to record fields. <<#type idref = { mutable id: 'a. 'a -> 'a };; type idref = { mutable id : 'a. 'a -> 'a; } #let r = {id = fun x -> x};; val r : idref = {id = } #let g s = (s.id 1, s.id true);; val g : idref -> int * bool = #r.id <- (fun x -> print_string "called id\n"; x);; - : unit = () #g r;; called id called id - : int * bool = (1, true) >> 1.6 Exceptions *=*=*=*=*=*=*=* Caml provides exceptions for signalling and handling exceptional conditions. Exceptions can also be used as a general-purpose non-local control structure. Exceptions are declared with the exception construct, and signalled with the raise operator. For instance, the function below for taking the head of a list uses an exception to signal the case where an empty list is given. <<#exception Empty_list;; exception Empty_list #let head l = # match l with # [] -> raise Empty_list # | hd :: tl -> hd;; val head : 'a list -> 'a = #head [1;2];; - : int = 1 #head [];; Exception: Empty_list. >> Exceptions are used throughout the standard library to signal cases where the library functions cannot complete normally. For instance, the List.assoc function, which returns the data associated with a given key in a list of (key, data) pairs, raises the predefined exception Not_found when the key does not appear in the list: <<#List.assoc 1 [(0, "zero"); (1, "one")];; - : string = "one" #List.assoc 2 [(0, "zero"); (1, "one")];; Exception: Not_found. >> Exceptions can be trapped with the try...with construct: <<#let name_of_binary_digit digit = # try # List.assoc digit [0, "zero"; 1, "one"] # with Not_found -> # "not a binary digit";; val name_of_binary_digit : int -> string = #name_of_binary_digit 0;; - : string = "zero" #name_of_binary_digit (-1);; - : string = "not a binary digit" >> The with part is actually a regular pattern-matching on the exception value. Thus, several exceptions can be caught by one try...with construct. Also, finalization can be performed by trapping all exceptions, performing the finalization, then raising again the exception: <<#let temporarily_set_reference ref newval funct = # let oldval = !ref in # try # ref := newval; # let res = funct () in # ref := oldval; # res # with x -> # ref := oldval; # raise x;; val temporarily_set_reference : 'a ref -> 'a -> (unit -> 'b) -> 'b = >> 1.7 Symbolic processing of expressions *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* We finish this introduction with a more complete example representative of the use of Caml for symbolic processing: formal manipulations of arithmetic expressions containing variables. The following variant type describes the expressions we shall manipulate: <<#type expression = # Const of float # | Var of string # | Sum of expression * expression (* e1 + e2 *) # | Diff of expression * expression (* e1 - e2 *) # | Prod of expression * expression (* e1 * e2 *) # | Quot of expression * expression (* e1 / e2 *) #;; type expression = Const of float | Var of string | Sum of expression * expression | Diff of expression * expression | Prod of expression * expression | Quot of expression * expression >> We first define a function to evaluate an expression given an environment that maps variable names to their values. For simplicity, the environment is represented as an association list. <<#exception Unbound_variable of string;; exception Unbound_variable of string #let rec eval env exp = # match exp with # Const c -> c # | Var v -> # (try List.assoc v env with Not_found -> raise(Unbound_variable v)) # | Sum(f, g) -> eval env f +. eval env g # | Diff(f, g) -> eval env f -. eval env g # | Prod(f, g) -> eval env f *. eval env g # | Quot(f, g) -> eval env f /. eval env g;; val eval : (string * float) list -> expression -> float = #eval [("x", 1.0); ("y", 3.14)] (Prod(Sum(Var "x", Const 2.0), Var "y"));; - : float = 9.42 >> Now for a real symbolic processing, we define the derivative of an expression with respect to a variable dv: <<#let rec deriv exp dv = # match exp with # Const c -> Const 0.0 # | Var v -> if v = dv then Const 1.0 else Const 0.0 # | Sum(f, g) -> Sum(deriv f dv, deriv g dv) # | Diff(f, g) -> Diff(deriv f dv, deriv g dv) # | Prod(f, g) -> Sum(Prod(f, deriv g dv), Prod(deriv f dv, g)) # | Quot(f, g) -> Quot(Diff(Prod(deriv f dv, g), Prod(f, deriv g dv)), # Prod(g, g)) #;; val deriv : expression -> string -> expression = #deriv (Quot(Const 1.0, Var "x")) "x";; - : expression = Quot (Diff (Prod (Const 0., Var "x"), Prod (Const 1., Const 1.)), Prod (Var "x", Var "x")) >> 1.8 Pretty-printing and parsing *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= As shown in the examples above, the internal representation (also called abstract syntax) of expressions quickly becomes hard to read and write as the expressions get larger. We need a printer and a parser to go back and forth between the abstract syntax and the concrete syntax, which in the case of expressions is the familiar algebraic notation (e.g. 2*x+1). For the printing function, we take into account the usual precedence rules (i.e. * binds tighter than +) to avoid printing unnecessary parentheses. To this end, we maintain the current operator precedence and print parentheses around an operator only if its precedence is less than the current precedence. <<#let print_expr exp = # (* Local function definitions *) # let open_paren prec op_prec = # if prec > op_prec then print_string "(" in # let close_paren prec op_prec = # if prec > op_prec then print_string ")" in # let rec print prec exp = (* prec is the current precedence *) # match exp with # Const c -> print_float c # | Var v -> print_string v # | Sum(f, g) -> # open_paren prec 0; # print 0 f; print_string " + "; print 0 g; # close_paren prec 0 # | Diff(f, g) -> # open_paren prec 0; # print 0 f; print_string " - "; print 1 g; # close_paren prec 0 # | Prod(f, g) -> # open_paren prec 2; # print 2 f; print_string " * "; print 2 g; # close_paren prec 2 # | Quot(f, g) -> # open_paren prec 2; # print 2 f; print_string " / "; print 3 g; # close_paren prec 2 # in print 0 exp;; val print_expr : expression -> unit = #let e = Sum(Prod(Const 2.0, Var "x"), Const 1.0);; val e : expression = Sum (Prod (Const 2., Var "x"), Const 1.) #print_expr e; print_newline();; 2. * x + 1. - : unit = () #print_expr (deriv e "x"); print_newline();; 2. * 1. + 0. * x + 0. - : unit = () >> Parsing (transforming concrete syntax into abstract syntax) is usually more delicate. Caml offers several tools to help write parsers: on the one hand, Caml versions of the lexer generator Lex and the parser generator Yacc (see chapter 12), which handle LALR(1) languages using push-down automata; on the other hand, a predefined type of streams (of characters or tokens) and pattern-matching over streams, which facilitate the writing of recursive-descent parsers for LL(1) languages. An example using ocamllex and ocamlyacc is given in chapter 12. Here, we will use stream parsers. The syntactic support for stream parsers is provided by the Camlp4 preprocessor, which can be loaded into the interactive toplevel via the #load directive below. <<##load "camlp4o.cma";; Camlp4 Parsing version 3.05 (2002-07-22) #open Genlex;; #let lexer = make_lexer ["("; ")"; "+"; "-"; "*"; "/"];; val lexer : char Stream.t -> Genlex.token Stream.t = >> For the lexical analysis phase (transformation of the input text into a stream of tokens), we use a ``generic'' lexer provided in the standard library module Genlex. The make_lexer function takes a list of keywords and returns a lexing function that ``tokenizes'' an input stream of characters. Tokens are either identifiers, keywords, or literals (integer, floats, characters, strings). Whitespace and comments are skipped. <<#let token_stream = lexer(Stream.of_string "1.0 +x");; val token_stream : Genlex.token Stream.t = #Stream.next token_stream;; - : Genlex.token = Float 1. #Stream.next token_stream;; - : Genlex.token = Kwd "+" #Stream.next token_stream;; - : Genlex.token = Ident "x" >> The parser itself operates by pattern-matching on the stream of tokens. As usual with recursive descent parsers, we use several intermediate parsing functions to reflect the precedence and associativity of operators. Pattern-matching over streams is more powerful than on regular data structures, as it allows recursive calls to parsing functions inside the patterns, for matching sub-components of the input stream. See chapter 7 for more details. In order to use stream parsers at toplevel, we must first load the camlp4 preprocessor. <<##load"camlp4o.cma";; Camlp4 Parsing version 3.05 (2002-07-22) >> Then we are ready to define our parser. <<#let rec parse_expr = parser # [< e1 = parse_mult; e = parse_more_adds e1 >] -> e #and parse_more_adds e1 = parser # [< 'Kwd "+"; e2 = parse_mult; e = parse_more_adds (Sum(e1, e2)) >] -> e # | [< 'Kwd "-"; e2 = parse_mult; e = parse_more_adds (Diff(e1, e2)) >] -> e # | [< >] -> e1 #and parse_mult = parser # [< e1 = parse_simple; e = parse_more_mults e1 >] -> e #and parse_more_mults e1 = parser # [< 'Kwd "*"; e2 = parse_simple; e = parse_more_mults (Prod(e1, e2)) >] -> e # | [< 'Kwd "/"; e2 = parse_simple; e = parse_more_mults (Quot(e1, e2)) >] -> e # | [< >] -> e1 #and parse_simple = parser # [< 'Ident s >] -> Var s # | [< 'Int i >] -> Const(float i) # | [< 'Float f >] -> Const f # | [< 'Kwd "("; e = parse_expr; 'Kwd ")" >] -> e;; val parse_expr : Genlex.token Stream.t -> expression = val parse_more_adds : expression -> Genlex.token Stream.t -> expression = val parse_mult : Genlex.token Stream.t -> expression = val parse_more_mults : expression -> Genlex.token Stream.t -> expression = val parse_simple : Genlex.token Stream.t -> expression = #let parse_expression = parser [< e = parse_expr; _ = Stream.empty >] -> e;; val parse_expression : Genlex.token Stream.t -> expression = >> Composing the lexer and parser, we finally obtain a function to read an expression from a character string: <<#let read_expression s = parse_expression(lexer(Stream.of_string s));; val read_expression : string -> expression = #read_expression "2*(x+y)";; - : expression = Prod (Const 2., Sum (Var "x", Var "y")) >> A small puzzle: why do we get different results in the following two examples? <<#read_expression "x - 1";; - : expression = Diff (Var "x", Const 1.) #read_expression "x-1";; Exception: Stream.Error "". >> Answer: the generic lexer provided by Genlex recognizes negative integer literals as one integer token. Hence, x-1 is read as the token Ident "x" followed by the token Int(-1); this sequence does not match any of the parser rules. On the other hand, the second space in x - 1 causes the lexer to return the three expected tokens: Ident "x", then Kwd "-", then Int(1). 1.9 Standalone Caml programs *=*=*=*=*=*=*=*=*=*=*=*=*=*=* All examples given so far were executed under the interactive system. Caml code can also be compiled separately and executed non-interactively using the batch compilers ocamlc or ocamlopt. The source code must be put in a file with extension .ml. It consists of a sequence of phrases, which will be evaluated at runtime in their order of appearance in the source file. Unlike in interactive mode, types and values are not printed automatically; the program must call printing functions explicitly to produce some output. Here is a sample standalone program to print Fibonacci numbers: << (* File fib.ml *) let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2);; let main () = let arg = int_of_string Sys.argv.(1) in print_int(fib arg); print_newline(); exit 0;; main ();; >> Sys.argv is an array of strings containing the command-line parameters. Sys.argv.(1) is thus the first command-line parameter. The program above is compiled and executed with the following shell commands: << $ ocamlc -o fib fib.ml $ ./fib 10 89 $ ./fib 20 10946 >> Chapter 2 The module system ****************************** This chapter introduces the module system of Objective Caml. 2.1 Structures *=*=*=*=*=*=*=* A primary motivation for modules is to package together related definitions (such as the definitions of a data type and associated operations over that type) and enforce a consistent naming scheme for these definitions. This avoids running out of names or accidentally confusing names. Such a package is called a structure and is introduced by the struct...end construct, which contains an arbitrary sequence of definitions. The structure is usually given a name with the module binding. Here is for instance a structure packaging together a type of priority queues and their operations: <<#module PrioQueue = # struct # type priority = int # type 'a queue = Empty | Node of priority * 'a * 'a queue * 'a queue # let empty = Empty # let rec insert queue prio elt = # match queue with # Empty -> Node(prio, elt, Empty, Empty) # | Node(p, e, left, right) -> # if prio <= p # then Node(prio, elt, insert right p e, left) # else Node(p, e, insert right prio elt, left) # exception Queue_is_empty # let rec remove_top = function # Empty -> raise Queue_is_empty # | Node(prio, elt, left, Empty) -> left # | Node(prio, elt, Empty, right) -> right # | Node(prio, elt, (Node(lprio, lelt, _, _) as left), # (Node(rprio, relt, _, _) as right)) -> # if lprio <= rprio # then Node(lprio, lelt, remove_top left, right) # else Node(rprio, relt, left, remove_top right) # let extract = function # Empty -> raise Queue_is_empty # | Node(prio, elt, _, _) as queue -> (prio, elt, remove_top queue) # end;; module PrioQueue : sig type priority = int and 'a queue = Empty | Node of priority * 'a * 'a queue * 'a queue val empty : 'a queue val insert : 'a queue -> priority -> 'a -> 'a queue exception Queue_is_empty val remove_top : 'a queue -> 'a queue val extract : 'a queue -> priority * 'a * 'a queue end >> Outside the structure, its components can be referred to using the ``dot notation'', that is, identifiers qualified by a structure name. For instance, PrioQueue.insert in a value context is the function insert defined inside the structure PrioQueue. Similarly, PrioQueue.queue in a type context is the type queue defined in PrioQueue. <<#PrioQueue.insert PrioQueue.empty 1 "hello";; - : string PrioQueue.queue = PrioQueue.Node (1, "hello", PrioQueue.Empty, PrioQueue.Empty) >> 2.2 Signatures *=*=*=*=*=*=*=* Signatures are interfaces for structures. A signature specifies which components of a structure are accessible from the outside, and with which type. It can be used to hide some components of a structure (e.g. local function definitions) or export some components with a restricted type. For instance, the signature below specifies the three priority queue operations empty, insert and extract, but not the auxiliary function remove_top. Similarly, it makes the queue type abstract (by not providing its actual representation as a concrete type). <<#module type PRIOQUEUE = # sig # type priority = int (* still concrete *) # type 'a queue (* now abstract *) # val empty : 'a queue # val insert : 'a queue -> int -> 'a -> 'a queue # val extract : 'a queue -> int * 'a * 'a queue # exception Queue_is_empty # end;; module type PRIOQUEUE = sig type priority = int and 'a queue val empty : 'a queue val insert : 'a queue -> int -> 'a -> 'a queue val extract : 'a queue -> int * 'a * 'a queue exception Queue_is_empty end >> Restricting the PrioQueue structure by this signature results in another view of the PrioQueue structure where the remove_top function is not accessible and the actual representation of priority queues is hidden: <<#module AbstractPrioQueue = (PrioQueue : PRIOQUEUE);; module AbstractPrioQueue : PRIOQUEUE #AbstractPrioQueue.remove_top;; Unbound value AbstractPrioQueue.remove_top #AbstractPrioQueue.insert AbstractPrioQueue.empty 1 "hello";; - : string AbstractPrioQueue.queue = >> The restriction can also be performed during the definition of the structure, as in << module PrioQueue = (struct ... end : PRIOQUEUE);; >> An alternate syntax is provided for the above: << module PrioQueue : PRIOQUEUE = struct ... end;; >> 2.3 Functors *=*=*=*=*=*=* Functors are ``functions'' from structures to structures. They are used to express parameterized structures: a structure A parameterized by a structure B is simply a functor F with a formal parameter B (along with the expected signature for B) which returns the actual structure A itself. The functor F can then be applied to one or several implementations B_1 ...B_n of B, yielding the corresponding structures A_1 ...A_n. For instance, here is a structure implementing sets as sorted lists, parameterized by a structure providing the type of the set elements and an ordering function over this type (used to keep the sets sorted): <<#type comparison = Less | Equal | Greater;; type comparison = Less | Equal | Greater #module type ORDERED_TYPE = # sig # type t # val compare: t -> t -> comparison # end;; module type ORDERED_TYPE = sig type t val compare : t -> t -> comparison end #module Set = # functor (Elt: ORDERED_TYPE) -> # struct # type element = Elt.t # type set = element list # let empty = [] # let rec add x s = # match s with # [] -> [x] # | hd::tl -> # match Elt.compare x hd with # Equal -> s (* x is already in s *) # | Less -> x :: s (* x is smaller than all elements of s *) # | Greater -> hd :: add x tl # let rec member x s = # match s with # [] -> false # | hd::tl -> # match Elt.compare x hd with # Equal -> true (* x belongs to s *) # | Less -> false (* x is smaller than all elements of s *) # | Greater -> member x tl # end;; module Set : functor (Elt : ORDERED_TYPE) -> sig type element = Elt.t and set = element list val empty : 'a list val add : Elt.t -> Elt.t list -> Elt.t list val member : Elt.t -> Elt.t list -> bool end >> By applying the Set functor to a structure implementing an ordered type, we obtain set operations for this type: <<#module OrderedString = # struct # type t = string # let compare x y = if x = y then Equal else if x < y then Less else Greater # end;; module OrderedString : sig type t = string val compare : 'a -> 'a -> comparison end #module StringSet = Set(OrderedString);; module StringSet : sig type element = OrderedString.t and set = element list val empty : 'a list val add : OrderedString.t -> OrderedString.t list -> OrderedString.t list val member : OrderedString.t -> OrderedString.t list -> bool end #StringSet.member "bar" (StringSet.add "foo" StringSet.empty);; - : bool = false >> 2.4 Functors and type abstraction *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= As in the PrioQueue example, it would be good style to hide the actual implementation of the type set, so that users of the structure will not rely on sets being lists, and we can switch later to another, more efficient representation of sets without breaking their code. This can be achieved by restricting Set by a suitable functor signature: <<#module type SETFUNCTOR = # functor (Elt: ORDERED_TYPE) -> # sig # type element = Elt.t (* concrete *) # type set (* abstract *) # val empty : set # val add : element -> set -> set # val member : element -> set -> bool # end;; module type SETFUNCTOR = functor (Elt : ORDERED_TYPE) -> sig type element = Elt.t and set val empty : set val add : element -> set -> set val member : element -> set -> bool end #module AbstractSet = (Set : SETFUNCTOR);; module AbstractSet : SETFUNCTOR #module AbstractStringSet = AbstractSet(OrderedString);; module AbstractStringSet : sig type element = OrderedString.t and set = AbstractSet(OrderedString).set val empty : set val add : element -> set -> set val member : element -> set -> bool end #AbstractStringSet.add "gee" AbstractStringSet.empty;; - : AbstractStringSet.set = >> In an attempt to write the type constraint above more elegantly, one may wish to name the signature of the structure returned by the functor, then use that signature in the constraint: <<#module type SET = # sig # type element # type set # val empty : set # val add : element -> set -> set # val member : element -> set -> bool # end;; module type SET = sig type element and set val empty : set val add : element -> set -> set val member : element -> set -> bool end #module WrongSet = (Set : functor(Elt: ORDERED_TYPE) -> SET);; module WrongSet : functor (Elt : ORDERED_TYPE) -> SET #module WrongStringSet = WrongSet(OrderedString);; module WrongStringSet : sig type element = WrongSet(OrderedString).element and set = WrongSet(OrderedString).set val empty : set val add : element -> set -> set val member : element -> set -> bool end #WrongStringSet.add "gee" WrongStringSet.empty;; This expression has type string but is here used with type WrongStringSet.element = WrongSet(OrderedString).element >> The problem here is that SET specifies the type element abstractly, so that the type equality between element in the result of the functor and t in its argument is forgotten. Consequently, WrongStringSet.element is not the same type as string, and the operations of WrongStringSet cannot be applied to strings. As demonstrated above, it is important that the type element in the signature SET be declared equal to Elt.t; unfortunately, this is impossible above since SET is defined in a context where Elt does not exist. To overcome this difficulty, Objective Caml provides a with type construct over signatures that allows to enrich a signature with extra type equalities: <<#module AbstractSet = # (Set : functor(Elt: ORDERED_TYPE) -> (SET with type element = Elt.t));; module AbstractSet : functor (Elt : ORDERED_TYPE) -> sig type element = Elt.t and set val empty : set val add : element -> set -> set val member : element -> set -> bool end >> As in the case of simple structures, an alternate syntax is provided for defining functors and restricting their result: << module AbstractSet(Elt: ORDERED_TYPE) : (SET with type element = Elt.t) = struct ... end;; >> Abstracting a type component in a functor result is a powerful technique that provides a high degree of type safety, as we now illustrate. Consider an ordering over character strings that is different from the standard ordering implemented in the OrderedString structure. For instance, we compare strings without distinguishing upper and lower case. <<#module NoCaseString = # struct # type t = string # let compare s1 s2 = # OrderedString.compare (String.lowercase s1) (String.lowercase s2) # end;; module NoCaseString : sig type t = string val compare : string -> string -> comparison end #module NoCaseStringSet = AbstractSet(NoCaseString);; module NoCaseStringSet : sig type element = NoCaseString.t and set = AbstractSet(NoCaseString).set val empty : set val add : element -> set -> set val member : element -> set -> bool end #NoCaseStringSet.add "FOO" AbstractStringSet.empty;; This expression has type AbstractStringSet.set = AbstractSet(OrderedString).set but is here used with type NoCaseStringSet.set = AbstractSet(NoCaseString).set >> Notice that the two types AbstractStringSet.set and NoCaseStringSet.set are not compatible, and values of these two types do not match. This is the correct behavior: even though both set types contain elements of the same type (strings), both are built upon different orderings of that type, and different invariants need to be maintained by the operations (being strictly increasing for the standard ordering and for the case-insensitive ordering). Applying operations from AbstractStringSet to values of type NoCaseStringSet.set could give incorrect results, or build lists that violate the invariants of NoCaseStringSet. 2.5 Modules and separate compilation *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* All examples of modules so far have been given in the context of the interactive system. However, modules are most useful for large, batch-compiled programs. For these programs, it is a practical necessity to split the source into several files, called compilation units, that can be compiled separately, thus minimizing recompilation after changes. In Objective Caml, compilation units are special cases of structures and signatures, and the relationship between the units can be explained easily in terms of the module system. A compilation unit a comprises two files: - the implementation file a.ml, which contains a sequence of definitions, analogous to the inside of a struct...end construct; - the interface file a.mli, which contains a sequence of specifications, analogous to the inside of a sig...end construct. Both files define a structure named A (same name as the base name a of the two files, with the first letter capitalized), as if the following definition was entered at top-level: << module A: sig (* contents of file a.mli *) end = struct (* contents of file a.ml *) end;; >> The files defining the compilation units can be compiled separately using the ocamlc -c command (the -c option means ``compile only, do not try to link''); this produces compiled interface files (with extension .cmi) and compiled object code files (with extension .cmo). When all units have been compiled, their .cmo files are linked together using the ocaml command. For instance, the following commands compile and link a program composed of two compilation units aux and main: << $ ocamlc -c aux.mli # produces aux.cmi $ ocamlc -c aux.ml # produces aux.cmo $ ocamlc -c main.mli # produces main.cmi $ ocamlc -c main.ml # produces main.cmo $ ocamlc -o theprogram aux.cmo main.cmo >> The program behaves exactly as if the following phrases were entered at top-level: << module Aux: sig (* contents of aux.mli *) end = struct (* contents of aux.ml *) end;; module Main: sig (* contents of main.mli *) end = struct (* contents of main.ml *) end;; >> In particular, Main can refer to Aux: the definitions and declarations contained in main.ml and main.mli can refer to definition in aux.ml, using the Aux.ident notation, provided these definitions are exported in aux.mli. The order in which the .cmo files are given to ocaml during the linking phase determines the order in which the module definitions occur. Hence, in the example above, Aux appears first and Main can refer to it, but Aux cannot refer to Main. Notice that only top-level structures can be mapped to separately-compiled files, but not functors nor module types. However, all module-class objects can appear as components of a structure, so the solution is to put the functor or module type inside a structure, which can then be mapped to a file. Chapter 3 Objects in Caml **************************** (Chapter written by Jérôme Vouillon and Didier Rémy) This chapter gives an overview of the object-oriented features of Objective Caml. 3.1 Classes and objects 3.2 Reference to self 3.3 Initializers 3.4 Virtual methods 3.5 Private methods 3.6 Class interfaces 3.7 Inheritance 3.8 Multiple inheritance 3.9 Parameterized classes 3.10 Polymorphic methods 3.11 Using coercions 3.12 Functional objects 3.13 Cloning objects 3.14 Recursive classes 3.15 Binary methods 3.16 Friends 3.1 Classes and objects *=*=*=*=*=*=*=*=*=*=*=*= The class point below defines one instance variable x and two methods get_x and move. The initial value of the instance variable is 0. The variable x is declared mutable, so the method move can change its value. <<#class point = # object # val mutable x = 0 # method get_x = x # method move d = x <- x + d # end;; class point : object val mutable x : int method get_x : int method move : int -> unit end >> We now create a new point p, instance of the point class. <<#let p = new point;; val p : point = >> Note that the type of p is point. This is an abbreviation automatically defined by the class definition above. It stands for the object type unit>, listing the methods of class point along with their types. We now invoke some methods to p: <<#p#get_x;; - : int = 0 #p#move 3;; - : unit = () #p#get_x;; - : int = 3 >> The evaluation of the body of a class only takes place at object creation time. Therefore, in the following example, the instance variable x is initialized to different values for two different objects. <<#let x0 = ref 0;; val x0 : int ref = {contents = 0} #class point = # object # val mutable x = incr x0; !x0 # method get_x = x # method move d = x <- x + d # end;; class point : object val mutable x : int method get_x : int method move : int -> unit end #new point#get_x;; - : int = 1 #new point#get_x;; - : int = 2 >> The class point can also be abstracted over the initial values of the x coordinate. <<#class point = fun x_init -> # object # val mutable x = x_init # method get_x = x # method move d = x <- x + d # end;; class point : int -> object val mutable x : int method get_x : int method move : int -> unit end >> Like in function definitions, the definition above can be abbreviated as: <<#class point x_init = # object # val mutable x = x_init # method get_x = x # method move d = x <- x + d # end;; class point : int -> object val mutable x : int method get_x : int method move : int -> unit end >> An instance of the class point is now a function that expects an initial parameter to create a point object: <<#new point;; - : int -> point = #let p = new point 7;; val p : point = >> The parameter x_init is, of course, visible in the whole body of the definition, including methods. For instance, the method get_offset in the class below returns the position of the object relative to its initial position. <<#class point x_init = # object # val mutable x = x_init # method get_x = x # method get_offset = x - x_init # method move d = x <- x + d # end;; class point : int -> object val mutable x : int method get_offset : int method get_x : int method move : int -> unit end >> Expressions can be evaluated and bound before defining the object body of the class. This is useful to enforce invariants. For instance, points can be automatically adjusted to the nearest point on a grid, as follows: <<#class adjusted_point x_init = # let origin = (x_init / 10) * 10 in # object # val mutable x = origin # method get_x = x # method get_offset = x - origin # method move d = x <- x + d # end;; class adjusted_point : int -> object val mutable x : int method get_offset : int method get_x : int method move : int -> unit end >> (One could also raise an exception if the x_init coordinate is not on the grid.) In fact, the same effect could here be obtained by calling the definition of class point with the value of the origin. <<#class adjusted_point x_init = point ((x_init / 10) * 10);; class adjusted_point : int -> point >> An alternative solution would have been to define the adjustment in a special allocation function: <<#let new_adjusted_point x_init = new point ((x_init / 10) * 10);; val new_adjusted_point : int -> point = >> However, the former pattern is generally more appropriate, since the code for adjustment is part of the definition of the class and will be inherited. This ability provides class constructors as can be found in other languages. Several constructors can be defined this way to build objects of the same class but with different initialization patterns; an alternative is to use initializers, as decribed below in section 3.3. 3.2 Reference to self *=*=*=*=*=*=*=*=*=*=*= A method or an initializer can send messages to self (that is, the current object). For that, self must be explicitly bound, here to the variable s (s could be any identifier, even though we will often choose the name self.) <<#class printable_point x_init = # object (s) # val mutable x = x_init # method get_x = x # method move d = x <- x + d # method print = print_int s#get_x # end;; class printable_point : int -> object val mutable x : int method get_x : int method move : int -> unit method print : unit end #let p = new printable_point 7;; val p : printable_point = #p#print;; 7- : unit = () >> Dynamically, the variable s is bound at the invocation of a method. In particular, when the class printable_point is inherited, the variable s will be correctly bound to the object of the subclass. 3.3 Initializers *=*=*=*=*=*=*=*=* Let-bindings within class definitions are evaluated before the object is constructed. It is also possible to evaluate an expression immediately after the object has been built. Such code is written as an anonymous hidden method called an initializer. Therefore, is can access self and the instance variables. <<#class printable_point x_init = # let origin = (x_init / 10) * 10 in # object (self) # val mutable x = origin # method get_x = x # method move d = x <- x + d # method print = print_int self#get_x # initializer print_string "new point at "; self#print; print_newline() # end;; class printable_point : int -> object val mutable x : int method get_x : int method move : int -> unit method print : unit end #let p = new printable_point 17;; new point at 10 val p : printable_point = >> Initializers cannot be overridden. On the contrary, all initializers are evaluated sequentially. Initializers are particularly useful to enforce invariants. Another example can be seen in section 5.1. 3.4 Virtual methods *=*=*=*=*=*=*=*=*=*= It is possible to declare a method without actually defining it, using the keyword virtual. This method will be provided later in subclasses. A class containing virtual methods must be flagged virtual, and cannot be instantiated (that is, no object of this class can be created). It still defines type abbreviations (treating virtual methods as other methods.) <<#class virtual abstract_point x_init = # object (self) # val mutable x = x_init # method virtual get_x : int # method get_offset = self#get_x - x_init # method virtual move : int -> unit # end;; class virtual abstract_point : int -> object val mutable x : int method get_offset : int method virtual get_x : int method virtual move : int -> unit end #class point x_init = # object # inherit abstract_point x_init # method get_x = x # method move d = x <- x + d # end;; class point : int -> object val mutable x : int method get_offset : int method get_x : int method move : int -> unit end >> 3.5 Private methods *=*=*=*=*=*=*=*=*=*= Private methods are methods that do not appear in object interfaces. They can only be invoked from other methods of the same object. <<#class restricted_point x_init = # object (self) # val mutable x = x_init # method get_x = x # method private move d = x <- x + d # method bump = self#move 1 # end;; class restricted_point : int -> object val mutable x : int method bump : unit method get_x : int method private move : int -> unit end #let p = new restricted_point 0;; val p : restricted_point = #p#move 10;; This expression has type restricted_point It has no method move #p#bump;; - : unit = () >> Private methods are inherited (they are by default visible in subclasses), unless they are hidden by signature matching, as described below. Private methods can be made public in a subclass. <<#class point_again x = # object (self) # inherit restricted_point x # method virtual move : _ # end;; class point_again : int -> object val mutable x : int method bump : unit method get_x : int method move : int -> unit end >> The annotation virtual here is only used to mention a method without providing its definition. Since we didn't add the private annotation, this makes the method public, keeping the original definition. An alternative definition is <<#class point_again x = # object (self : < move : _; ..> ) # inherit restricted_point x # end;; class point_again : int -> object val mutable x : int method bump : unit method get_x : int method move : int -> unit end >> The constraint on self's type is requiring a public move method, and this is sufficient to override private. One could think that a private method should remain private in a subclass. However, since the method is visible in a subclass, it is always possible to pick its code and define a method of the same name that runs that code, so yet another (heavier) solution would be: <<#class point_again x = # object # inherit restricted_point x as super # method move = super#move # end;; class point_again : int -> object val mutable x : int method bump : unit method get_x : int method move : int -> unit end >> Of course, private methods can also be virtual. Then, the keywords must appear in this order method private virtual. 3.6 Class interfaces *=*=*=*=*=*=*=*=*=*=* Class interfaces are inferred from class definitions. They may also be defined directly and used to restrict the type of a class. Like class declarations, they also define a new type abbreviation. <<#class type restricted_point_type = # object # method get_x : int # method bump : unit #end;; class type restricted_point_type = object method bump : unit method get_x : int end #fun (x : restricted_point_type) -> x;; - : restricted_point_type -> restricted_point_type = >> In addition to program documentation, class interfaces can be used to constrain the type of a class. Both instance variables and concrete private methods can be hidden by a class type constraint. Public and virtual methods, however, cannot. <<#class restricted_point' x = (restricted_point x : restricted_point_type);; class restricted_point' : int -> restricted_point_type >> Or, equivalently: <<#class restricted_point' = (restricted_point : int -> restricted_point_type);; class restricted_point' : int -> restricted_point_type >> The interface of a class can also be specified in a module signature, and used to restrict the inferred signature of a module. <<#module type POINT = sig # class restricted_point' : int -> # object # method get_x : int # method bump : unit # end #end;; module type POINT = sig class restricted_point' : int -> object method bump : unit method get_x : int end end #module Point : POINT = struct # class restricted_point' = restricted_point #end;; module Point : POINT >> 3.7 Inheritance *=*=*=*=*=*=*=*= We illustrate inheritance by defining a class of colored points that inherits from the class of points. This class has all instance variables and all methods of class point, plus a new instance variable c and a new method color. <<#class colored_point x (c : string) = # object # inherit point x # val c = c # method color = c # end;; class colored_point : int -> string -> object val c : string val mutable x : int method color : string method get_offset : int method get_x : int method move : int -> unit end #let p' = new colored_point 5 "red";; val p' : colored_point = #p'#get_x, p'#color;; - : int * string = (5, "red") >> A point and a colored point have incompatible types, since a point has no method color. However, the function get_x below is a generic function applying method get_x to any object p that has this method (and possibly some others, which are represented by an ellipsis in the type). Thus, it applies to both points and colored points. <<#let get_succ_x p = p#get_x + 1;; val get_succ_x : < get_x : int; .. > -> int = #get_succ_x p + get_succ_x p';; - : int = 8 >> Methods need not be declared previously, as shown by the example: <<#let set_x p = p#set_x;; val set_x : < set_x : 'a; .. > -> 'a = #let incr p = set_x p (get_succ_x p);; val incr : < get_x : int; set_x : int -> 'a; .. > -> 'a = >> 3.8 Multiple inheritance *=*=*=*=*=*=*=*=*=*=*=*=* Multiple inheritance is allowed. Only the last definition of a method is kept: the redefinition in a subclass of a method that was visible in the parent class overrides the definition in the parent class. Previous definitions of a method can be reused by binding the related ancestor. Below, super is bound to the ancestor printable_point. The name super is a pseudo value identifier that can only be used to invoke a super-class method, as in super#print. <<#class printable_colored_point y c = # object (self) # val c = c # method color = c # inherit printable_point y as super # method print = # print_string "("; # super#print; # print_string ", "; # print_string (self#color); # print_string ")" # end;; class printable_colored_point : int -> string -> object val c : string val mutable x : int method color : string method get_x : int method move : int -> unit method print : unit end #let p' = new printable_colored_point 17 "red";; new point at (10, red) val p' : printable_colored_point = #p'#print;; (10, red)- : unit = () >> A private method that has been hidden in the parent class is no longer visible, and is thus not overridden. Since initializers are treated as private methods, all initializers along the class hierarchy are evaluated, in the order they are introduced. 3.9 Parameterized classes *=*=*=*=*=*=*=*=*=*=*=*=*= Reference cells can be implemented as objects. The naive definition fails to typecheck: <<#class ref x_init = # object # val mutable x = x_init # method get = x # method set y = x <- y # end;; Some type variables are unbound in this type: class ref : 'a -> object val mutable x : 'a method get : 'a method set : 'a -> unit end The method get has type 'a where 'a is unbound >> The reason is that at least one of the methods has a polymorphic type (here, the type of the value stored in the reference cell), thus either the class should be parametric, or the method type should be constrained to a monomorphic type. A monomorphic instance of the class could be defined by: <<#class ref (x_init:int) = # object # val mutable x = x_init # method get = x # method set y = x <- y # end;; class ref : int -> object val mutable x : int method get : int method set : int -> unit end >> A class for polymorphic references must explicitly list the type parameters in its declaration. Class type parameters are always listed between [ and ]. The type parameters must also be bound somewhere in the class body by a type constraint. <<#class ['a] ref x_init = # object # val mutable x = (x_init : 'a) # method get = x # method set y = x <- y # end;; class ['a] ref : 'a -> object val mutable x : 'a method get : 'a method set : 'a -> unit end #let r = new ref 1 in r#set 2; (r#get);; - : int = 2 >> The type parameter in the declaration may actually be constrained in the body of the class definition. In the class type, the actual value of the type parameter is displayed in the constraint clause. <<#class ['a] ref_succ (x_init:'a) = # object # val mutable x = x_init + 1 # method get = x # method set y = x <- y # end;; class ['a] ref_succ : 'a -> object constraint 'a = int val mutable x : int method get : int method set : int -> unit end >> Let us consider a more complex example: define a circle, whose center may be any kind of point. We put an additional type constraint in method move, since no free variables must remain unaccounted for by the class type parameters. <<#class ['a] circle (c : 'a) = # object # val mutable center = c # method center = center # method set_center c = center <- c # method move = (center#move : int -> unit) # end;; class ['a] circle : 'a -> object constraint 'a = < move : int -> unit; .. > val mutable center : 'a method center : 'a method move : int -> unit method set_center : 'a -> unit end >> An alternate definition of circle, using a constraint clause in the class definition, is shown below. The type #point used below in the constraint clause is an abbreviation produced by the definition of class point. This abbreviation unifies with the type of any object belonging to a subclass of class point. It actually expands to < get_x : int; move : int -> unit; .. >. This leads to the following alternate definition of circle, which has slightly stronger constraints on its argument, as we now expect center to have a method get_x. <<#class ['a] circle (c : 'a) = # object # constraint 'a = #point # val mutable center = c # method center = center # method set_center c = center <- c # method move = center#move # end;; class ['a] circle : 'a -> object constraint 'a = #point val mutable center : 'a method center : 'a method move : int -> unit method set_center : 'a -> unit end >> The class colored_circle is a specialized version of class circle that requires the type of the center to unify with #colored_point, and adds a method color. Note that when specializing a parameterized class, the instance of type parameter must always be explicitly given. It is again written between [ and ]. <<#class ['a] colored_circle c = # object # constraint 'a = #colored_point # inherit ['a] circle c # method color = center#color # end;; class ['a] colored_circle : 'a -> object constraint 'a = #colored_point val mutable center : 'a method center : 'a method color : string method move : int -> unit method set_center : 'a -> unit end >> 3.10 Polymorphic methods *=*=*=*=*=*=*=*=*=*=*=*=* While parameterized classes may be polymorphic in their contents, they are not enough to allow polymorphism of method use. A classical example is defining an iterator. <<#List.fold_left;; - : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a = #class ['a] intlist (l : int list) = # object # method empty = (l = []) # method fold f (accu : 'a) = List.fold_left f accu l # end;; class ['a] intlist : int list -> object method empty : bool method fold : ('a -> int -> 'a) -> 'a -> 'a end >> At first look, we seem to have a polymorphic iterator, however this does not work in practice. <<#let l = new intlist [1; 2; 3];; val l : '_a intlist = #l#fold (fun x y -> x+y) 0;; - : int = 6 #l;; - : int intlist = #l#fold (fun s x -> s ^ string_of_int x ^ " ") "";; This expression has type int but is here used with type string >> Our iterator works, as shows its first use for summation. However, since objects themselves are not polymorphic (only their constructors are), using the fold method fixes its type for this individual object. Our next attempt to use it as a string iterator fails. The problem here is that quantification was wrongly located: this is not the class we want to be polymorphic, but the fold method. This can be achieved by giving an explicitly polymorphic type in the method definition. <<#class intlist (l : int list) = # object # method empty = (l = []) # method fold : 'a. ('a -> int -> 'a) -> 'a -> 'a = # fun f accu -> List.fold_left f accu l # end;; class intlist : int list -> object method empty : bool method fold : ('a -> int -> 'a) -> 'a -> 'a end #let l = new intlist [1; 2; 3];; val l : intlist = #l#fold (fun x y -> x+y) 0;; - : int = 6 #l#fold (fun s x -> s ^ string_of_int x ^ " ") "";; - : string = "1 2 3 " >> As you can see in the class type shown by the compiler, while polymorphic method types must be fully explicit in class definitions (appearing immediately after the method name), they can be left implicit in class descriptions. However, the type can be completely omitted in the class definition if it is already known, through inheritance or type constraints on self. Here is an example of method overriding. <<#class intlist_rev l = # object # inherit intlist l # method fold f accu = List.fold_left f accu (List.rev l) # end;; >> The following idiom separates description and definition. <<#class type ['a] iterator = # object method fold : ('b -> 'a -> 'b) -> 'b -> 'b end;; #class intlist l = # object (self : int #iterator) # method empty = (l = []) # method fold f accu = List.fold_left f accu l # end;; >> Note here the (self : int #iterator) idiom, which ensures that this object implements the interface iterator. Polymorphic methods are called in exactly the same way as normal methods, but you should be aware of some limitations of type inference. Namely, a polymorphic method can only be called if its type is known at the call site. Otherwise, the method will be assumed to be monomorphic, and given an incompatible type. <<#let sum lst = lst#fold (fun x y -> x+y) 0;; val sum : < fold : (int -> int -> int) -> int -> 'a; .. > -> 'a = #sum l;; This expression has type intlist = < empty : bool; fold : 'a. ('a -> int -> 'a) -> 'a -> 'a > but is here used with type < empty : bool; fold : (int -> int -> int) -> int -> 'b > >> The workaround is easy: you should put a type constraint on the parameter. <<#let sum (lst : _ #iterator) = lst#fold (fun x y -> x+y) 0;; val sum : int #iterator -> int = >> Of course the constraint may also be an explicit method type. Only occurences of quantified variables are required. <<#let sum lst = # (lst : < fold : 'a. ('a -> _ -> 'a) -> 'a -> 'a; .. >)#fold (+) 0;; val sum : < fold : 'a. ('a -> int -> 'a) -> 'a -> 'a; .. > -> int = >> Another use of polymorphic methods is to allow some form of implicit subtyping in method arguments. We have already seen in section 3.7 how some functions may be polymorphic in the class of their argument. This can be extended to methods. <<#class type point0 = object method get_x : int end;; class type point0 = object method get_x : int end #class distance_point x = # object # inherit point x # method distance : 'a. (#point0 as 'a) -> int = # fun other -> abs (other#get_x - x) # end;; class distance_point : int -> object val mutable x : int method distance : #point0 -> int method get_offset : int method get_x : int method move : int -> unit end #let p = new distance_point 3 in #(p#distance (new point 8), p#distance (new colored_point 1 "blue"));; - : int * int = (5, 2) >> Note here the special syntax (#point0 as 'a) we have to use to quantify the extensible part of #point0. As for the variable binder, it can be omitted in class specifications. If you want polymorphism inside object field it must be quantified independently. <<#class multi_poly = # object # method m1 : 'a. (< n1 : 'b. 'b -> 'b; .. > as 'a) -> _ = # fun o -> o#n1 true, o#n1 "hello" # method m2 : 'a 'b. (< n2 : 'b -> bool; .. > as 'a) -> 'b -> _ = # fun o x -> o#n2 x # end;; class multi_poly : object method m1 : < n1 : 'a. 'a -> 'a; .. > -> bool * string method m2 : < n2 : 'b -> bool; .. > -> 'b -> bool end >> In method m1, o must be an object with at least a method n1, itself polymorphic. In method m2, the argument of n2 and x must have the same type, which is quantified at the same level as 'a. 3.11 Using coercions *=*=*=*=*=*=*=*=*=*=* Subtyping is never implicit. There are, however, two ways to perform subtyping. The most general construction is fully explicit: both the domain and the codomain of the type coercion must be given. We have seen that points and colored points have incompatible types. For instance, they cannot be mixed in the same list. However, a colored point can be coerced to a point, hiding its color method: <<#let colored_point_to_point cp = (cp : colored_point :> point);; val colored_point_to_point : colored_point -> point = #let p = new point 3 and q = new colored_point 4 "blue";; val p : point = val q : colored_point = #let l = [p; (colored_point_to_point q)];; val l : point list = [; ] >> An object of type t can be seen as an object of type t' only if t is a subtype of t'. For instance, a point cannot be seen as a colored point. <<#(p : point :> colored_point);; Type point = < get_offset : int; get_x : int; move : int -> unit > is not a subtype of type colored_point = < color : string; get_offset : int; get_x : int; move : int -> unit > >> Indeed, narrowing coercions would be unsafe, and could only be combined with a type case, possibly raising a runtime error. However, there is no such operation available in the language. Be aware that subtyping and inheritance are not related. Inheritance is a syntactic relation between classes while subtyping is a semantic relation between types. For instance, the class of colored points could have been defined directly, without inheriting from the class of points; the type of colored points would remain unchanged and thus still be a subtype of points. The domain of a coercion can usually be omitted. For instance, one can define: <<#let to_point cp = (cp :> point);; val to_point : #point -> point = >> In this case, the function colored_point_to_point is an instance of the function to_point. This is not always true, however. The fully explicit coercion is more precise and is sometimes unavoidable. Consider, for example, the following class: <<#class c0 = object method m = {< >} method n = 0 end;; class c0 : object ('a) method m : 'a method n : int end >> The object type c is an abbreviation for as 'a. Consider now the type declaration: <<#class type c1 = object method m : c1 end;; class type c1 = object method m : c1 end >> The object type c1 is an abbreviation for the type as 'a. The coercion from an object of type c0 to an object of type c1 is correct: <<#fun (x:c0) -> (x : c0 :> c1);; - : c0 -> c1 = >> However, the domain of the coercion cannot be omitted here: <<#fun (x:c0) -> (x :> c1);; This expression cannot be coerced to type c1 = < m : c1 >; it has type c0 = < m : c0; n : int > as 'a but is here used with type 'a Type c0 = 'a is not compatible with type 'a Type c0 = 'a is not compatible with type c1 = < m : c1 > Only the first object type has a method n. This simple coercion was not fully general. Consider using a double coercion. >> The solution is to use the explicit form. Sometimes, a change in the class-type definition can also solve the problem <<#class type c2 = object ('a) method m : 'a end;; class type c2 = object ('a) method m : 'a end #fun (x:c0) -> (x :> c2);; - : c0 -> c2 = >> While class types c1 and c2 are different, both object types c1 and c2 expand to the same object type (same method names and types). Yet, when the domain of a coercion is left implicit and its co-domain is an abbreviation of a known class type, then the class type, rather than the object type, is used to derive the coercion function. This allows to leave the domain implicit in most cases when coercing form a subclass to its superclass. The type of a coercion can always be seen as below: <<#let to_c1 x = (x :> c1);; val to_c1 : < m : #c1; .. > -> c1 = #let to_c2 x = (x :> c2);; val to_c2 : #c2 -> c2 = >> Note the difference between the two coercions: in the second case, the type #c2 = < m : 'a; .. > as 'a is polymorphically recursive (according to the explicit recursion in the class type of c2); hence the success of applying this coercion to an object of class c0. On the other hand, in the first case, c1 was only expanded and unrolled twice to obtain < m : < m : c1; .. >; .. > (remember #c1 = < m : c1; .. >), without introducing recursion. You may also note that the type of to_c2 is #c2 -> c2 while the type of to_c1 is more general than #c1 -> c1. This is not always true, since there are class types for which some instances of #c are not subtypes of c, as explained in section 3.15. Yet, for parameterless classes the coercion (_ :> c) is always more general than (_ : #c :> c). A common problem may occur when one tries to define a coercion to a class c while defining class c. The problem is due to the type abbreviation not being completely defined yet, and so its subtypes are not clearly known. Then, a coercion (_ :> c) or (_ : #c :> c) is taken to be the identity function, as in <<#function x -> (x :> 'a);; - : 'a -> 'a = >> As a consequence, if the coercion is applied to self, as in the following example, the type of self is unified with the closed type c (a closed object type is an object type without ellipsis). This would constrain the type of self be closed and is thus rejected. Indeed, the type of self cannot be closed: this would prevent any further extension of the class. Therefore, a type error is generated when the unification of this type with another type would result in a closed object type. <<#class c = object method m = 1 end #and d = object (self) # inherit c # method n = 2 # method as_c = (self :> c) #end;; This expression cannot be coerced to type c = < m : int >; it has type < as_c : 'a; m : int; n : int; .. > but is here used with type c = < m : int > Self type cannot be unified with a closed object type >> However, the most common instance of this problem, coercing self to its current class, is detected as a special case by the type checker, and properly typed. <<#class c = object (self) method m = (self :> c) end;; class c : object method m : c end >> This allows the following idiom, keeping a list of all objects belonging to a class or its subclasses: <<#let all_c = ref [];; val all_c : '_a list ref = {contents = []} #class c (m : int) = # object (self) # method m = m # initializer all_c := (self :> c) :: !all_c # end;; class c : int -> object method m : int end >> This idiom can in turn be used to retrieve an object whose type has been weakened: <<#let rec lookup_obj obj = function [] -> raise Not_found # | obj' :: l -> # if (obj :> < >) = (obj' :> < >) then obj' else lookup_obj obj l ;; val lookup_obj : < .. > -> (< .. > as 'a) list -> 'a = #let lookup_c obj = lookup_obj obj !all_c;; val lookup_c : < .. > -> < m : int > = >> The type < m : int > we see here is just the expansion of c, due to the use of a reference; we have succeeded in getting back an object of type c. The previous coercion problem can often be avoided by first defining the abbreviation, using a class type: <<#class type c' = object method m : int end;; class type c' = object method m : int end #class c : c' = object method m = 1 end #and d = object (self) # inherit c # method n = 2 # method as_c = (self :> c') #end;; class c : c' class d : object method as_c : c' method m : int method n : int end >> It is also possible to use a virtual class. Inheriting from this class simultaneously allows to enforce all methods of c to have the same type as the methods of c'. <<#class virtual c' = object method virtual m : int end;; class virtual c' : object method virtual m : int end #class c = object (self) inherit c' method m = 1 end;; class c : object method m : int end >> One could think of defining the type abbreviation directly: <<#type c' = ;; >> However, the abbreviation #c' cannot be defined directly in a similar way. It can only be defined by a class or a class-type definition. This is because # sharp abbreviations carry an implicit anonymous variable .. that cannot be explicitly named. The closer you get to it is: <<#type 'a c'_class = 'a constraint 'a = < m : int; .. >;; >> with an extra type variable capturing the open object type. 3.12 Functional objects *=*=*=*=*=*=*=*=*=*=*=*= It is possible to write a version of class point without assignments on the instance variables. The construct {< ... >} returns a copy of ``self'' (that is, the current object), possibly changing the value of some instance variables. <<#class functional_point y = # object # val x = y # method get_x = x # method move d = {< x = x + d >} # end;; class functional_point : int -> object ('a) val x : int method get_x : int method move : int -> 'a end #let p = new functional_point 7;; val p : functional_point = #p#get_x;; - : int = 7 #(p#move 3)#get_x;; - : int = 10 #p#get_x;; - : int = 7 >> Note that the type abbreviation functional_point is recursive, which can be seen in the class type of functional_point: the type of self is 'a and 'a appears inside the type of the method move. The above definition of functional_point is not equivalent to the following: <<#class bad_functional_point y = # object # val x = y # method get_x = x # method move d = new functional_point (x+d) # end;; class bad_functional_point : int -> object val x : int method get_x : int method move : int -> functional_point end #let p = new functional_point 7;; val p : functional_point = #p#get_x;; - : int = 7 #(p#move 3)#get_x;; - : int = 10 #p#get_x;; - : int = 7 >> While objects of either class will behave the same, objects of their subclasses will be different. In a subclass of the latter, the method move will keep returning an object of the parent class. On the contrary, in a subclass of the former, the method move will return an object of the subclass. Functional update is often used in conjunction with binary methods as illustrated in section 5.2.1. 3.13 Cloning objects *=*=*=*=*=*=*=*=*=*=* Objects can also be cloned, whether they are functional or imperative. The library function Oo.copy makes a shallow copy of an object. That is, it returns an object that is equal to the previous one. The instance variables have been copied but their contents are shared. Assigning a new value to an instance variable of the copy (using a method call) will not affect instance variables of the original, and conversely. A deeper assignment (for example if the instance variable if a reference cell) will of course affect both the original and the copy. The type of Oo.copy is the following: <<#Oo.copy;; - : (< .. > as 'a) -> 'a = >> The keyword as in that type binds the type variable 'a to the object type < .. >. Therefore, Oo.copy takes an object with any methods (represented by the ellipsis), and returns an object of the same type. The type of Oo.copy is different from type < .. > -> < .. > as each ellipsis represents a different set of methods. Ellipsis actually behaves as a type variable. <<#let p = new point 5;; val p : point = #let q = Oo.copy p;; val q : < get_offset : int; get_x : int; move : int -> unit > = #q#move 7; (p#get_x, q#get_x);; - : int * int = (5, 12) >> In fact, Oo.copy p will behave as p#copy assuming that a public method copy with body {< >} has been defined in the class of p. Objects can be compared using the generic comparison functions = and <>. Two objects are equal if and only if they are physically equal. In particular, an object and its copy are not equal. <<#let q = Oo.copy p;; val q : < get_offset : int; get_x : int; move : int -> unit > = #p = q, p = p;; - : bool * bool = (false, true) >> Other generic comparissons such as (<, <=,...) can also be used on objects. The relation < defines an unspecified but strict ordering on objets. The ordering relationship between two objects is fixed once for all after the two objects have been created and it is not affected by mutation of fields. Cloning and override have a non empty intersection. They are interchangeable when used within an object and without overriding any field: <<#class copy = # object # method copy = {< >} # end;; class copy : object ('a) method copy : 'a end #class copy = # object (self) # method copy = Oo.copy self # end;; class copy : object ('a) method copy : 'a end >> Only the override can be used to actually override fields, and only the Oo.copy primitive can be used externally. Cloning can also be used to provide facilities for saving and restoring the state of objects. <<#class backup = # object (self : 'mytype) # val mutable copy = None # method save = copy <- Some {< copy = None >} # method restore = match copy with Some x -> x | None -> self # end;; class backup : object ('a) val mutable copy : 'a option method restore : 'a method save : unit end >> The above definition will only backup one level. The backup facility can be added to any class using multiple inheritance. <<#class ['a] backup_ref x = object inherit ['a] ref x inherit backup end;; class ['a] backup_ref : 'a -> object ('b) val mutable copy : 'b option val mutable x : 'a method get : 'a method restore : 'b method save : unit method set : 'a -> unit end #let rec get p n = if n = 0 then p # get else get (p # restore) (n-1);; val get : (< get : 'b; restore : 'a; .. > as 'a) -> int -> 'b = #let p = new backup_ref 0 in #p # save; p # set 1; p # save; p # set 2; #[get p 0; get p 1; get p 2; get p 3; get p 4];; - : int list = [2; 1; 1; 1; 1] >> A variant of backup could retain all copies. (We then add a method clear to manually erase all copies.) <<#class backup = # object (self : 'mytype) # val mutable copy = None # method save = copy <- Some {< >} # method restore = match copy with Some x -> x | None -> self # method clear = copy <- None # end;; class backup : object ('a) val mutable copy : 'a option method clear : unit method restore : 'a method save : unit end >> <<#class ['a] backup_ref x = object inherit ['a] ref x inherit backup end;; class ['a] backup_ref : 'a -> object ('b) val mutable copy : 'b option val mutable x : 'a method clear : unit method get : 'a method restore : 'b method save : unit method set : 'a -> unit end #let p = new backup_ref 0 in #p # save; p # set 1; p # save; p # set 2; #[get p 0; get p 1; get p 2; get p 3; get p 4];; - : int list = [2; 1; 0; 0; 0] >> 3.14 Recursive classes *=*=*=*=*=*=*=*=*=*=*=* Recursive classes can be used to define objects whose types are mutually recursive. <<#class window = # object # val mutable top_widget = (None : widget option) # method top_widget = top_widget # end #and widget (w : window) = # object # val window = w # method window = window # end;; class window : object val mutable top_widget : widget option method top_widget : widget option end class widget : window -> object val window : window method window : window end >> Although their types are mutually recursive, the classes widget and window are themselves independent. 3.15 Binary methods *=*=*=*=*=*=*=*=*=*= A binary method is a method which takes an argument of the same type as self. The class comparable below is a template for classes with a binary method leq of type 'a -> bool where the type variable 'a is bound to the type of self. Therefore, #comparable expands to < leq : 'a -> bool; .. > as 'a. We see here that the binder as also allows to write recursive types. <<#class virtual comparable = # object (_ : 'a) # method virtual leq : 'a -> bool # end;; class virtual comparable : object ('a) method virtual leq : 'a -> bool end >> We then define a subclass money of comparable. The class money simply wraps floats as comparable objects. We will extend it below with more operations. There is a type constraint on the class parameter x as the primitive <= is a polymorphic comparison function in Objective Caml. The inherit clause ensures that the type of objects of this class is an instance of #comparable. <<#class money (x : float) = # object # inherit comparable # val repr = x # method value = repr # method leq p = repr <= p#value # end;; class money : float -> object ('a) val repr : float method leq : 'a -> bool method value : float end >> Note that the type money1 is not a subtype of type comparable, as the self type appears in contravariant position in the type of method leq. Indeed, an object m of class money has a method leq that expects an argument of type money since it accesses its value method. Considering m of type comparable would allow to call method leq on m with an argument that does not have a method value, which would be an error. Similarly, the type money2 below is not a subtype of type money. <<#class money2 x = # object # inherit money x # method times k = {< repr = k *. repr >} # end;; class money2 : float -> object ('a) val repr : float method leq : 'a -> bool method times : float -> 'a method value : float end >> It is however possible to define functions that manipulate objects of type either money or money2: the function min will return the minimum of any two objects whose type unifies with #comparable. The type of min is not the same as #comparable -> #comparable -> #comparable, as the abbreviation #comparable hides a type variable (an ellipsis). Each occurrence of this abbreviation generates a new variable. <<#let min (x : #comparable) y = # if x#leq y then x else y;; val min : (#comparable as 'a) -> 'a -> 'a = >> This function can be applied to objects of type money or money2. <<#(min (new money 1.3) (new money 3.1))#value;; - : float = 1.3 #(min (new money2 5.0) (new money2 3.14))#value;; - : float = 3.14 >> More examples of binary methods can be found in sections 5.2.1 and 5.2.3. Notice the use of functional update for method times. Writing new money2 (k *. repr) instead of {< repr = k *. repr >} would not behave well with inheritance: in a subclass money3 of money2 the times method would return an object of class money2 but not of class money3 as would be expected. The class money could naturally carry another binary method. Here is a direct definition: <<#class money x = # object (self : 'a) # val repr = x # method value = repr # method print = print_float repr # method times k = {< repr = k *. x >} # method leq (p : 'a) = repr <= p#value # method plus (p : 'a) = {< repr = x +. p#value >} # end;; class money : float -> object ('a) val repr : float method leq : 'a -> bool method plus : 'a -> 'a method print : unit method times : float -> 'a method value : float end >> 3.16 Friends *=*=*=*=*=*=* The above class money reveals a problem that often occurs with binary methods. In order to interact with other objects of the same class, the representation of money objects must be revealed, using a method such as value. If we remove all binary methods (here plus and leq), the representation can easily be hidden inside objects by removing the method value as well. However, this is not possible as long as some binary requires access to the representation on object of the same class but different from self. <<#class safe_money x = # object (self : 'a) # val repr = x # method print = print_float repr # method times k = {< repr = k *. x >} # end;; class safe_money : float -> object ('a) val repr : float method print : unit method times : float -> 'a end >> Here, the representation of the object is known only to a particular object. To make it available to other objects of the same class, we are forced to make it available to the whole world. However we can easily restrict the visibility of the representation using the module system. <<#module type MONEY = # sig # type t # class c : float -> # object ('a) # val repr : t # method value : t # method print : unit # method times : float -> 'a # method leq : 'a -> bool # method plus : 'a -> 'a # end # end;; #module Euro : MONEY = # struct # type t = float # class c x = # object (self : 'a) # val repr = x # method value = repr # method print = print_float repr # method times k = {< repr = k *. x >} # method leq (p : 'a) = repr <= p#value # method plus (p : 'a) = {< repr = x +. p#value >} # end # end;; >> Another example of friend functions may be found in section 5.2.3. These examples occur when a group of objects (here objects of the same class) and functions should see each others internal representation, while their representation should be hidden from the outside. The solution is always to define all friends in the same module, give access to the representation and use a signature constraint to make the representation abstract outside of the module. Chapter 4 Labels and variants ******************************** (Chapter written by Jacques Garrigue) This chapter gives an overview of the new features in Objective Caml 3: labels, and polymorphic variants. 4.1 Labels *=*=*=*=*=* If you have a look at modules ending in Labels in the standard library, you will see that function types have annotations you did not have in the functions you defined yourself. <<#ListLabels.map;; - : f:('a -> 'b) -> 'a list -> 'b list = #StringLabels.sub;; - : string -> pos:int -> len:int -> string = >> Such annotations of the form name: are called labels. They are meant to document the code, allow more checking, and give more flexibility to function application. You can give such names to arguments in your programs, by prefixing them with a tilde ~. <<#let f ~x ~y = x - y;; val f : x:int -> y:int -> int = #let x = 3 and y = 2 in f ~x ~y;; - : int = 1 >> When you want to use distinct names for the variable and the label appearing in the type, you can use a naming label of the form ~name:. This also applies when the argument is not a variable. <<#let f ~x:x1 ~y:y1 = x1 - y1;; val f : x:int -> y:int -> int = #f ~x:3 ~y:2;; - : int = 1 >> Labels obey the same rules as other identifiers in Caml, that is you cannot use a reserved keyword (like in or to) as label. Formal parameters and arguments are matched according to their respective labels (1), the absence of label being interpreted as the empty label. This allows commuting arguments in applications. One can also partially apply a function on any argument, creating a new function of the remaining parameters. <<#let f ~x ~y = x - y;; val f : x:int -> y:int -> int = #f ~y:2 ~x:3;; - : int = 1 #ListLabels.fold_left;; - : f:('a -> 'b -> 'a) -> init:'a -> 'b list -> 'a = #ListLabels.fold_left [1;2;3] ~init:0 ~f:(+);; - : int = 6 #ListLabels.fold_left ~init:0;; - : f:(int -> 'a -> int) -> 'a list -> int = >> If in a function several arguments bear the same label (or no label), they will not commute among themselves, and order matters. But they can still commute with other arguments. <<#let hline ~x:x1 ~x:x2 ~y = (x1, x2, y);; val hline : x:'a -> x:'b -> y:'c -> 'a * 'b * 'c = #hline ~x:3 ~y:2 ~x:5;; - : int * int * int = (3, 5, 2) >> As an exception to the above parameter matching rules, if an application is total, labels may be omitted. In practice, most applications are total, so that labels can be omitted in applications. <<#f 3 2;; - : int = 1 #ListLabels.map succ [1;2;3];; - : int list = [2; 3; 4] >> But beware that functions like ListLabels.fold_left whose result type is a type variable will never be considered as totally applied. <<#ListLabels.fold_left (+) 0 [1;2;3];; This expression has type int -> int -> int but is here used with type 'a list >> When a function is passed as an argument to an higher-order function, labels must match in both types. Neither adding nor removing labels are allowed. <<#let h g = g ~x:3 ~y:2;; val h : (x:int -> y:int -> 'a) -> 'a = #h f;; - : int = 1 #h (+);; This expression has type int -> int -> int but is here used with type x:int -> y:int -> 'a >> 4.1.1 Optional arguments ========================= An interesting feature of labeled arguments is that they can be made optional. For optional parameters, the question mark ? replaces the tilde ~ of non-optional ones, and the label is also prefixed by ? in the function type. Default values may be given for such optional parameters. <<#let bump ?(step = 1) x = x + step;; val bump : ?step:int -> int -> int = #bump 2;; - : int = 3 #bump ~step:3 2;; - : int = 5 >> A function taking some optional arguments must also take at least one non-labeled argument. This is because the criterion for deciding whether an optional has been omitted is the application on a non-labeled argument appearing after this optional argument in the function type. <<#let test ?(x = 0) ?(y = 0) () ?(z = 0) () = (x, y, z);; val test : ?x:int -> ?y:int -> unit -> ?z:int -> unit -> int * int * int = #test ();; - : ?z:int -> unit -> int * int * int = #test ~x:2 () ~z:3 ();; - : int * int * int = (2, 0, 3) >> Optional parameters may also commute with non-optional or unlabelled ones, as long as they are applied simultaneously. By nature, optional arguments do not commute with unlabeled arguments applied independently. <<#test ~y:2 ~x:3 () ();; - : int * int * int = (3, 2, 0) #test () () ~z:1 ~y:2 ~x:3;; - : int * int * int = (3, 2, 1) #(test () ()) ~z:1;; This expression is not a function, it cannot be applied >> Here (test () ()) is already (0,0,0) and cannot be further applied. Optional arguments are actually implemented as option types. If you do not give a default value, you have access to their internal representation, type 'a option = None | Some of 'a. You can then provide different behaviors when an argument is present or not. <<#let bump ?step x = # match step with # | None -> x * 2 # | Some y -> x + y #;; val bump : ?step:int -> int -> int = >> It may also be useful to relay an optional argument from a function call to another. This can be done by prefixing the applied argument with ?. This question mark disables the wrapping of optional argument in an option type. <<#let test2 ?x ?y () = test ?x ?y () ();; val test2 : ?x:int -> ?y:int -> unit -> int * int * int = #test2 ?x:None;; - : ?y:int -> unit -> int * int * int = >> 4.1.2 Labels and type inference ================================ While they provide an increased comfort for writing function applications, labels and optional arguments have the pitfall that they cannot be inferred as completely as the rest of the language. You can see it in the following two examples. <<#let h' g = g ~y:2 ~x:3;; val h' : (y:int -> x:int -> 'a) -> 'a = #h' f;; This expression has type x:int -> y:int -> int but is here used with type y:int -> x:int -> 'a #let bump_it bump x = # bump ~step:2 x;; val bump_it : (step:int -> 'a -> 'b) -> 'a -> 'b = #bump_it bump 1;; This expression has type ?step:int -> int -> int but is here used with type step:int -> 'a -> 'b >> The first case is simple: g is passed ~y and then ~x, but f expects ~x and then ~y. This is correctly handled if we know the type of g to be x:int -> y:int -> int in advance, but otherwise this causes the above type clash. The simplest workaround is to apply formal parameters in a standard order. The second example is more subtle: while we intended the argument bump to be of type ?step:int -> int -> int, it is inferred as step:int -> int -> 'a. These two types being incompatible (internally normal and optional arguments are different), a type error occurs when applying bump_it to the real bump. We will not try here to explain in detail how type inference works. One must just understand that there is not enough information in the above program to deduce the correct type of g or bump. That is, there is no way to know whether an argument is optional or not, or which is the correct order, by looking only at how a function is applied. The strategy used by the compiler is to assume that there are no optional arguments, and that applications are done in the right order. The right way to solve this problem for optional parameters is to add a type annotation to the argument bump. <<#let bump_it (bump : ?step:int -> int -> int) x = # bump ~step:2 x;; val bump_it : (?step:int -> int -> int) -> int -> int = #bump_it bump 1;; - : int = 3 >> In practive, such problems appear mostly when using objects whose methods have optional arguments, so that writing the type of object arguments is often a good idea. Normally the compiler generates a type error if you attempt to pass to a function a parameter whose type is different from the expected one. However, in the specific case where the expected type is a non-labeled function type, and the argument is a function expecting optional parameters, the compiler will attempt to transform the argument to have it match the expected type, by passing None for all optional parameters. <<#let twice f (x : int) = f(f x);; val twice : (int -> int) -> int -> int = #twice bump 2;; - : int = 8 >> This transformation is coherent with the intended semantics, including side-effects. That is, if the application of optional parameters shall produce side-effects, these are delayed until the received function is really applied to an argument. 4.1.3 Suggestions for labeling =============================== Like for names, choosing labels for functions is not an easy task. A good labeling is a labeling which - makes programs more readable, - is easy to remember, - when possible, allows useful partial applications. We explain here the rules we applied when labeling Objective Caml libraries. To speak in an ``object-oriented'' way, one can consider that each function has a main argument, its object, and other arguments related with its action, the parameters. To permit the combination of functions through functionals in commuting label mode, the object will not be labeled. Its role is clear by the function itself. The parameters are labeled with names reminding either of their nature or role. Best labels combine in their meaning nature and role. When this is not possible the role is to prefer, since the nature will often be given by the type itself. Obscure abbreviations should be avoided. < 'b) -> 'a list -> 'b list UnixLabels.write : file_descr -> buf:string -> pos:int -> len:int -> unit >> When there are several objects of same nature and role, they are all left unlabeled. < 'b -> 'c) -> 'a list -> 'b list -> unit >> When there is no preferable object, all arguments are labeled. < src_pos:int -> dst:string -> dst_pos:int -> len:int -> unit >> However, when there is only one argument, it is often left unlabeled. < string >> This principle also applies to functions of several arguments whose return type is a type variable, as long as the role of each argument is not ambiguous. Labeling such functions may lead to awkward error messages when one attempts to omit labels in an application, as we have seen with ListLabels.fold_left. Here are some of the label names you will find throughout the libraries. ------------------------------------------------------ |Label| Meaning | ------------------------------------------------------ |f: |a function to be applied | |pos: |a position in a string or array | |len: |a length | |buf: |a string used as buffer | |src: |the source of an operation | |dst: |the destination of an operation | |init:|the initial value for an iterator | |cmp: |a comparison function, e.g. Pervasives.compare| |mode:|an operation mode or a flag list | ------------------------------------------------------ All these are only suggestions, but one shall keep in mind that the choice of labels is essential for readability. Bizarre choices will make the program harder to maintain. In the ideal, the right function name with right labels shall be enough to understand the function's meaning. Since one can get this information with OCamlBrowser or the ocaml toplevel, the documentation is only used when a more detailed specification is needed. 4.2 Polymorphic variants *=*=*=*=*=*=*=*=*=*=*=*=* Variants as presented in section 1.4 are a powerful tool to build data structures and algorithms. However they sometimes lack flexibility when used in modular programming. This is due to the fact every constructor reserves a name to be used with a unique type. On cannot use the same name in another type, or consider a value of some type to belong to some other type with more constructors. With polymorphic variants, this original assumption is removed. That is, a variant tag does not belong to any type in particular, the type system will just check that it is an admissible value according to its use. You need not define a type before using a variant tag. A variant type will be inferred independently for each of its uses. Basic use ========= In programs, polymorphic variants work like usual ones. You just have to prefix their names with a backquote character `. <<#[`On; `Off];; - : [> `Off | `On] list = [`On; `Off] #`Number 1;; - : [> `Number of int] = `Number 1 #let f = function `On -> 1 | `Off -> 0 | `Number n -> n;; val f : [< `Number of int | `Off | `On] -> int = #List.map f [`On; `Off];; - : int list = [1; 0] >> [>`Off|`On] list means that to match this list, you should at least be able to match `Off and `On, without argument. [<`On|`Off|`Number of int] means that f may be applied to `Off, `On (both without argument), or `Number n where n is an integer. The > and < inside the variant type shows that they may still be refined, either by defining more tags or allowing less. As such they contain an implicit type variable. Both variant types appearing only once in the type, the implicit type variables they constrain are not shown. The above variant types were polymorphic, allowing further refinement. When writing type annotations, one will most often describe fixed variant types, that is types that can be no longer refined. This is also the case for type abbreviations. Such types do not contain < or >, but just an enumeration of the tags and their associated types, just like in a normal datatype definition. <<#type 'a vlist = [`Nil | `Cons of 'a * 'a vlist];; type 'a vlist = [ `Cons of 'a * 'a vlist | `Nil] #let rec map f : 'a vlist -> 'b vlist = function # | `Nil -> `Nil # | `Cons(a, l) -> `Cons(f a, map f l) #;; val map : ('a -> 'b) -> 'a vlist -> 'b vlist = >> Advanced use ============ Type-checking polymorphic variants is a subtle thing, and some expressions may result in more complex type information. <<#let f = function `A -> `C | `B -> `D | x -> x;; val f : ([> `A | `B | `C | `D] as 'a) -> 'a = #f `E;; - : _[> `A | `B | `C | `D | `E] = `E >> Here we are seeing two phenomena. First, since this matching is open (the last case catches any tag), we obtain the type [> `A | `B] rather than [< `A | `B] in a closed matching. Then, since x is returned as is, input and return types are identical. The notation as 'a denotes such type sharing. If we apply f to yet another tag `E, it gets added to the list. <<#let f1 = function `A x -> x = 1 | `B -> true | `C -> false #let f2 = function `A x -> x = "a" | `B -> true ;; val f1 : [< `A of int | `B | `C] -> bool = val f2 : [< `A of string | `B] -> bool = #let f x = f1 x && f2 x;; val f : [< `A of string & int | `B] -> bool = >> Here f1 and f2 both accept the variant tags `A and `B, but the argument of `A is int for f1 and string for f2. In f's type `C, only accepted by f1, disappears, but both argument types appear for `A as int & string. This means that if we pass the variant tag `A to f, its argument should be both int and string. Since there is no such value, f cannot be applied to `A, and `B is the only accepted input. Even if a value has a fixed variant type, one can still give it a larger type through coercions. Coercions are normally written with both the source type and the destination type, but in simple cases the source type may be omitted. <<#type 'a wlist = [`Nil | `Cons of 'a * 'a wlist | `Snoc of 'a wlist * 'a];; type 'a wlist = [ `Cons of 'a * 'a wlist | `Nil | `Snoc of 'a wlist * 'a] #let wlist_of_vlist l = (l : 'a vlist :> 'a wlist);; val wlist_of_vlist : 'a vlist -> 'a wlist = #let open_vlist l = (l : 'a vlist :> [> 'a vlist]);; val open_vlist : 'a vlist -> [> 'a vlist] = #fun x -> (x :> [`A|`B|`C]);; - : [< `A | `B | `C] -> [ `A | `B | `C] = >> You may also selectively coerce values through pattern matching. <<#let split_cases = function # | `Nil | `Cons _ as x -> `A x # | `Snoc _ as x -> `B x #;; val split_cases : [< `Cons of 'a | `Nil | `Snoc of 'b] -> [> `A of [> `Cons of 'a | `Nil] | `B of [> `Snoc of 'b]] = >> When an or-pattern composed of variant tags is wrapped inside an alias-pattern, the alias is given a type containing only the tags enumerated in the or-pattern. This allows for many useful idioms, like incremental definition of functions. <<#let num x = `Num x #let eval1 eval (`Num x) = x #let rec eval x = eval1 eval x ;; val num : 'a -> [> `Num of 'a] = val eval1 : 'a -> [ `Num of 'b] -> 'b = val eval : [ `Num of 'a] -> 'a = #let plus x y = `Plus(x,y) #let eval2 eval = function # | `Plus(x,y) -> eval x + eval y # | `Num _ as x -> eval1 eval x #let rec eval x = eval2 eval x ;; val plus : 'a -> 'b -> [> `Plus of 'a * 'b] = val eval2 : ('a -> int) -> [< `Num of int | `Plus of 'a * 'a] -> int = val eval : ([< `Num of int | `Plus of 'a * 'a] as 'a) -> int = >> To make this even more confortable, you may use type definitions as abbreviations for or-patterns. That is, if you have defined type myvariant = [`Tag1 int | `Tag2 bool], then the pattern #myvariant is equivalent to writing (`Tag1(_ : int) | `Tag2(_ : bool)). Such abbreviations may be used alone, <<#let f = function # | #myvariant -> "myvariant" # | `Tag3 -> "Tag3";; val f : [< `Tag1 of int | `Tag2 of bool | `Tag3] -> string = >> or combined with with aliases. <<#let g1 = function `Tag1 _ -> "Tag1" | `Tag2 _ -> "Tag2";; val g1 : [< `Tag1 of 'a | `Tag2 of 'b] -> string = #let g = function # | #myvariant as x -> g1 x # | `Tag3 -> "Tag3";; val g : [< `Tag1 of int | `Tag2 of bool | `Tag3] -> string = >> 4.2.1 Weaknesses of polymorphic variants ========================================= After seeing the power of polymorphic variants, one may wonder why they were added to core language variants, rather than replacing them. The answer is two fold. One first aspect is that while being pretty efficient, the lack of static type information allows for less optimizations, and makes polymorphic variants slightly heavier than core language ones. However noticeable differences would only appear on huge data structures. More important is the fact that polymorphic variants, while being type-safe, result in a weaker type discipline. That is, core language variants do actually much more than ensuring type-safety, they also check that you use only declared constructors, that all constructors present in a data-structure are compatible, and they enforce typing constraints to their parameters. For this reason, you must be more careful about making types explicit when you use polymorphic variants. When you write a library, this is easy since you can describe exact types in interfaces, but for simple programs you are probably better off with core language variants. Beware also that certain idioms make trivial errors very hard to find. For instance, the following code is probably wrong but the compiler has no way to see it. <<#type abc = [`A | `B | `C] ;; type abc = [ `A | `B | `C] #let f = function # | `As -> "A" # | #abc -> "other" ;; val f : [< `A | `As | `B | `C] -> string = #let f : abc -> string = f ;; val f : abc -> string = >> You can avoid such risks by annotating the definition itself. <<#let f : abc -> string = function # | `As -> "A" # | #abc -> "other" ;; Warning: this match case is unused. val f : abc -> string = >> --------------------------------------- (1) This correspond to the commuting label mode of Objective Caml 3.00 through 3.02, with some additional flexibility on total applications. The so-called classic mode (-nolabels options) is now deprecated for normal use. Chapter 5 Advanced examples with classes and modules ******************************************************* (Chapter written by Didier Rémy) In this chapter, we show some larger examples using objects, classes and modules. We review many of the object features simultaneously on the example of a bank account. We show how modules taken from the standard library can be expressed as classes. Lastly, we describe a programming pattern know of as virtual types through the example of window managers. 5.1 Extended example: bank accounts *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= In this section, we illustrate most aspects of Object and inheritance by refining, debugging, and specializing the following initial naive definition of a simple bank account. (We reuse the module Euro defined at the end of chapter 3.) <<#let euro = new Euro.c;; val euro : float -> Euro.c = #let zero = euro 0.;; val zero : Euro.c = #let neg x = x#times (-1.);; val neg : < times : float -> 'a; .. > -> 'a = #class account = # object # val mutable balance = zero # method balance = balance # method deposit x = balance <- balance # plus x # method withdraw x = # if x#leq balance then (balance <- balance # plus (neg x); x) else zero # end;; class account : object val mutable balance : Euro.c method balance : Euro.c method deposit : Euro.c -> unit method withdraw : Euro.c -> Euro.c end #let c = new account in c # deposit (euro 100.); c # withdraw (euro 50.);; - : Euro.c = >> We now refine this definition with a method to compute interest. <<#class account_with_interests = # object (self) # inherit account # method private interest = self # deposit (self # balance # times 0.03) # end;; class account_with_interests : object val mutable balance : Euro.c method balance : Euro.c method deposit : Euro.c -> unit method private interest : unit method withdraw : Euro.c -> Euro.c end >> We make the method interest private, since clearly it should not be called freely from the outside. Here, it is only made accessible to subclasses that will manage monthly or yearly updates of the account. We should soon fix a bug in the current definition: the deposit method can be used for withdrawing money by depositing negative amounts. We can fix this directly: <<#class safe_account = # object # inherit account # method deposit x = if zero#leq x then balance <- balance#plus x # end;; class safe_account : object val mutable balance : Euro.c method balance : Euro.c method deposit : Euro.c -> unit method withdraw : Euro.c -> Euro.c end >> However, the bug might be fixed more safely by the following definition: <<#class safe_account = # object # inherit account as unsafe # method deposit x = # if zero#leq x then unsafe # deposit x # else raise (Invalid_argument "deposit") # end;; class safe_account : object val mutable balance : Euro.c method balance : Euro.c method deposit : Euro.c -> unit method withdraw : Euro.c -> Euro.c end >> In particular, this does not require the knowledge of the implementation of the method deposit. To keep trace of operations, we extend the class with a mutable field history and a private method trace to add an operation in the log. Then each method to be traced is redefined. <<#type 'a operation = Deposit of 'a | Retrieval of 'a;; type 'a operation = Deposit of 'a | Retrieval of 'a #class account_with_history = # object (self) # inherit safe_account as super # val mutable history = [] # method private trace x = history <- x :: history # method deposit x = self#trace (Deposit x); super#deposit x # method withdraw x = self#trace (Retrieval x); super#withdraw x # method history = List.rev history # end;; class account_with_history : object val mutable balance : Euro.c val mutable history : Euro.c operation list method balance : Euro.c method deposit : Euro.c -> unit method history : Euro.c operation list method private trace : Euro.c operation -> unit method withdraw : Euro.c -> Euro.c end >> One may wish to open an account and simultaneously deposit some initial amount. Although the initial implementation did not address this requirement, it can be achieved by using an initializer. <<#class account_with_deposit x = # object # inherit account_with_history # initializer balance <- x # end;; class account_with_deposit : Euro.c -> object val mutable balance : Euro.c val mutable history : Euro.c operation list method balance : Euro.c method deposit : Euro.c -> unit method history : Euro.c operation list method private trace : Euro.c operation -> unit method withdraw : Euro.c -> Euro.c end >> A better alternative is: <<#class account_with_deposit x = # object (self) # inherit account_with_history # initializer self#deposit x # end;; class account_with_deposit : Euro.c -> object val mutable balance : Euro.c val mutable history : Euro.c operation list method balance : Euro.c method deposit : Euro.c -> unit method history : Euro.c operation list method private trace : Euro.c operation -> unit method withdraw : Euro.c -> Euro.c end >> Indeed, the latter is safer since the call to deposit will automatically benefit from safety checks and from the trace. Let's test it: <<#let ccp = new account_with_deposit (euro 100.) in #let balance = ccp#withdraw (euro 50.) in #ccp#history;; - : Euro.c operation list = [Deposit ; Retrieval ] >> Closing an account can be done with the following polymorphic function: <<#let close c = c#withdraw (c#balance);; val close : < balance : 'a; withdraw : 'a -> 'b; .. > -> 'b = >> Of course, this applies to all sorts of accounts. Finally, we gather several versions of the account into a module Account abstracted over some currency. <<#let today () = (01,01,2000) (* an approximation *) #module Account (M:MONEY) = # struct # type m = M.c # let m = new M.c # let zero = m 0. # # class bank = # object (self) # val mutable balance = zero # method balance = balance # val mutable history = [] # method private trace x = history <- x::history # method deposit x = # self#trace (Deposit x); # if zero#leq x then balance <- balance # plus x # else raise (Invalid_argument "deposit") # method withdraw x = # if x#leq balance then # (balance <- balance # plus (neg x); self#trace (Retrieval x); x) # else zero # method history = List.rev history # end # # class type client_view = # object # method deposit : m -> unit # method history : m operation list # method withdraw : m -> m # method balance : m # end # # class virtual check_client x = # let y = if (m 100.)#leq x then x # else raise (Failure "Insufficient initial deposit") in # object (self) initializer self#deposit y end # # module Client (B : sig class bank : client_view end) = # struct # class account x : client_view = # object # inherit B.bank # inherit check_client x # end # # let discount x = # let c = new account x in # if today() < (1998,10,30) then c # deposit (m 100.); c # end # end;; >> This shows the use of modules to group several class definitions that can in fact be thought of as a single unit. This unit would be provided by a bank for both internal and external uses. This is implemented as a functor that abstracts over the currency so that the same code can be used to provide accounts in different currencies. The class bank is the real implementation of the bank account (it could have been inlined). This is the one that will be used for further extensions, refinements, etc. Conversely, the client will only be given the client view. <<#module Euro_account = Account(Euro);; #module Client = Euro_account.Client (Euro_account);; #new Client.account (new Euro.c 100.);; >> Hence, the clients do not have direct access to the balance, nor the history of their own accounts. Their only way to change their balance is to deposit or withdraw money. It is important to give the clients a class and not just the ability to create accounts (such as the promotional discount account), so that they can personalize their account. For instance, a client may refine the deposit and withdraw methods so as to do his own financial bookkeeping, automatically. On the other hand, the function discount is given as such, with no possibility for further personalization. It is important that to provide the client's view as a functor Client so that client accounts can still be build after a possible specialization of the bank. The functor Client may remain unchanged and be passed the new definition to initialize a client's view of the extended account. <<#module Investment_account (M : MONEY) = # struct # type m = M.c # module A = Account(M) # # class bank = # object # inherit A.bank as super # method deposit x = # if (new M.c 1000.)#leq x then # print_string "Would you like to invest?"; # super#deposit x # end # # module Client = A.Client # end;; >> The functor Client may also be redefined when some new features of the account can be given to the client. <<#module Internet_account (M : MONEY) = # struct # type m = M.c # module A = Account(M) # class bank = # object # inherit A.bank # method mail s = print_string s # end # # class type client_view = # object # method deposit : m -> unit # method history : m operation list # method withdraw : m -> m # method balance : m # method mail : string -> unit # end # # module Client (B : sig class bank : client_view end) = # struct # class account x : client_view = # object # inherit B.bank # inherit A.check_client x # end # end # end;; >> 5.2 Simple modules as classes *=*=*=*=*=*=*=*=*=*=*=*=*=*=*= One may wonder whether it is possible to treat primitive types such as integers and strings as objects. Although this is usually uninteresting for integers or strings, there may be some situations where this is desirable. The class money above is such an example. We show here how to do it for strings. 5.2.1 Strings ============== A naive definition of strings as objects could be: <<#class ostring s = # object # method get n = String.get n # method set n c = String.set n c # method print = print_string s # method copy = new ostring (String.copy s) # end;; class ostring : string -> object method copy : ostring method get : string -> int -> char method print : unit method set : string -> int -> char -> unit end >> However, the method copy returns an object of the class string, and not an objet of the current class. Hence, if the class is further extended, the method copy will only return an object of the parent class. <<#class sub_string s = # object # inherit ostring s # method sub start len = new sub_string (String.sub s start len) # end;; class sub_string : string -> object method copy : ostring method get : string -> int -> char method print : unit method set : string -> int -> char -> unit method sub : int -> int -> sub_string end >> As seen in section 3.15, the solution is to use functional update instead. We need to create an instance variable containing the representation s of the string. <<#class better_string s = # object # val repr = s # method get n = String.get n # method set n c = String.set n c # method print = print_string repr # method copy = {< repr = String.copy repr >} # method sub start len = {< repr = String.sub s start len >} # end;; class better_string : string -> object ('a) val repr : string method copy : 'a method get : string -> int -> char method print : unit method set : string -> int -> char -> unit method sub : int -> int -> 'a end >> As shown in the inferred type, the methods copy and sub now return objects of the same type as the one of the class. Another difficulty is the implementation of the method concat. In order to concatenate a string with another string of the same class, one must be able to access the instance variable externally. Thus, a method repr returning s must be defined. Here is the correct definition of strings: <<#class ostring s = # object (self : 'mytype) # val repr = s # method repr = repr # method get n = String.get n # method set n c = String.set n c # method print = print_string repr # method copy = {< repr = String.copy repr >} # method sub start len = {< repr = String.sub s start len >} # method concat (t : 'mytype) = {< repr = repr ^ t#repr >} # end;; class ostring : string -> object ('a) val repr : string method concat : 'a -> 'a method copy : 'a method get : string -> int -> char method print : unit method repr : string method set : string -> int -> char -> unit method sub : int -> int -> 'a end >> Another constructor of the class string can be defined to return an uninitialized string of a given length: <<#class cstring n = ostring (String.create n);; class cstring : int -> ostring >> Here, exposing the representation of strings is probably harmless. We do could also hide the representation of strings as we hid the currency in the class money of section 3.16. Stacks ------ There is sometimes an alternative between using modules or classes for parametric data types. Indeed, there are situations when the two approaches are quite similar. For instance, a stack can be straightforwardly implemented as a class: <<#exception Empty;; exception Empty #class ['a] stack = # object # val mutable l = ([] : 'a list) # method push x = l <- x::l # method pop = match l with [] -> raise Empty | a::l' -> l <- l'; a # method clear = l <- [] # method length = List.length l # end;; class ['a] stack : object val mutable l : 'a list method clear : unit method length : int method pop : 'a method push : 'a -> unit end >> However, writing a method for iterating over a stack is more problematic. A method fold would have type ('b -> 'a -> 'b) -> 'b -> 'b. Here 'a is the parameter of the stack. The parameter 'b is not related to the class 'a stack but to the argument that will be passed to the method fold. A naive approach is to make 'b an extra parameter of class stack: <<#class ['a, 'b] stack2 = # object # inherit ['a] stack # method fold f (x : 'b) = List.fold_left f x l # end;; class ['a, 'b] stack2 : object val mutable l : 'a list method clear : unit method fold : ('b -> 'a -> 'b) -> 'b -> 'b method length : int method pop : 'a method push : 'a -> unit end >> However, the method fold of a given object can only be applied to functions that all have the same type: <<#let s = new stack2;; val s : ('_a, '_b) stack2 = #s#fold (+) 0;; - : int = 0 #s;; - : (int, int) stack2 = >> A better solution is to use polymorphic methods, which were introduced in Objective Caml version 3.05. Polymorphic methods makes it possible to treat the type variable 'b in the type of fold as universally quantified, giving fold the polymorphic type Forall 'b. ('b -> 'a -> 'b) -> 'b -> 'b. An explicit type declaration on the method fold is required, since the type checker cannot infer the polymorphic type by itself. <<#class ['a] stack3 = # object # inherit ['a] stack # method fold : 'b. ('b -> 'a -> 'b) -> 'b -> 'b # = fun f x -> List.fold_left f x l # end;; class ['a] stack3 : object val mutable l : 'a list method clear : unit method fold : ('b -> 'a -> 'b) -> 'b -> 'b method length : int method pop : 'a method push : 'a -> unit end >> 5.2.2 Hashtbl ============== A simplified version of object-oriented hash tables should have the following class type. <<#class type ['a, 'b] hash_table = # object # method find : 'a -> 'b # method add : 'a -> 'b -> unit # end;; class type ['a, 'b] hash_table = object method add : 'a -> 'b -> unit method find : 'a -> 'b end >> A simple implementation, which is quite reasonable for small hastables is to use an association list: <<#class ['a, 'b] small_hashtbl : ['a, 'b] hash_table = # object # val mutable table = [] # method find key = List.assoc key table # method add key valeur = table <- (key, valeur) :: table # end;; class ['a, 'b] small_hashtbl : ['a, 'b] hash_table >> A better implementation, and one that scales up better, is to use a true hash tables... whose elements are small hash tables! <<#class ['a, 'b] hashtbl size : ['a, 'b] hash_table = # object (self) # val table = Array.init size (fun i -> new small_hashtbl) # method private hash key = # (Hashtbl.hash key) mod (Array.length table) # method find key = table.(self#hash key) # find key # method add key = table.(self#hash key) # add key # end;; class ['a, 'b] hashtbl : int -> ['a, 'b] hash_table >> 5.2.3 Sets =========== Implementing sets leads to another difficulty. Indeed, the method union needs to be able to access the internal representation of another object of the same class. This is another instance of friend functions as seen in section 3.16. Indeed, this is the same mechanism used in the module Set in the absence of objects. In the object-oriented version of sets, we only need to add an additional method tag to return the representation of a set. Since sets are parametric in the type of elements, the method tag has a parametric type 'a tag, concrete within the module definition but abstract in its signature. From outside, it will then be guaranteed that two objects with a method tag of the same type will share the same representation. <<#module type SET = # sig # type 'a tag # class ['a] c : # object ('b) # method is_empty : bool # method mem : 'a -> bool # method add : 'a -> 'b # method union : 'b -> 'b # method iter : ('a -> unit) -> unit # method tag : 'a tag # end # end;; #module Set : SET = # struct # let rec merge l1 l2 = # match l1 with # [] -> l2 # | h1 :: t1 -> # match l2 with # [] -> l1 # | h2 :: t2 -> # if h1 < h2 then h1 :: merge t1 l2 # else if h1 > h2 then h2 :: merge l1 t2 # else merge t1 l2 # type 'a tag = 'a list # class ['a] c = # object (_ : 'b) # val repr = ([] : 'a list) # method is_empty = (repr = []) # method mem x = List.exists ((=) x) repr # method add x = {< repr = merge [x] repr >} # method union (s : 'b) = {< repr = merge repr s#tag >} # method iter (f : 'a -> unit) = List.iter f repr # method tag = repr # end # end;; >> 5.3 The subject/observer pattern *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The following example, known as the subject/observer pattern, is often presented in the literature as a difficult inheritance problem with inter-connected classes. The general pattern amounts to the definition a pair of two classes that recursively interact with one another. The class observer has a distinguished method notify that requires two arguments, a subject and an event to execute an action. <<#class virtual ['subject, 'event] observer = # object # method virtual notify : 'subject -> 'event -> unit # end;; class virtual ['a, 'b] observer : object method virtual notify : 'a -> 'b -> unit end >> The class subject remembers a list of observers in an instance variable, and has a distinguished method notify_observers to broadcast the message notify to all observers with a particular event e. <<#class ['observer, 'event] subject = # object (self) # val mutable observers = ([]:'observer list) # method add_observer obs = observers <- (obs :: observers) # method notify_observers (e : 'event) = # List.iter (fun x -> x#notify self e) observers # end;; class ['a, 'b] subject : object ('c) constraint 'a = < notify : 'c -> 'b -> unit; .. > val mutable observers : 'a list method add_observer : 'a -> unit method notify_observers : 'b -> unit end >> The difficulty usually relies in defining instances of the pattern above by inheritance. This can be done in a natural and obvious manner in Ocaml, as shown on the following example manipulating windows. <<#type event = Raise | Resize | Move;; type event = Raise | Resize | Move #let string_of_event = function # Raise -> "Raise" | Resize -> "Resize" | Move -> "Move";; val string_of_event : event -> string = #let count = ref 0;; val count : int ref = {contents = 0} #class ['observer] window_subject = # let id = count := succ !count; !count in # object (self) # inherit ['observer, event] subject # val mutable position = 0 # method identity = id # method move x = position <- position + x; self#notify_observers Move # method draw = Printf.printf "{Position = %d}\n" position; # end;; class ['a] window_subject : object ('b) constraint 'a = < notify : 'b -> event -> unit; .. > val mutable observers : 'a list val mutable position : int method add_observer : 'a -> unit method draw : unit method identity : int method move : int -> unit method notify_observers : event -> unit end #class ['subject] window_observer = # object # inherit ['subject, event] observer # method notify s e = s#draw # end;; class ['a] window_observer : object constraint 'a = < draw : unit; .. > method notify : 'a -> event -> unit end >> Unsurprisingly the type of window is recursive. <<#let window = new window_subject;; val window : < notify : 'a -> event -> unit; _.. > window_subject as 'a = >> However, the two classes of window_subject and window_observer are not mutually recursive. <<#let window_observer = new window_observer;; val window_observer : < draw : unit; _.. > window_observer = #window#add_observer window_observer;; - : unit = () #window#move 1;; {Position = 1} - : unit = () >> Classes window_observer and window_subject can still be extended by inheritance. For instance, one may enrich the subject with new behaviors and refined the behavior of the observer. <<#class ['observer] richer_window_subject = # object (self) # inherit ['observer] window_subject # val mutable size = 1 # method resize x = size <- size + x; self#notify_observers Resize # val mutable top = false # method raise = top <- true; self#notify_observers Raise # method draw = Printf.printf "{Position = %d; Size = %d}\n" position size; # end;; class ['a] richer_window_subject : object ('b) constraint 'a = < notify : 'b -> event -> unit; .. > val mutable observers : 'a list val mutable position : int val mutable size : int val mutable top : bool method add_observer : 'a -> unit method draw : unit method identity : int method move : int -> unit method notify_observers : event -> unit method raise : unit method resize : int -> unit end #class ['subject] richer_window_observer = # object # inherit ['subject] window_observer as super # method notify s e = if e <> Raise then s#raise; super#notify s e # end;; class ['a] richer_window_observer : object constraint 'a = < draw : unit; raise : unit; .. > method notify : 'a -> event -> unit end >> We can also create a different kind of observer: <<#class ['subject] trace_observer = # object # inherit ['subject, event] observer # method notify s e = # Printf.printf # "\n" s#identity (string_of_event e) # end;; class ['a] trace_observer : object constraint 'a = < identity : int; .. > method notify : 'a -> event -> unit end >> and attached several observers to the same object: <<#let window = new richer_window_subject;; val window : < notify : 'a -> event -> unit; _.. > richer_window_subject as 'a = #window#add_observer (new richer_window_observer);; - : unit = () #window#add_observer (new trace_observer);; - : unit = () #window#move 1; window#resize 2;; {Position = 1; Size = 1} {Position = 1; Size = 1} {Position = 1; Size = 3} {Position = 1; Size = 3} - : unit = () >> Part: II ******** The Objective Caml language *************************** Chapter 6 The Objective Caml language **************************************** Foreword *=*=*=*= This document is intended as a reference manual for the Objective Caml language. It lists the language constructs, and gives their precise syntax and informal semantics. It is by no means a tutorial introduction to the language: there is not a single example. A good working knowledge of Caml is assumed. No attempt has been made at mathematical rigor: words are employed with their intuitive meaning, without further definition. As a consequence, the typing rules have been left out, by lack of the mathematical framework required to express them, while they are definitely part of a full formal definition of the language. Notations ========= The syntax of the language is given in BNF-like notation. Terminal symbols are set in typewriter font (like this). Non-terminal symbols are set in italic font (like that). Square brackets [...] denote optional components. Curly brackets {...} denotes zero, one or several repetitions of the enclosed components. Curly bracket with a trailing plus sign {...}^+ denote one or several repetitions of the enclosed components. Parentheses (...) denote grouping. 6.1 Lexical conventions *=*=*=*=*=*=*=*=*=*=*=*= Blanks ------ The following characters are considered as blanks: space, newline, horizontal tabulation, carriage return, line feed and form feed. Blanks are ignored, but they separate adjacent identifiers, literals and keywords that would otherwise be confused as one single identifier, literal or keyword. Comments -------- Comments are introduced by the two characters (*, with no intervening blanks, and terminated by the characters *), with no intervening blanks. Comments are treated as blank characters. Comments do not occur inside string or character literals. Nested comments are handled correctly. Identifiers ----------- ident ::= (letter|_) { letter|0...9|_|' } letter ::= A ... Z | a ... z Identifiers are sequences of letters, digits, _ (the underscore character), and ' (the single quote), starting with a letter or an underscore. Letters contain at least the 52 lowercase and uppercase letters from the ASCII set. The current implementation (except on MacOS) also recognizes as letters all accented characters from the ISO 8859-1 (``ISO Latin 1'') set. All characters in an identifier are meaningful. The current implementation places no limits on the number of characters of an identifier. Integer literals ---------------- [+] --- integer-literal ::= [-] { 0...9 }[ ] --- [+] --- | [-] (0x|0X) { 0...9|A...F|a...f }[ ] --- [+] --- | [-] (0o|0O) { 0...7 }[ ] --- [+] --- | [-] (0b|0B) { 0...1 }[ ] --- An integer literal is a sequence of one or more digits, optionally preceded by a minus sign. By default, integer literals are in decimal (radix 10). The following prefixes select a different radix: ------------------------------- |Prefix| Radix | ------------------------------- |0x, 0X|hexadecimal (radix 16)| |0o, 0O|octal (radix 8) | |0b, 0B|binary (radix 2) | ------------------------------- (The initial 0 is the digit zero; the O for octal is the letter O.) The interpretation of integer literals that fall outside the range of representable integer values is undefined. Floating-point literals ----------------------- [+] [+] --- --- float-literal ::= [-] { 0...9 }[ ] [. { 0...9 }] [(e|E) [+|-] { 0...9 }[ ]] --- --- Floating-point decimals consist in an integer part, a decimal part and an exponent part. The integer part is a sequence of one or more digits, optionally preceded by a minus sign. The decimal part is a decimal point followed by zero, one or more digits. The exponent part is the character e or E followed by an optional + or - sign, followed by one or more digits. The decimal part or the exponent part can be omitted, but not both to avoid ambiguity with integer literals. The interpretation of floating-point literals that fall outside the range of representable floating-point values is undefined. Character literals ------------------ char-literal ::= ' regular-char ' | ' escape-sequence ' escape-sequence ::= \ (\ | " | ' | n | t | b | r) | \ (0...9) (0...9) (0...9) Character literals are delimited by ' (single quote) characters. The two single quotes enclose either one character different from ' and \, or one of the escape sequences below: ------------------------------------------------------- |Sequence| Character denoted | ------------------------------------------------------- |\\ |backslash (\) | |\" |double quote (") | |\' |single quote (') | |\n |linefeed (LF) | |\r |carriage return (CR) | |\t |horizontal tabulation (TAB) | |\b |backspace (BS) | |\ddd |the character with ASCII code ddd in decimal| ------------------------------------------------------- String literals --------------- string-literal ::= " { string-character } " string-character ::= regular-char-str | escape-sequence String literals are delimited by " (double quote) characters. The two double quotes enclose a sequence of either characters different from " and \, or escape sequences from the table given above for character literals. To allow splitting long string literals across lines, the sequence \newline blanks (a \ at end-of-line followed by any number of blanks at the beginning of the next line) is ignored inside string literals. The current implementation places practically no restrictions on the length of string literals. Naming labels ------------- To avoid ambiguities, naming labels cannot just be defined syntactically as the sequence of the three tokens ~, ident and :, and have to be defined at the lexical level. label ::= ~ (a ... z) { letter|0...9|_|' } : optlabel ::= ? (a ... z) { letter|0...9|_|' } : Naming labels come in two flavours: label for normal arguments and optlabel for optional ones. They are simply distinguished by their first character, either ~ or ?. Prefix and infix symbols ------------------------ infix-symbol ::= (= | < | > | @ | ^ | | | & | + | - | * | / | $ | %) { operator-char } prefix-symbol ::= (! | ? | ~) { operator-char } operator-char ::= ! | $ | % | & | * | + | - | . | / | : | < | = | > | ? | @ | ^ | | | ~ Sequences of ``operator characters'', such as <=> or !!, are read as a single token from the infix-symbol or prefix-symbol class. These symbols are parsed as prefix and infix operators inside expressions, but otherwise behave much as identifiers. Keywords -------- The identifiers below are reserved as keywords, and cannot be employed otherwise: << and as assert asr begin class closed constraint do done downto else end exception external false for fun function functor if in include inherit land lazy let lor lsl lsr lxor match method mod module mutable new of open or parser private rec sig struct then to true try type val virtual when while with >> The following character sequences are also keywords: << # & ' ( ) * , -> ? ?? . .. .( .[ : :: := ; ;; <- = [ [| [< {< ] |] >] >} _ ` { | } ~ >> Ambiguities ----------- Lexical ambiguities are resolved according to the ``longest match'' rule: when a character sequence can be decomposed into two tokens in several different ways, the decomposition retained is the one with the longest first token. Line number directives ---------------------- [+] --- linenum-directive ::= # {0 ... 9}[ ] --- [+] --- | # {0 ... 9}[ ] " { string-character } " --- Preprocessors that generate Caml source code can insert line number directives in their output so that error messages produced by the compiler contain line numbers and file names referring to the source file before preprocessing, instead of after preprocessing. A line number directive is composed of a # (sharp sign), followed by a positive integer (the source line number), optionally followed by a character string (the source file name). Line number directives are treated as blank characters during lexical analysis. 6.2 Values *=*=*=*=*=* This section describes the kinds of values that are manipulated by Objective Caml programs. 6.2.1 Base values ================== Integer numbers --------------- Integer values are integer numbers from -2^30 to 2^30-1, that is -1073741824 to 1073741823. The implementation may support a wider range of integer values: on 64-bit platforms, the current implementation supports integers ranging from -2^62 to 2^62-1. Floating-point numbers ---------------------- Floating-point values are numbers in floating-point representation. The current implementation uses double-precision floating-point numbers conforming to the IEEE 754 standard, with 53 bits of mantissa and an exponent ranging from -1022 to 1023. Characters ---------- Character values are represented as 8-bit integers between 0 and 255. Character codes between 0 and 127 are interpreted following the ASCII standard. The current implementation interprets character codes between 128 and 255 following the ISO 8859-1 standard. Character strings ----------------- String values are finite sequences of characters. The current implementation supports strings containing up to 2^24 - 6 characters (16777210 characters). 6.2.2 Tuples ============= Tuples of values are written (v_1, ..., v_n), standing for the n-tuple of values v_1 to v_n. The current implementation supports tuple of up to 2^22 - 1 elements (4194303 elements). 6.2.3 Records ============== Record values are labeled tuples of values. The record value written { field_1 = v_1; ...; field_n = v_n } associates the value v_i to the record field field_i, for i = 1 ... n. The current implementation supports records with up to 2^22 - 1 fields (4194303 fields). 6.2.4 Arrays ============= Arrays are finite, variable-sized sequences of values of the same type. The current implementation supports arrays containing to 2^22 - 1 elements (4194303 elements). 6.2.5 Variant values ===================== Variant values are either a constant constructor, or a pair of a non-constant constructor and a value. The former case is written cconstr; the latter case is written ncconstr(v), where v is said to be the argument of the non-constant constructor ncconstr. The following constants are treated like built-in constant constructors: ----------------------------- |Constant| Constructor | ----------------------------- |false |the boolean false | |true |the boolean true | |() |the ``unit'' value| |[] |the empty list | ----------------------------- The current implementation limits the number of distinct constructors in a given variant type to at most 249. 6.2.6 Polymorphic variants =========================== Polymorphic variants are an alternate form of variant values, not belonging explicitly to a predefined variant type, and following specific typing rules. They can be either constant, written `tag-name, or non-constant, written `tag-name(v). 6.2.7 Functions ================ Functional values are mappings from values to values. 6.2.8 Objects ============== Objects are composed of a hidden internal state which is a record of instance variables, and a set of methods for accessing and modifying these variables. The structure of an object is described by the toplevel class that created it. 6.3 Names *=*=*=*=*= Identifiers are used to give names to several classes of language objects and refer to these objects by name later: - value names (syntactic class value-name), - value constructors (constant -- class cconstr-name -- or non-constant -- class ncconstr-name), - labels (label-name), - variant tags (tag-name), - type constructors (typeconstr-name), - record fields (field-name), - class names (class-name), - method names (method-name), - instance variable names (inst-var-name), - module names (module-name), - module type names (modtype-name). These nine name spaces are distinguished both by the context and by the capitalization of the identifier: whether the first letter of the identifier is in lowercase (written lowercase-ident below) or in uppercase (written capitalized-ident). Underscore is considered a lowercase letter for this purpose. Naming objects -------------- value-name ::= lowercase-ident | ( operator-name ) operator-name ::= prefix-symbol | infix-symbol | * | = | or | & | := cconstr-name ::= capitalized-ident | false | true | [ ] | ( ) ncconstr-name ::= capitalized-ident | :: label-name ::= lowercase-ident tag-name ::= capitalized-ident typeconstr-name ::= lowercase-ident field-name ::= lowercase-ident module-name ::= capitalized-ident modtype-name ::= ident class-name ::= lowercase-ident inst-var-name ::= lowercase-ident method-name ::= lowercase-ident As shown above, prefix and infix symbols as well as some keywords can be used as value names, provided they are written between parentheses. Keywords such as '::' and 'false' are also constructor names. The capitalization rules are summarized in the table below. ---------------------------------------- | Name space |Case of first letter| ---------------------------------------- |Values |lowercase | |Constructors |uppercase | |Labels |lowercase | |Variant tag |uppercase | |Type constructors|lowercase | |Record fields |lowercase | |Classes |lowercase | |Methods |lowercase | |Modules |uppercase | |Module types |any | ---------------------------------------- Note on variant tags: the current implementation accepts lowercase variant tags in addition to uppercase variant tags, but we suggest you avoid lowercase variant tags for portability and compatibility with future OCaml versions. Referring to named objects -------------------------- value-path ::= value-name | module-path . lowercase-ident cconstr ::= cconstr-name | module-path . capitalized-ident ncconstr ::= ncconstr-name | module-path . capitalized-ident typeconstr ::= typeconstr-name | extended-module-path . lowercase-ident field ::= field-name | module-path . lowercase-ident module-path ::= module-name | module-path . capitalized-ident extended-module-path ::= module-name | extended-module-path . capitalized-ident | extended-module-path ( extended-module-path ) modtype-path ::= modtype-name | extended-module-path . ident class-path ::= class-name | module-path . lowercase-ident A named object can be referred to either by its name (following the usual static scoping rules for names) or by an access path prefix . name, where prefix designates a module and name is the name of an object defined in that module. The first component of the path, prefix, is either a simple module name or an access path name_1 . name_2 ..., in case the defining module is itself nested inside other modules. For referring to type constructors or module types, the prefix can also contain simple functor applications (as in the syntactic class extended-module-path above), in case the defining module is the result of a functor application. Label names, tag names, method names and instance variable names need not be qualified: the former three are global labels, while the latter are local to a class. 6.4 Type expressions *=*=*=*=*=*=*=*=*=*=* typexpr ::= ' ident | _ | ( typexpr ) | [[?]ident:] typexpr -> typexpr [+] --- | typexpr { * typexpr }[ ] --- | typeconstr | typexpr typeconstr | ( typexpr { , typexpr } ) typeconstr | typexpr as ' ident | [ variant-type ] | < [..] > | < method-type { ; method-type } [; ..] > | # class-path | typexpr # class-path | ( typexpr { , typexpr } ) # class-path poly-typexpr ::= typexpr [+] --- | { ' ident }[ ] . typexpr --- method-type ::= method-name : poly-typexpr The table below shows the relative precedences and associativity of operators and non-closed type constructions. The constructions with higher precedences come first. -------------------------------------------- | Operator |Associativity| -------------------------------------------- |Type constructor application|-- | |* |-- | |-> |right | |as |-- | -------------------------------------------- Type expressions denote types in definitions of data types as well as in type constraints over patterns and expressions. Type variables -------------- The type expression ' ident stands for the type variable named ident. The type expression _ stands for an anonymous type variable. In data type definitions, type variables are names for the data type parameters. In type constraints, they represent unspecified types that can be instantiated by any type to satisfy the type constraint. In general the scope of a named type variable is the whole enclosing definition; they can only be generalized when leaving this scope. Anonymous variables have no such restriction. Parenthesized types ------------------- The type expression ( typexpr ) denotes the same type as typexpr. Function types -------------- The type expression typexpr_1 -> typexpr_2 denotes the type of functions mapping arguments of type typexpr_1 to results of type typexpr_2. label typexpr_1 -> typexpr_2 denotes the same function type, but the argument is labeled label. ?label typexpr_1 -> typexpr_2 denotes the type of functions mapping an optional labeled argument of type typexpr_1 to results of type typexpr_2. That is, the physical type of the function will be typexpr_1 option -> typexpr_2. Tuple types ----------- The type expression typexpr_1 * ... * typexpr_n denotes the type of tuples whose elements belong to types typexpr_1, ... typexpr_n respectively. Constructed types ----------------- Type constructors with no parameter, as in typeconstr, are type expressions. The type expression typexpr typeconstr, where typeconstr is a type constructor with one parameter, denotes the application of the unary type constructor typeconstr to the type typexpr. The type expression (typexpr_1,...,typexpr_n) typeconstr, where typeconstr is a type constructor with n parameters, denotes the application of the n-ary type constructor typeconstr to the types typexpr_1 through typexpr_n. Aliased and recursive types --------------------------- The type expression typexpr as ' ident denotes the same type as typexpr, and also binds the type variable ident to type typexpr both in typexpr and in the remaining part of the type. If the type variable ident actually occurs in typexpr, a recursive type is created. Recursive types for which there exists a recursive path that does not contain an object or variant type constructor are rejected, except when the -rectypes mode is selected. If ' ident denotes an explicit polymorphic variable, and typexpr denotes either an object or variant type, the row variable of typexpr is captured by ' ident, and quantified upon. Variant types ------------- variant-type ::= [ | ] tag-spec { | tag-spec } | > [ tag-spec ] { | tag-spec } [+] --- | < [ | ] tag-spec-full { | tag-spec-full } [ > { `tag-name }[ ] ] --- tag-spec ::= `tag-name [ of typexpr ] | typexpr tag-spec-full ::= `tag-name [ of typexpr ] { & typexpr } | typexpr Variant types describe the values a polymorphic variant may take. The first case is an exact variant type: all possible tags are known, with their associated types, and they can all be present. Its structure is fully known. The second case is an open variant type, describing a polymorphic variant value: it gives the list of all tags the value could take, with their associated types. This type is still compatible with a variant type containing more tags. A special case is the unknown type, which does not define any tag, and is compatible with any variant type. The third case is a closed variant type. It gives information about all the possible tags and their associated types, and which tags are known to potentially appear in values. The above exact variant type is just an abbreviation for a closed variant type where all possible tags are also potentially present. In all three cases, tags may be either specified directly in the `tag-name [...] form, or indirectly through a type expression. In this last case, the type expression must expand to an exact variant type, whose tag specifications are inserted in its place. Full specification of variant tags are only used for non-exact closed types. They can be understood as a conjunctive type for the argument: it is intended to have all the types enumerated in the specification. Such conjunctive constraints may be unsatisfiable. In such a case the corresponding tag may not be used in a value of this type. This does not mean that the whole type is not valid: one can still use other available tags. Object types ------------ An object type < method-type { ; method-type } > is a record of method types. Each method may have an explicit polymorphic type: { ' ident }^+ . typexpr. Explicit polymorphic variables have a local scope, and an explicit polymorphic type can only be unified to an equivalent one, with polymorphic variables at the same positions. The type < method-type { ; method-type } ; .. > is the type of an object with methods and their associated types are described by method-type_1, ..., method-type_n, and possibly some other methods represented by the ellipsis. This ellipsis actually is a special kind of type variable (also called row variable in the literature) that stands for any number of extra method types. #-types ------- The type # class-path is a special kind of abbreviation. This abbreviation unifies with the type of any object belonging to a subclass of class class-path. It is handled in a special way as it usually hides a type variable (an ellipsis, representing the methods that may be added in a subclass). In particular, it vanishes when the ellipsis gets instantiated. Each type expression # class-path defines a new type variable, so type # class-path -> # class-path is usually not the same as type (# class-path as ' ident) -> ' ident. Use of #-types to abbreviate variant types is deprecated. If t is an exact variant type then #t translates to [< t], and #t[> `tag_1 ...`tag_k] translates to [< t > `tag_1 ...`tag_k] Variant and record types ------------------------ There are no type expressions describing (defined) variant types nor record types, since those are always named, i.e. defined before use and referred to by name. Type definitions are described in section 6.8.1. 6.5 Constants *=*=*=*=*=*=*= constant ::= integer-literal | float-literal | char-literal | string-literal | cconstr | `tag-name The syntactic class of constants comprises literals from the four base types (integers, floating-point numbers, characters, character strings), and constant constructors from both normal and polymorphic variants. 6.6 Patterns *=*=*=*=*=*=* pattern ::= value-name | _ | constant | pattern as value-name | ( pattern ) | ( pattern : typexpr ) | pattern | pattern | ncconstr pattern | `tag-name pattern | #typeconstr-name | pattern { , pattern } | { field = pattern { ; field = pattern } } | [ pattern { ; pattern } ] | pattern :: pattern | [| pattern { ; pattern } |] The table below shows the relative precedences and associativity of operators and non-closed pattern constructions. The constructions with higher precedences come first. --------------------------------------- | Operator |Associativity| --------------------------------------- |Constructor application|-- | |:: |right | |, |-- | || |left | |as |-- | --------------------------------------- Patterns are templates that allow selecting data structures of a given shape, and binding identifiers to components of the data structure. This selection operation is called pattern matching; its outcome is either ``this value does not match this pattern'', or ``this value matches this pattern, resulting in the following bindings of names to values''. Variable patterns ----------------- A pattern that consists in a value name matches any value, binding the name to the value. The pattern _ also matches any value, but does not bind any name. Patterns are linear: a variable cannot appear several times in a given pattern. In particular, there is no way to test for equality between two parts of a data structure using only a pattern (but when guards can be used for this purpose). Constant patterns ----------------- A pattern consisting in a constant matches the values that are equal to this constant. Alias patterns -------------- The pattern pattern_1 as value-name matches the same values as pattern_1. If the matching against pattern_1 is successful, the name name is bound to the matched value, in addition to the bindings performed by the matching against pattern_1. Parenthesized patterns ---------------------- The pattern ( pattern_1 ) matches the same values as pattern_1. A type constraint can appear in a parenthesized pattern, as in ( pattern_1 : typexpr ). This constraint forces the type of pattern_1 to be compatible with type. ``Or'' patterns --------------- The pattern pattern_1 | pattern_2 represents the logical ``or'' of the two patterns pattern_1 and pattern_2. A value matches pattern_1 | pattern_2 either if it matches pattern_1 or if it matches pattern_2. The two sub-patterns pattern_1 and pattern_2 must bind exactly the same identifiers to values having the same types. The bindings performed by matching against an ``or'' pattern are either those performed by the matching against pattern_1, if it succeeds, or those performed by the matching against pattern_2, if it succeeds. If both matchings succeed, it is undefined which set of bindings is selected. Variant patterns ---------------- The pattern ncconstr pattern_1 matches all variants whose constructor is equal to ncconstr, and whose argument matches pattern_1. The pattern pattern_1 :: pattern_2 matches non-empty lists whose heads match pattern_1, and whose tails match pattern_2. This pattern behaves like ( :: ) ( pattern_1 , pattern_2 ). The pattern [ pattern_1 ; ... ; pattern_n ] matches lists of length n whose elements match pattern_1 ...pattern_n, respectively. This pattern behaves like pattern_1 :: ... :: pattern_n :: []. Polymorphic variant patterns ---------------------------- The pattern `tag-name pattern_1 matches all polymorphic variants whose tag is equal to tag-name, and whose argument matches pattern_1. Variant abbreviation patterns ----------------------------- If the type [('a,'b,...)] typeconstr = [`tag_1 t_1 | ... | `tag_n t_n] is defined, then the pattern #typeconstr is a shorthand for the or-pattern (`tag_1(_ : t_1) | ... | `tag_n(_ : t_n)). It matches all values of type #typeconstr. Tuple patterns -------------- The pattern pattern_1 , ... , pattern_n matches n-tuples whose components match the patterns pattern_1 through pattern_n. That is, the pattern matches the tuple values (v_1, ..., v_n) such that pattern_i matches v_i for i = 1,... , n. Record patterns --------------- The pattern { field_1 = pattern_1 ; ... ; field_n = pattern_n } matches records that define at least the fields field_1 through field_n, and such that the value associated to field_i matches the pattern pattern_i, for i = 1,... , n. The record value can define more fields than field_1 ...field_n; the values associated to these extra fields are not taken into account for matching. Array patterns -------------- The pattern [| pattern_1 ; ... ; pattern_n |] matches arrays of length n such that the i-th array element matches the pattern pattern_i, for i = 1,... , n. 6.7 Expressions *=*=*=*=*=*=*=*= expr ::= value-path | constant | ( expr ) | begin expr end | ( expr : typexpr ) | expr , expr { , expr } | ncconstr expr | `tag-name expr | expr :: expr | [ expr { ; expr } ] | [| expr { ; expr } |] | { field = expr { ; field = expr } } | { expr with field = expr { ; field = expr } } [+] --- | expr { argument }[ ] --- | prefix-symbol expr | expr infix-op expr | expr . field | expr . field <- expr | expr .( expr ) | expr .( expr ) <- expr | expr .[ expr ] | expr .[ expr ] <- expr | if expr then expr [ else expr ] | while expr do expr done | for ident = expr ( to | downto ) expr do expr done | expr ; expr | match expr with pattern-matching | function pattern-matching | fun multiple-matching | try expr with pattern-matching | let [rec] let-binding { and let-binding } in expr | new class-path | expr # method-name | ( expr :> typexpr ) | ( expr : typexpr :> typexpr ) | {< inst-var-name = expr { ; inst-var-name = expr } >} argument ::= expr | ~ label-name | ~ label-name : expr | ? label-name | ? label-name : expr pattern-matching ::= [ | ] pattern [when expr] -> expr { | pattern [when expr] -> expr } [+] --- multiple-matching ::= { parameter }[ ] [when expr] -> expr --- let-binding ::= pattern [: typexpr] = expr [+] --- | value-name { parameter }[ ] [: typexpr] = expr --- parameter ::= pattern | ~ label-name | ~ ( label-name [: typexpr] ) | ~ label-name : pattern | ? label-name | ? ( label-name [: typexpr] [= expr] ) | ? label-name : pattern | ? label-name : ( pattern [: typexpr] [= expr] ) infix-op ::= infix-symbol | * | = | or | & The table below shows the relative precedences and associativity of operators and non-closed constructions. The constructions with higher precedence come first. For infix and prefix symbols, we write ``*...'' to mean ``any symbol starting with *''. --------------------------------------------------------------------- | Construction or operator |Associativity| --------------------------------------------------------------------- |prefix-symbol |-- | |. .( .[ |-- | |function application |left | |constructor application |-- | |- -. (prefix) |-- | |**... |right | |*... /... %... mod |left | |+... -... |left | |:: |right | |@...^... |right | |comparisons (= == < etc.), all other infix symbols|left | |not |-- | |& && |left | |or || |left | |, |-- | |<- := |right | |if |-- | |; |right | |let match fun function try |-- | --------------------------------------------------------------------- 6.7.1 Basic expressions ======================== Constants --------- Expressions consisting in a constant evaluate to this constant. Value paths ----------- Expressions consisting in an access path evaluate to the value bound to this path in the current evaluation environment. The path can be either a value name or an access path to a value component of a module. Parenthesized expressions ------------------------- The expressions ( expr ) and begin expr end have the same value as expr. Both constructs are semantically equivalent, but it is good style to use begin ... end inside control structures: << if ... then begin ... ; ... end else begin ... ; ... end >> and ( ... ) for the other grouping situations. Parenthesized expressions can contain a type constraint, as in ( expr : type ). This constraint forces the type of expr to be compatible with type. Parenthesized expressions can also contain coercions ( expr [: type] :> type) (see subsection 6.7.5 below). Function application -------------------- Function application is denoted by juxtaposition of (possibly labeled) expressions. The expression expr argument_1 ... argument_n evaluates the expression expr and those appearing in argument_1 to argument_n. The expression expr must evaluate to a functional value f, which is then applied to the values of argument_1, ..., argument_n. The order in which the expressions expr, argument_1, ..., argument_n are evaluated is not specified. Arguments and parameters are matched according to their respective labels. Argument order is irrelevant, except among arguments with the same label, or no label. If a parameter is specified as optional (label prefixed by ?) in the type of expr, the corresponding argument will be automatically wrapped with the constructor Some, except if the argument itself is also prefixed by ?, in which case it is passed as is. If a non-labeled argument is passed, and its corresponding parameter is preceded by one or several optional parameters, then these parameters are defaulted, i.e. the value None will be passed for them. All other missing parameters (without corresponding argument), both optional and non-optional, will be kept, and the result of the function will still be a function of these missing parameters to the body of f. As a special case, if the function has a known arity, all the arguments are unlabeled, and their number matches the number of non-optional parameters, then labels are ignored and non-optional parameters are matched in their definition order. Optional arguments are defaulted. In all cases but exact match of order and labels, without optional parameters, the function type should be known at the application point. This can be ensured by adding a type constraint. Principality of the derivation can be checked in the -principal mode. Function definition ------------------- Two syntactic forms are provided to define functions. The first form is introduced by the keyword function: [ ] [ ] --- --- [ ] [ ] function pattern--- -> expr--- [1] [1] --- --- | ... [ ] [ ] --- --- [ ] [ ] | pattern--- -> expr--- [n] [n] --- --- This expression evaluates to a functional value with one argument. When this function is applied to a value v, this value is matched against each pattern pattern_1 to pattern_n. If one of these matchings succeeds, that is, if the value v matches the pattern pattern_i for some i, then the expression expr_i associated to the selected pattern is evaluated, and its value becomes the value of the function application. The evaluation of expr_i takes place in an environment enriched by the bindings performed during the matching. If several patterns match the argument v, the one that occurs first in the function definition is selected. If none of the patterns matches the argument, the exception Match_failure is raised. The other form of function definition is introduced by the keyword fun: fun parameter_1 ... parameter_n -> expr This expression is equivalent to: fun parameter_1 -> ... fun parameter_n -> expr Functions of the form fun optlabel ( pattern = expr_0 ) -> expr are equivalent to [ ] --- [ ] fun optlabel x -> let pattern = match x with Some x -> x | None -> expr--- in expr [0] --- where x is a fresh variable. When expr_0 will be evaluated is left unspecified. After these two transformations, expressions are of the form fun [label_1] pattern_1 -> ... fun [label_n] pattern_n -> expr If we ignore labels, which will only be meaningful at function application, this is equivalent to function pattern_1 -> ... function pattern_n -> expr That is, the fun expression above evaluates to a curried function with n arguments: after applying this function n times to the values v_1 ... v_m, the values will be matched in parallel against the patterns pattern_1 ... pattern_n. If the matching succeeds, the function returns the value of expr in an environment enriched by the bindings performed during the matchings. If the matching fails, the exception Match_failure is raised. Guards in pattern-matchings --------------------------- Cases of a pattern matching (in the function, fun, match and try constructs) can include guard expressions, which are arbitrary boolean expressions that must evaluate to true for the match case to be selected. Guards occur just before the -> token and are introduced by the when keyword: [ ] [ ] [ ] --- --- --- [ ] [ ] [ ] function pattern--- [when cond---] -> expr--- [1] [1] [1] --- --- --- | ... [ ] [ ] [ ] --- --- --- [ ] [ ] [ ] | pattern--- [when cond---] -> expr--- [n] [n] [n] --- --- --- Matching proceeds as described before, except that if the value matches some pattern pattern_i which has a guard cond_i, then the expression cond_i is evaluated (in an environment enriched by the bindings performed during matching). If cond_i evaluates to true, then expr_i is evaluated and its value returned as the result of the matching, as usual. But if cond_i evaluates to false, the matching is resumed against the patterns following pattern_i. Local definitions ----------------- The let and let rec constructs bind value names locally. The construct let pattern_1 = expr_1 and ... and pattern_n = expr_n in expr evaluates expr_1 ... expr_n in some unspecified order, then matches their values against the patterns pattern_1 ... pattern_n. If the matchings succeed, expr is evaluated in the environment enriched by the bindings performed during matching, and the value of expr is returned as the value of the whole let expression. If one of the matchings fails, the exception Match_failure is raised. An alternate syntax is provided to bind variables to functional values: instead of writing let ident = fun parameter_1 ... parameter_m -> expr in a let expression, one may instead write let ident parameter_1 ... parameter_m = expr Recursive definitions of names are introduced by let rec: let rec pattern_1 = expr_1 and ... and pattern_n = expr_n in expr The only difference with the let construct described above is that the bindings of names to values performed by the pattern-matching are considered already performed when the expressions expr_1 to expr_n are evaluated. That is, the expressions expr_1 to expr_n can reference identifiers that are bound by one of the patterns pattern_1, ..., pattern_n, and expect them to have the same value as in expr, the body of the let rec construct. The recursive definition is guaranteed to behave as described above if the expressions expr_1 to expr_n are function definitions (fun ... or function ...), and the patterns pattern_1 ... pattern_n are just value names, as in: let rec name_1 = fun ... and ... and name_n = fun ... in expr This defines name_1 ... name_n as mutually recursive functions local to expr. The behavior of other forms of let rec definitions is implementation-dependent. The current implementation also supports a certain class of recursive definitions of non-functional values, such as let rec name_1 = 1 :: name_2 and name_2 = 2 :: name_1 in expr which binds name_1 to the cyclic list 1::2::1::2::..., and name_2 to the cyclic list 2::1::2::1::...Informally, the class of accepted definitions consists of those definitions where the defined names occur only inside function bodies or as argument to a data constructor. 6.7.2 Control structures ========================= Sequence -------- The expression expr_1 ; expr_2 evaluates expr_1 first, then expr_2, and returns the value of expr_2. Conditional ----------- The expression if expr_1 then expr_2 else expr_3 evaluates to the value of expr_2 if expr_1 evaluates to the boolean true, and to the value of expr_3 if expr_1 evaluates to the boolean false. The else expr_3 part can be omitted, in which case it defaults to else (). Case expression --------------- The expression match expr [ ] [ ] --- --- [ ] [ ] with pattern--- -> expr--- [1] [1] --- --- | ... [ ] [ ] --- --- [ ] [ ] | pattern--- -> expr--- [n] [n] --- --- matches the value of expr against the patterns pattern_1 to pattern_n. If the matching against pattern_i succeeds, the associated expression expr_i is evaluated, and its value becomes the value of the whole match expression. The evaluation of expr_i takes place in an environment enriched by the bindings performed during matching. If several patterns match the value of expr, the one that occurs first in the match expression is selected. If none of the patterns match the value of expr, the exception Match_failure is raised. Boolean operators ----------------- The expression expr_1 && expr_2 evaluates to true if both expr_1 and expr_2 evaluate to true; otherwise, it evaluates to false. The first component, expr_1, is evaluated first. The second component, expr_2, is not evaluated if the first component evaluates to false. Hence, the expression expr_1 && expr_2 behaves exactly as if expr_1 then expr_2 else false. The expression expr_1 || expr_2 evaluates to true if one of expr_1 and expr_2 evaluates to true; otherwise, it evaluates to false. The first component, expr_1, is evaluated first. The second component, expr_2, is not evaluated if the first component evaluates to true. Hence, the expression expr_1 || expr_2 behaves exactly as if expr_1 then true else expr_2. The boolean operator & is synonymous for &&. The boolean operator or is synonymous for ||. Loops ----- The expression while expr_1 do expr_2 done repeatedly evaluates expr_2 while expr_1 evaluates to true. The loop condition expr_1 is evaluated and tested at the beginning of each iteration. The whole while ... done expression evaluates to the unit value (). The expression for name = expr_1 to expr_2 do expr_3 done first evaluates the expressions expr_1 and expr_2 (the boundaries) into integer values n and p. Then, the loop body expr_3 is repeatedly evaluated in an environment where name is successively bound to the values n, n+1, ..., p-1, p. The loop body is never evaluated if n > p. The expression for name = expr_1 downto expr_2 do expr_3 done evaluates similarly, except that name is successively bound to the values n, n-1, ..., p+1, p. The loop body is never evaluated if n < p. In both cases, the whole for expression evaluates to the unit value (). Exception handling ------------------ The expression try expr [ ] [ ] --- --- [ ] [ ] with pattern--- -> expr--- [1] [1] --- --- | ... [ ] [ ] --- --- [ ] [ ] | pattern--- -> expr--- [n] [n] --- --- evaluates the expression expr and returns its value if the evaluation of expr does not raise any exception. If the evaluation of expr raises an exception, the exception value is matched against the patterns pattern_1 to pattern_n. If the matching against pattern_i succeeds, the associated expression expr_i is evaluated, and its value becomes the value of the whole try expression. The evaluation of expr_i takes place in an environment enriched by the bindings performed during matching. If several patterns match the value of expr, the one that occurs first in the try expression is selected. If none of the patterns matches the value of expr, the exception value is raised again, thereby transparently ``passing through'' the try construct. 6.7.3 Operations on data structures ==================================== Products -------- The expression expr_1 , ... , expr_n evaluates to the n-tuple of the values of expressions expr_1 to expr_n. The evaluation order for the subexpressions is not specified. Variants -------- The expression ncconstr expr evaluates to the variant value whose constructor is ncconstr, and whose argument is the value of expr. For lists, some syntactic sugar is provided. The expression expr_1 :: expr_2 stands for the constructor ( :: ) applied to the argument ( expr_1 , expr_2 ), and therefore evaluates to the list whose head is the value of expr_1 and whose tail is the value of expr_2. The expression [ expr_1 ; ... ; expr_n ] is equivalent to expr_1 :: ... :: expr_n :: [], and therefore evaluates to the list whose elements are the values of expr_1 to expr_n. Polymorphic variants -------------------- The expression `tag-name expr evaluates to the variant value whose tag is tag-name, and whose argument is the value of expr. Records ------- The expression { field_1 = expr_1 ; ... ; field_n = expr_n } evaluates to the record value { field_1 = v_1 ; ... ; field_n = v_n }, where v_i is the value of expr_i for i = 1,... , n. The fields field_1 to field_n must all belong to the same record types; all fields belonging to this record type must appear exactly once in the record expression, though they can appear in any order. The order in which expr_1 to expr_n are evaluated is not specified. The expression { expr with field_1 = expr_1 ; ... ; field_n = expr_n } builds a fresh record with fields field_1 ... field_n equal to expr_1 ... expr_n, and all other fields having the same value as in the record expr. In other terms, it returns a shallow copy of the record expr, except for the fields field_1 ... field_n, which are initialized to expr_1 ... expr_n. The expression expr_1 . field evaluates expr_1 to a record value, and returns the value associated to field in this record value. The expression expr_1 . field <- expr_2 evaluates expr_1 to a record value, which is then modified in-place by replacing the value associated to field in this record by the value of expr_2. This operation is permitted only if field has been declared mutable in the definition of the record type. The whole expression expr_1 . field <- expr_2 evaluates to the unit value (). Arrays ------ The expression [| expr_1 ; ... ; expr_n |] evaluates to a n-element array, whose elements are initialized with the values of expr_1 to expr_n respectively. The order in which these expressions are evaluated is unspecified. The expression expr_1 .( expr_2 ) returns the value of element number expr_2 in the array denoted by expr_1. The first element has number 0; the last element has number n-1, where n is the size of the array. The exception Invalid_argument is raised if the access is out of bounds. The expression expr_1 .( expr_2 ) <- expr_3 modifies in-place the array denoted by expr_1, replacing element number expr_2 by the value of expr_3. The exception Invalid_argument is raised if the access is out of bounds. The value of the whole expression is (). Strings ------- The expression expr_1 .[ expr_2 ] returns the value of character number expr_2 in the string denoted by expr_1. The first character has number 0; the last character has number n-1, where n is the length of the string. The exception Invalid_argument is raised if the access is out of bounds. The expression expr_1 .[ expr_2 ] <- expr_3 modifies in-place the string denoted by expr_1, replacing character number expr_2 by the value of expr_3. The exception Invalid_argument is raised if the access is out of bounds. The value of the whole expression is (). 6.7.4 Operators ================ Symbols from the class infix-symbols, as well as the keywords *, =, or and &, can appear in infix position (between two expressions). Symbols from the class prefix-symbols can appear in prefix position (in front of an expression). Infix and prefix symbols do not have a fixed meaning: they are simply interpreted as applications of functions bound to the names corresponding to the symbols. The expression prefix-symbol expr is interpreted as the application ( prefix-symbol ) expr. Similarly, the expression expr_1 infix-symbol expr_2 is interpreted as the application ( infix-symbol ) expr_1 expr_2. The table below lists the symbols defined in the initial environment and their initial meaning. (See the description of the standard library module Pervasive in chapter 20 for more details). Their meaning may be changed at any time using let ( infix-op ) name_1 name_2 = ... ------------------------------------------------ | Operator | Initial meaning | ------------------------------------------------ |+ |Integer addition. | |- (infix) |Integer subtraction. | |- (prefix) |Integer negation. | |* |Integer multiplication. | |/ |Integer division. Raise | | |Division_by_zero if second | | |argument is zero. The result is | | |unspecified if either argument is | | |negative. | |mod |Integer modulus. Raise | | |Division_by_zero if second | | |argument is zero. The result is | | |unspecified if either argument is | | |negative. | |land |Bitwise logical ``and'' on | | |integers. | |lor |Bitwise logical ``or'' on | | |integers. | |lxor |Bitwise logical ``exclusive or'' | | |on integers. | |lsl |Bitwise logical shift left on | | |integers. | |lsr |Bitwise logical shift right on | | |integers. | |asr |Bitwise arithmetic shift right on | | |integers. | |+. |Floating-point addition. | |-. (infix) |Floating-point subtraction. | |-. (prefix)|Floating-point negation. | |*. |Floating-point multiplication. | |/. |Floating-point division. | |** |Floating-point exponentiation. | |@ |List concatenation. | |^ |String concatenation. | |! |Dereferencing (return the current | | |contents of a reference). | |:= |Reference assignment (update the | | |reference given as first argument | | |with the value of the second | | |argument). | |= |Structural equality test. | |<> |Structural inequality test. | |== |Physical equality test. | |!= |Physical inequality test. | |< |Test ``less than''. | |<= |Test ``less than or equal''. | |> |Test ``greater than''. | |>= |Test ``greater than or equal''. | ------------------------------------------------ 6.7.5 Objects ============== Object creation --------------- When class-path evaluates to a class body, new class-path evaluates to an object containing the instance variables and methods of this class. When class-path evaluates to a class function, new class-path evaluates to a function expecting the same number of arguments and returning a new object of this class. Message sending --------------- The expression expr # method-name invokes the method method-name of the object denoted by expr. If method-name is a polymorphic method, its type should be known at the invocation site. This is true for instance if expr is the name of a fresh object (let ident = new class-path ... ) or if there is a type constraint. Principality of the derivation can be checked in the -principal mode. Coercion -------- The type of an object can be coerced (weakened) to a supertype. The expression ( expr :> typexpr ) coerces the expression expr to type typexpr. The expression ( expr : typexpr_1 :> typexpr_2 ) coerces the expression expr from type typexpr_1 to type typexpr_2. The former operator will sometimes fail to coerce an expression expr from a type t_1 to a type t_2 even if type t_1 is a subtype of type t_2: in the current implementation it only expands two levels of type abbreviations containing objects and/or variants, keeping only recursion when it is explicit in the class type. In case of failure, the latter operator should be used. In a class definition, coercion to the type this class defines is the identity, as this type abbreviation is not yet completely defined. Object duplication ------------------ An object can be duplicated using the library function Oo.copy (see section 20.21 ). Inside a method, the expression {< inst-var-name = expr { ; inst-var-name = expr } >} returns a copy of self with the given instance variables replaced by the values of the associated expressions; other instance variables have the same value in the returned object as in self. 6.8 Type and exception definitions *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* 6.8.1 Type definitions ======================= Type definitions bind type constructors to data types: either variant types, record types, type abbreviations, or abstract data types. They also bind the value constructors and record fields associated with the definition. type-definition ::= type typedef { and typedef } typedef ::= [type-params] typeconstr-name [type-information] type-information ::= [type-equation] [type-representation] { type-constraint } type-equation ::= = typexpr type-representation ::= = constr-decl { | constr-decl } | = { field-decl { ; field-decl } } type-params ::= type-param | ( type-param { , type-param } ) type-param ::= ' ident | + ' ident | - ' ident constr-decl ::= cconstr-name | ncconstr-name of typexpr field-decl ::= field-name : poly-typexpr | mutable field-name : poly-typexpr type-constraint ::= constraint ' ident = typexpr Type definitions are introduced by the type keyword, and consist in one or several simple definitions, possibly mutually recursive, separated by the and keyword. Each simple definition defines one type constructor. A simple definition consists in a lowercase identifier, possibly preceded by one or several type parameters, and followed by an optional type equation, then an optional type representation, and then a constraint clause. The identifier is the name of the type constructor being defined. The optional type parameters are either one type variable ' ident, for type constructors with one parameter, or a list of type variables ('ident_1,...,'ident_n), for type constructors with several parameters. Each type parameter may be prefixed by a variance constraint + (resp. -) indicating that the parameter is covariant (resp. contravariant). These type parameters can appear in the type expressions of the right-hand side of the definition, restricted eventually by a variance constraint ; i.e. a covariant parameter may only appear on the right side of a functional arrow (more precisely, follow the left branch of an even number of arrows), and a convariant parameter only the left side (left branch of an odd number of arrows). The optional type equation = typexpr makes the defined type equivalent to the type expression typexpr on the right of the = sign: one can be substituted for the other during typing. If no type equation is given, a new type is generated: the defined type is incompatible with any other type. The optional type representation describes the data structure representing the defined type, by giving the list of associated constructors (if it is a variant type) or associated fields (if it is a record type). If no type representation is given, nothing is assumed on the structure of the type besides what is stated in the optional type equation. The type representation = constr-decl { | constr-decl } describes a variant type. The constructor declarations constr-decl_1, ..., constr-decl_n describe the constructors associated to this variant type. The constructor declaration ncconstr-name of typexpr declares the name ncconstr-name as a non-constant constructor, whose argument has type typexpr. The constructor declaration cconstr-name declares the name cconstr-name as a constant constructor. Constructor names must be capitalized. The type representation = { field-decl { ; field-decl } } describes a record type. The field declarations field-decl_1, ..., field-decl_n describe the fields associated to this record type. The field declaration field-name : poly-typexpr declares field-name as a field whose argument has type poly-typexpr. The field declaration mutable field-name : poly-typexpr behaves similarly; in addition, it allows physical modification over the argument to this field. Immutable fields are covariant, but mutable fields are neither covariant nor contravariant. Both mutable and immutable field may have an explicitly polymorphic type. The polymorphism of the contents is statically checked whenever a record value is created or modified. Extracted values may have their types instanciated. The two components of a type definition, the optional equation and the optional representation, can be combined independently, giving rise to four typical situations: Abstract type: no equation, no representation. When appearing in a module signature, this definition specifies nothing on the type constructor, besides its number of parameters: its representation is hidden and it is assumed incompatible with any other type. Type abbreviation: an equation, no representation. This defines the type constructor as an abbreviation for the type expression on the right of the = sign. New variant type or record type: no equation, a representation. This generates a new type constructor and defines associated constructors or fields, through which values of that type can be directly built or inspected. Re-exported variant type or record type: an equation, a representation. In this case, the type constructor is defined as an abbreviation for the type expression given in the equation, but in addition the constructors or fields given in the representation remain attached to the defined type constructor. The type expression in the equation part must agree with the representation: it must be of the same kind (record or variant) and have exactly the same constructors or fields, in the same order, with the same arguments. The type variables appearing as type parameters can optionally be prefixed by + or - to indicate that the type constructor is covariant or contravariant with respect to this parameter. This variance information is used to decide subtyping relations when checking the validity of >: coercions (see section 6.7.5). For instance, type +'a t declares t as an abstract type that is covariant in its parameter; this means that if the type tau is a subtype of the type sigma, then tau t is a subtype of sigma t. Similarly, type -'a t declares that the abstract type t is contravariant in its parameter: if tau is subtype of sigma, then sigma t is subtype of tau t. If no + or - variance annotation is given, the type constructor is assumed invariant in the corresponding parameter. For instance, the abstract type declaration type 'a t means that tau t is neither a subtype nor a supertype of sigma t if tau is subtype of sigma. The variance indicated by the + and - annotations on parameters are required only for abstract types. For abbreviations, variant types or record types, the variance properties of the type constructor are inferred from its definition, and the variance annotations are only checked for conformance with the definition. The construct constraint ' ident = typexpr allows to specify type parameters. Any actual type argument corresponding to the type parameter ident has to be an instance of typexpr (more precisely, ident and typexpr are unified). Type variables of typexpr can appear in the type equation and the type declaration. 6.8.2 Exception definitions ============================ exception-definition ::= exception constr-decl | exception cconstr-name = cconstr | exception ncconstr-name = ncconstr Exception definitions add new constructors to the built-in variant type `exn' of exception values. The constructors are declared as for a definition of a variant type. The form exception constr-decl generates a new exception, distinct from all other exceptions in the system. The form exception name = constr gives an alternate name to an existing exception. 6.9 Classes *=*=*=*=*=*= Classes are defined using a small language, similar to the module language. 6.9.1 Class types ================== Class types are the class-level equivalent of type expressions: they specify the general shape and type properties of classes. class-type ::= | class-body-type | [[?]label]typexpr -> class-type class-body-type ::= object [( typexpr )] {class-field-spec} end | class-path | [ typexpr {, typexpr} ] class-path class-field-spec ::= inherit class-type | val [mutable] inst-var-name : typexpr | method [private] method-name : poly-typexpr | method [private] virtual method-name : poly-typexpr | constraint typexpr = typexpr Simple class expressions ------------------------ The expression class-path is equivalent to the class type bound to the name class-path. Similarly, the expression [ typexpr_1 , ... typexpr_n ] class-path is equivalent to the parametric class type bound to the name class-path, in which type parameters have been instanciated to respectively typexpr_1, ...typexpr_n. Class function type ------------------- The class type expression typexpr -> class-type is the type of class functions (functions from values to classes) that take as argument a value of type typexpr and return as result a class of type class-type. Class body type --------------- The class type expression object [( typexpr )] {class-field-spec} end is the type of a class body. It specifies its instance variables and methods. In this type, typexpr is matched against the self type, therefore providing a binding for the self type. A class body will match a class body type if it provides definitions for all the components specified in the class type, and these definitions meet the type requirements given in the class type. Furthermore, all methods either virtual or public present in the class body must also be present in the class type (on the other hand, some instance variables and concrete private methods may be omitted). A virtual method will match a concrete method, which makes it possible to forget its implementation. An immutable instance variable will match a mutable instance variable. Inheritance ----------- The inheritance construct inherit class-type allows to include methods and instance variables from other classes types. The instance variable and method types from this class type are added into the current class type. Instance variable specification ------------------------------- A specification of an instance variable is written val [mutable] inst-var-name : typexpr, where inst-var-name is the name of the instance variable and typexpr its expected type. The flag mutable indicates whether this instance variable can be physically modified. An instance variable specification will hide any previous specification of an instance variable of the same name. Method specification -------------------- The specification of a method is written method [private] method-name : poly-typexpr, where method-name is the name of the method and poly-typexpr its expected type, possibly polymorphic. The flag private indicates whether the method can be accessed from outside the class. The polymorphism may be left implicit in method specifications: any type variable which is not bound to a class parameter and does not appear elsewhere inside the class specification will be assumed to be polymorphic, and made explicit in the resulting method type. Writing an explicit polymorphic type will disable this behaviour. Several specification for the same method must have compatible types. Any non-private specification of a method forces it to be public. Virtual method specification ---------------------------- Virtual method specification is written method [private] virtual method-name : poly-typexpr, where method-name is the name of the method and poly-typexpr its expected type. Constraints on type parameters ------------------------------ The construct constraint typexpr_1 = typexpr_2 forces the two type expressions to be equals. This is typically used to specify type parameters: they can be that way be bound to a specified type expression. 6.9.2 Class expressions ======================== Class expressions are the class-level equivalent of value expressions: they evaluate to classes, thus providing implementations for the specifications expressed in class types. class-expr ::= class-path | [ typexpr {, typexpr} ] class-path | ( class-expr ) | ( class-expr : class-type ) [+] --- | class-expr {argument}[ ] --- [+] --- | fun {parameter}[ ] -> class-expr --- | let [rec] let-binding {and let-binding} in class-expr | object [( pattern [: typexpr] )] { class-field } end class-field ::= inherit class-expr [as value-name] | val [mutable] inst-var-name [: typexpr] = expr | method [private] method-name {pattern} [: typexpr] = expr | method [private] method-name : poly-typexpr = expr | method [private] virtual method-name : poly-typexpr | constraint typexpr = typexpr | initializer expr Simple class expressions ------------------------ The expression class-path evaluates to the class bound to the name class-path. Similarly, the expression [ typexpr_1 , ... typexpr_n ] class-path evaluates to the parametric class bound to the name class-path, in which type parameters have been instanciated to respectively typexpr_1, ...typexpr_n. The expression ( class-expr ) evaluates to the same module as class-expr. The expression ( class-expr : class-type ) checks that class-type match the type of class-expr (that is, that the implementation class-expr meets the type specification class-type). The whole expression evaluates to the same class as class-expr, except that all components not specified in class-type are hidden and can no longer be accessed. Class application ----------------- Class application is denoted by juxtaposition of (possibly labeled) expressions. Evaluation works as for expression application. Class function -------------- The expression fun [[?]label]pattern -> class-expr evaluates to a function from values to classes. When this function is applied to a value v, this value is matched against the pattern pattern and the result is the result of the evaluation of class-expr in the extended environment. Conversion from functions with default values to functions with patterns only works identically for class functions as for normal functions. The expression fun parameter_1 ... parameter_n -> class-expr is a short form for fun parameter_1 -> ... fun parameter_n -> expr Local definitions ----------------- The let and let rec constructs bind value names locally, as for the core language expressions. Class body ---------- The expression object ( pattern [: typexpr] ) { class-field } end denotes a class body. This is the prototype for an object : it lists the instance variables and methods of an objet of this class. A class body is a class value: it is not evaluated at once. Rather, its components are evaluated each time an object is created. In a class body, the pattern ( pattern [: typexpr] ) is matched against self, therefore provinding a binding for self and self type. Self can only be used in method and initializers. Self type cannot be a closed object type, so that the class remains extensible. Inheritance ----------- The inheritance construct inherit class-expr allows to reuse methods and instance variables from other classes. The class expression class-expr must evaluate to a class body. The instance variables, methods and initializers from this class body are added into the current class. The addition of a method will override any previously defined methods of the same name. An ancestor can be bound by prepending the construct as value-name to the inheritance construct above. value-name is not a true variable and can only be used to select a method, i.e. in an expression value-name # method-name. This gives access to the method method-name as it was defined in the parent class even if it is redefined in the current class. The scope of an ancestor binding is limited to the current class. The ancestor method may be called from a subclass but only indirectly. Instance variable definition ---------------------------- The definition val [mutable] inst-var-name = expr adds an instance variable inst-var-name whose initial value is the value of expression expr. Several variables of the same name can be defined in the same class. The flag mutable allows physical modification of this variable by methods. An instance variables can only be used in the following methods and initializers of the class. Method definition ----------------- Method definition is written method method-name = expr. The definition of a method overrides any previous definition of this method. The method will be public (that is, not private) if any of the definition states so. A private method, method private method-name = expr, is a method that can only be invoked on self (from other methods of the current class as well as of subclasses of the current class). This invocation is performed using the expression value-name # method-name, where value-name is directly bound to self at the beginning of the class definition. Private methods do not appear in object types. A method may have both public and private definitions, but as soon as there is a public one, all subsequent definitions will be made public. Methods may have an explicitly polymorphic type, allowing them to be used polymorphically in programs (even for the same object). The explicit declaration may be done in one of three ways: (1) by giving an explicit polymorphic type in the method definition, immediately after the method name, i.e. method [private] method-name : {' ident}^+ . typexpr = expr; (2) by a forward declaration of the explicit polymorphic type through a virtual method definition; (3) by importing such a declaration through inheritance and/or constraining the type of self. Some special expressions are available in method bodies for manipulating instance variables and duplicating self: expr ::= ... | inst-var-name <- expr | {< [ inst-var-name = expr { ; inst-var-name = expr } ] >} The expression inst-var-name <- expr modifies in-place the current object by replacing the value associated to inst-var-name by the value of expr. Of course, this instance variable must have been declared mutable. The expression {< [ inst-var-name = expr { ; inst-var-name = expr } ] >} evaluates to a copy of the current object in which the values of instance variables inst-var-name_1, ..., inst-var-name_n have been replaced by the values of the corresponding expressions expr_1, ..., expr_n. Virtual method definition ------------------------- Method specification is written method [private] virtual method-name : poly-typexpr. It specifies whether the method is public or private, and gives its type. If the method is intended to be polymorphic, the type should be explicit. Constraints on type parameters ------------------------------ The construct constraint typexpr_1 = typexpr_2 forces the two type expressions to be equals. This is typically used to specify type parameters: they can be that way be bound to a specified type expression. Initializers ------------ A class initializer initializer expr specifies an expression that will be evaluated when an object will be created from the class, once all the instance variables have been initialized. 6.9.3 Class definitions ======================== class-definition ::= class class-binding { and class-binding } class-binding ::= [virtual] [[ type-parameters ]] class-name { pattern } [: class-type] = class-expr type-parameters ::= ' ident { , ' ident } A class definition class class-binding { and class-binding } is recursive. Each class-binding defines a class-name that can be used in the whole expression except for inheritance. It can also be used for inheritance, but only in the definitions that follow its own. A class binding binds the class name class-name to the value of expression class-expr. It also binds the class type class-name to the type of the class, and defines two type abbreviations : class-name and # class-name. The first one is the type of objects of this class, while the second is more general as it unifies with the type of any object belonging to a subclass (see section 6.4). Virtual class ------------- A class must be flagged virtual if one of its methods is virtual (that is, appears in the class type, but is not actually defined). Objects cannot be created from a virtual class. Type parameters --------------- The class type parameters correspond to the ones of the class type and of the two type abbreviations defined by the class binding. They must be bound to actual types in the class definition using type constraints. So that the abbreviations are well-formed, type variables of the inferred type of the class must either be type parameters or be bound in the constraint clause. 6.9.4 Class specification ========================== class-specification ::= class class-spec { and class-spec } class-spec ::= [virtual] [[ type-parameters ]] class-name : class-type This is the counterpart in signatures of class definitions. A class specification matches a class definition if they have the same type parameters and their types match. 6.9.5 Class type definitions ============================= classtype-definition ::= class type classtype-def { and classtype-def } classtype-def ::= [virtual] [[ type-parameters ]] class-name = class-body-type A class type definition class class-name = class-body-type defines an abbreviation class-name for the class body type class-body-type. As for class definitions, two type abbreviations class-name and # class-name are also defined. The definition can be parameterized by some type parameters. If any method in the class type body is virtual, the definition must be flagged virtual. Two class type definitions match if they have the same type parameters and the types they expand to match. 6.10 Module types (module specifications) *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= Module types are the module-level equivalent of type expressions: they specify the general shape and type properties of modules. module-type ::= modtype-path | sig { specification [;;] } end | functor ( module-name : module-type ) -> module-type | module-type with mod-constraint { and mod-constraint } | ( module-type ) specification ::= val value-name : typexpr | external value-name : typexpr = external-declaration | type-definition | exception constr-decl | class-specification | classtype-definition | module module-name : module-type | module module-name { ( module-name : module-type ) } : module-type | module type modtype-name | module type modtype-name = module-type | open module-path | include module-type mod-constraint ::= type [type-parameters] typeconstr = typexpr | module module-path = extended-module-path 6.10.1 Simple module types =========================== The expression modtype-path is equivalent to the module type bound to the name modtype-path. The expression ( module-type ) denotes the same type as module-type. 6.10.2 Signatures ================== Signatures are type specifications for structures. Signatures sig ... end are collections of type specifications for value names, type names, exceptions, module names and module type names. A structure will match a signature if the structure provides definitions (implementations) for all the names specified in the signature (and possibly more), and these definitions meet the type requirements given in the signature. For compatibility with Caml Light, an optional ;; is allowed after each specification in a signature. The ;; has no semantic meaning. Value specifications -------------------- A specification of a value component in a signature is written val value-name : typexpr, where value-name is the name of the value and typexpr its expected type. The form external value-name : typexpr = external-declaration is similar, except that it requires in addition the name to be implemented as the external function specified in external-declaration (see chapter 18). Type specifications ------------------- A specification of one or several type components in a signature is written type typedef { and typedef } and consists of a sequence of mutually recursive definitions of type names. Each type definition in the signature specifies an optional type equation = typexp and an optional type representation = constr-decl ... or = { label-decl ... }. The implementation of the type name in a matching structure must be compatible with the type expression specified in the equation (if given), and have the specified representation (if given). Conversely, users of that signature will be able to rely on the type equation or type representation, if given. More precisely, we have the following four situations: Abstract type: no equation, no representation. Names that are defined as abstract types in a signature can be implemented in a matching structure by any kind of type definition (provided it has the same number of type parameters). The exact implementation of the type will be hidden to the users of the structure. In particular, if the type is implemented as a variant type or record type, the associated constructors and fields will not be accessible to the users; if the type is implemented as an abbreviation, the type equality between the type name and the right-hand side of the abbreviation will be hidden from the users of the structure. Users of the structure consider that type as incompatible with any other type: a fresh type has been generated. Type abbreviation: an equation = typexp, no representation. The type name must be implemented by a type compatible with typexp. All users of the structure know that the type name is compatible with typexp. New variant type or record type: no equation, a representation. The type name must be implemented by a variant type or record type with exactly the constructors or fields specified. All users of the structure have access to the constructors or fields, and can use them to create or inspect values of that type. However, users of the structure consider that type as incompatible with any other type: a fresh type has been generated. Re-exported variant type or record type: an equation, a representation. This case combines the previous two: the representation of the type is made visible to all users, and no fresh type is generated. Exception specification ----------------------- The specification exception constr-decl in a signature requires the matching structure to provide an exception with the name and arguments specified in the definition, and makes the exception available to all users of the structure. Class specifications -------------------- A specification of one or several classes in a signature is written class class-spec { and class-spec } and consists of a sequence of mutually recursive definitions of class names. Class specifications are described more precisely in section 6.9.4. Class type specifications ------------------------- A specification of one or several classe types in a signature is written class type classtype-def { and classtype-def } and consists of a sequence of mutually recursive definitions of class type names. Class type specifications are described more precisely in section 6.9.5. Module specifications --------------------- A specification of a module component in a signature is written module module-name : module-type, where module-name is the name of the module component and module-type its expected type. Modules can be nested arbitrarily; in particular, functors can appear as components of structures and functor types as components of signatures. For specifying a module component that is a functor, one may write module module-name ( name_1 : module-type_1 ) ... ( name_n : module-type_n ) : module-type instead of module module-name : functor ( name_1 : module-type_1 ) -> ... -> module-type Module type specifications -------------------------- A module type component of a signature can be specified either as a manifest module type or as an abstract module type. An abstract module type specification module type modtype-name allows the name modtype-name to be implemented by any module type in a matching signature, but hides the implementation of the module type to all users of the signature. A manifest module type specification module type modtype-name = module-type requires the name modtype-name to be implemented by the module type module-type in a matching signature, but makes the equality between modtype-name and module-type apparent to all users of the signature. Opening a module path --------------------- The expression open module-path in a signature does not specify any components. It simply affects the parsing of the following items of the signature, allowing components of the module denoted by module-path to be referred to by their simple names name instead of path accesses module-path . name. The scope of the open stops at the end of the signature expression. Including a signature --------------------- The expression include module-type in a signature performs textual inclusion of the components of the signature denoted by module-type. It behaves as if the components of the included signature were copied at the location of the include. The module-type argument must refer to a module type that is a signature, not a functor type. 6.10.3 Functor types ===================== The module type expression functor ( module-name : module-type_1 ) -> module-type_2 is the type of functors (functions from modules to modules) that take as argument a module of type module-type_1 and return as result a module of type module-type_2. The module type module-type_2 can use the name module-name to refer to type components of the actual argument of the functor. No restrictions are placed on the type of the functor argument; in particular, a functor may take another functor as argument (``higher-order'' functor). 6.10.4 The with operator ========================= Assuming module-type denotes a signature, the expression module-type with mod-constraint { and mod-constraint } denotes the same signature where type equations have been added to some of the type specifications, as described by the constraints following the with keyword. The constraint type [type-parameters] typeconstr = typexp adds the type equation = typexp to the specification of the type component named typeconstr of the constrained signature. The constraint module module-path = extended-module-path adds type equations to all type components of the sub-structure denoted by module-path, making them equivalent to the corresponding type components of the structure denoted by extended-module-path. For instance, if the module type name S is bound to the signature << sig type t module M: (sig type u end) end >> then S with type t=int denotes the signature << sig type t=int module M: (sig type u end) end >> and S with module M = N denotes the signature << sig type t module M: (sig type u=N.u end) end >> A functor taking two arguments of type S that share their t component is written << functor (A: S) (B: S with type t = A.t) ... >> 6.11 Module expressions (module implementations) *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* Module expressions are the module-level equivalent of value expressions: they evaluate to modules, thus providing implementations for the specifications expressed in module types. module-expr ::= module-path | struct { definition [;;] } end | functor ( module-name : module-type ) -> module-expr | module-expr ( module-expr ) | ( module-expr ) | ( module-expr : module-type ) definition ::= let [rec] let-binding { and let-binding } | external value-name : typexpr = external-declaration | type-definition | exception-definition | class-definition | classtype-definition | module module-name { ( module-name : module-type ) } [ : module-type ] = module-expr | module type modtype-name = module-type | open module-path | include module-expr 6.11.1 Simple module expressions ================================= The expression module-path evaluates to the module bound to the name module-path. The expression ( module-expr ) evaluates to the same module as module-expr. The expression ( module-expr : module-type ) checks that the type of module-expr is a subtype of module-type, that is, that all components specified in module-type are implemented in module-expr, and their implementation meets the requirements given in module-type. In other terms, it checks that the implementation module-expr meets the type specification module-type. The whole expression evaluates to the same module as module-expr, except that all components not specified in module-type are hidden and can no longer be accessed. 6.11.2 Structures ================== Structures struct ... end are collections of definitions for value names, type names, exceptions, module names and module type names. The definitions are evaluated in the order in which they appear in the structure. The scope of the bindings performed by the definitions extend to the end of the structure. As a consequence, a definition may refer to names bound by earlier definitions in the same structure. For compatibility with toplevel phrases (chapter 9) and with Caml Light, an optional ;; is allowed after each definition in a structure. The ;; has no semantic meaning. Also for compatibility, ;; expr is allowed as a component of a structure, meaning let _ = expr, i.e. evaluate expr for its side-effects. Value definitions ----------------- A value definition let [rec] let-binding { and let-binding } bind value names in the same way as a let ... in ... expression (see section 6.7.1). The value names appearing in the left-hand sides of the bindings are bound to the corresponding values in the right-hand sides. A value definition external value-name : typexpr = external-declaration implements value-name as the external function specified in external-declaration (see chapter 18). Type definitions ---------------- A definition of one or several type components is written type typedef { and typedef } and consists of a sequence of mutually recursive definitions of type names. Exception definitions --------------------- Exceptions are defined with the syntax exception constr-decl or exception constr-name = constr. Class definitions ----------------- A definition of one or several classes is written class class-binding { and class-binding } and consists of a sequence of mutually recursive definitions of class names. Class definitions are described more precisely in section 6.9.3. Class type definitions ---------------------- A definition of one or several classes is written class type classtype-def { and classtype-def } and consists of a sequence of mutually recursive definitions of class type names. Class type definitions are described more precisely in section 6.9.5. Module definitions ------------------ The basic form for defining a module component is module module-name = module-expr, which evaluates module-expr and binds the result to the name module-name. One can write module module-name : module-type = module-expr instead of module module-name = ( module-expr : module-type ). Another derived form is module module-name ( name_1 : module-type_1 ) ... ( name_n : module-type_n ) = module-expr which is equivalent to module module-name = functor ( name_1 : module-type_1 ) -> ... -> module-expr Module type definitions ----------------------- A definition for a module type is written module type modtype-name = module-type. It binds the name modtype-name to the module type denoted by the expression module-type. Opening a module path --------------------- The expression open module-path in a structure does not define any components nor perform any bindings. It simply affects the parsing of the following items of the structure, allowing components of the module denoted by module-path to be referred to by their simple names name instead of path accesses module-path . name. The scope of the open stops at the end of the structure expression. Including the components of another structure --------------------------------------------- The expression include module-expr in a structure re-exports in the current structure all definitions of the structure denoted by module-expr. For instance, if the identifier S is bound to the module << struct type t = int let x = 2 end >> the module expression << struct include S let y = (x + 1 : t) end >> is equivalent to the module expression << struct type t = int let x = 2 let y = (x + 1 : t) end >> The difference between open and include is that open simply provides short names for the components of the opened structure, without defining any components of the current structure, while include also adds definitions for the components of the included structure. 6.11.3 Functors ================ Functor definition ------------------ The expression functor ( module-name : module-type ) -> module-expr evaluates to a functor that takes as argument modules of the type module-type_1, binds module-name to these modules, evaluates module-expr in the extended environment, and returns the resulting modules as results. No restrictions are placed on the type of the functor argument; in particular, a functor may take another functor as argument (``higher-order'' functor). Functor application ------------------- The expression module-expr_1 ( module-expr_2 ) evaluates module-expr_1 to a functor and module-expr_2 to a module, and applies the former to the latter. The type of module-expr_2 must match the type expected for the arguments of the functor module-expr_1. 6.12 Compilation units *=*=*=*=*=*=*=*=*=*=*=* unit-interface ::= { specification [;;] } unit-implementation ::= { definition [;;] } Compilation units bridge the module system and the separate compilation system. A compilation unit is composed of two parts: an interface and an implementation. The interface contains a sequence of specifications, just as the inside of a sig ... end signature expression. The implementation contains a sequence of definitions, just as the inside of a struct ... end module expression. A compilation unit also has a name unit-name, derived from the names of the files containing the interface and the implementation (see chapter 8 for more details). A compilation unit behaves roughly as the module definition module unit-name : sig unit-interface end = struct unit-implementation end A compilation unit can refer to other compilation units by their names, as if they were regular modules. For instance, if U is a compilation unit that defines a type t, other compilation units can refer to that type under the name U.t; they can also refer to U as a whole structure. Except for names of other compilation units, a unit interface or unit implementation must not have any other free variables. In other terms, the type-checking and compilation of an interface or implementation proceeds in the initial environment name_1 : sig interface_1 end ... name_n : sig interface_n end where name_1 ... name_n are the names of the other compilation units available in the search path (see chapter 8 for more details) and interface_1 ... interface_n are their respective interfaces. Chapter 7 Language extensions ******************************** This chapter describes syntactic extensions and convenience features that are implemented in Objective Caml, but not described in the Objective Caml reference manual. 7.1 Streams and stream parsers *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* Streams and stream parsers are no longer part of the Objective Caml language, but available through a CamlP4 syntax extension. See the CamlP4 reference manual for more information. Objective Caml programs that use streams and stream parsers can be compiled with the -pp camlp4o option to ocamlc and ocamlopt. For interactive use, run ocaml and issue the `#load "camlp4o.cma";;' command. 7.2 Range patterns *=*=*=*=*=*=*=*=*=* In patterns, Objective Caml recognizes the form ' c ' .. ' d ' (two character literals separated by ..) as shorthand for the pattern ' c ' | ' c_1 ' | ' c_2 ' | ... | ' c_n ' | ' d ' where c_1, c_2, ..., c_n are the characters that occur between c and d in the ASCII character set. For instance, the pattern '0'..'9' matches all characters that are digits. 7.3 Assertion checking *=*=*=*=*=*=*=*=*=*=*=* Objective Caml supports the assert construct to check debugging assertions. The expression assert expr evaluates the expression expr and returns () if expr evaluates to true. Otherwise, the exception Assert_failure is raised with the source file name and the location of expr as arguments. Assertion checking can be turned off with the -noassert compiler option. As a special case, assert false is reduced to raise (Assert_failure ...), which is polymorphic (and is not turned off by the -noassert option). 7.4 Deferred computations *=*=*=*=*=*=*=*=*=*=*=*=*= The expression lazy expr returns a value v of type Lazy.t that encapsulates the computation of expr. The argument expr is not evaluated at this point in the program. Instead, its evaluation will be performed the first time Lazy.force is applied to the value v, returning the actual value of expr. Subsequent applications of Lazy.force to v do not evaluate expr again. For more information, see the description of module Lazy in the standard library (section 20.15). 7.5 Local modules *=*=*=*=*=*=*=*=*= The expression let module module-name = module-expr in expr locally binds the module expression module-expr to the identifier module-name during the evaluation of the expression expr. It then returns the value of expr. For example: << let remove_duplicates comparison_fun string_list = let module StringSet = Set.Make(struct type t = string let compare = comparison_fun end) in StringSet.elements (List.fold_right StringSet.add string_list StringSet.empty) >> 7.6 Grouping in integer and floating-point literals *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= In integer and floating-point literals, the character _ (underscore) can be used to separate groups of digits, as in 1_000_000, 0x45_FF, or 1_234.567_89. The underscore characters are simply ignored when reading the literal. Part: III ********* The Objective Caml tools ************************ Chapter 8 Batch compilation (ocamlc) *************************************** This chapter describes the Objective Caml batch compiler ocamlc, which compiles Caml source files to bytecode object files and link these object files to produce standalone bytecode executable files. These executable files are then run by the bytecode interpreter ocamlrun. 8.1 Overview of the compiler *=*=*=*=*=*=*=*=*=*=*=*=*=*=* The ocamlc command has a command-line interface similar to the one of most C compilers. It accepts several types of arguments: - Arguments ending in .mli are taken to be source files for compilation unit interfaces. Interfaces specify the names exported by compilation units: they declare value names with their types, define public data types, declare abstract data types, and so on. From the file x.mli, the ocamlc compiler produces a compiled interface in the file x.cmi. - Arguments ending in .ml are taken to be source files for compilation unit implementations. Implementations provide definitions for the names exported by the unit, and also contain expressions to be evaluated for their side-effects. From the file x.ml, the ocamlc compiler produces compiled object bytecode in the file x.cmo. If the interface file x.mli exists, the implementation x.ml is checked against the corresponding compiled interface x.cmi, which is assumed to exist. If no interface x.mli is provided, the compilation of x.ml produces a compiled interface file x.cmi in addition to the compiled object code file x.cmo. The file x.cmi produced corresponds to an interface that exports everything that is defined in the implementation x.ml. - Arguments ending in .cmo are taken to be compiled object bytecode. These files are linked together, along with the object files obtained by compiling .ml arguments (if any), and the Objective Caml standard library, to produce a standalone executable program. The order in which .cmo and .ml arguments are presented on the command line is relevant: compilation units are initialized in that order at run-time, and it is a link-time error to use a component of a unit before having initialized it. Hence, a given x.cmo file must come before all .cmo files that refer to the unit x. - Arguments ending in .cma are taken to be libraries of object bytecode. A library of object bytecode packs in a single file a set of object bytecode files (.cmo files). Libraries are built with ocamlc -a (see the description of the -a option below). The object files contained in the library are linked as regular .cmo files (see above), in the order specified when the .cma file was built. The only difference is that if an object file contained in a library is not referenced anywhere in the program, then it is not linked in. - Arguments ending in .c are passed to the C compiler, which generates a .o object file. This object file is linked with the program if the -custom flag is set (see the description of -custom below). - Arguments ending in .o or .a (.obj or .lib under Windows) are assumed to be C object files and libraries. They are passed to the C linker when linking in -custom mode (see the description of -custom below). - Arguments ending in .so (.dll under Windows) are assumed to be C shared libraries (DLLs). During linking, they are searched for external C functions referenced from the Caml code, and their names are written in the generated bytecode executable. The run-time system ocamlrun then loads them dynamically at program start-up time. The output of the linking phase is a file containing compiled bytecode that can be executed by the Objective Caml bytecode interpreter: the command named ocamlrun. If caml.out is the name of the file produced by the linking phase, the command << ocamlrun caml.out arg_1 arg_2 ... arg_n >> executes the compiled code contained in caml.out, passing it as arguments the character strings arg_1 to arg_n. (See chapter 10 for more details.) On most systems, the file produced by the linking phase can be run directly, as in: << ./caml.out arg_1 arg_2 ... arg_n >> The produced file has the executable bit set, and it manages to launch the bytecode interpreter by itself. 8.2 Options *=*=*=*=*=*= The following command-line options are recognized by ocamlc. -a Build a library (.cma file) with the object files (.cmo files) given on the command line, instead of linking them into an executable file. The name of the library can be set with the -o option. The default name is library.cma. If -custom, -cclib or -ccopt options are passed on the command line, these options are stored in the resulting .cma library. Then, linking with this library automatically adds back the -custom, -cclib and -ccopt options as if they had been provided on the command line, unless the -noautolink option is given. -c Compile only. Suppress the linking phase of the compilation. Source code files are turned into compiled files, but no executable file is produced. This option is useful to compile modules separately. -cc ccomp Use ccomp as the C linker called by ocamlc -custom and as the C compiler for compiling .c source files. -cclib -llibname Pass the -llibname option to the C linker when linking in ``custom runtime'' mode (see the -custom option). This causes the given C library to be linked with the program. -ccopt option Pass the given option to the C compiler and linker, when linking in ``custom runtime'' mode (see the -custom option). For instance, -ccopt -Ldir causes the C linker to search for C libraries in directory dir. -custom Link in ``custom runtime'' mode. In the default linking mode, the linker produces bytecode that is intended to be executed with the shared runtime system, ocamlrun. In the custom runtime mode, the linker produces an output file that contains both the runtime system and the bytecode for the program. The resulting file is larger, but it can be executed directly, even if the ocamlrun command is not installed. Moreover, the ``custom runtime'' mode enables static linking of Caml code with user-defined C functions, as described in chapter 18. Unix: Never use the strip command on executables produced by ocamlc -custom. This would remove the bytecode part of the executable. -dllib -llibname Arrange for the C shared library dlllibname.so (dlllibname.dll under Windows) to be loaded dynamically by the run-time system ocamlrun at program start-up time. -dllpath dir Adds the directory dir to the run-time search path for shared C libraries. At link-time, shared libraries are searched in the standard search path (the one corresponding to the -I option). The -dllpath option simply stores dir in the produced executable file, where ocamlrun can find it and exploit it as described in section 10.3. -g Add debugging information while compiling and linking. This option is required in order to be able to debug the program with ocamldebug (see chapter 16). -i Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (.ml file). This can be useful to check the types inferred by the compiler. Also, since the output follows the syntax of interfaces, it can help in writing an explicit interface (.mli file) for a file: just redirect the standard output of the compiler to a .mli file, and edit that file to remove all declarations of unexported names. -I directory Add the given directory to the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmo), libraries (.cma), and C libraries specified with -cclib -lxxx. By default, the current directory is searched first, then the standard library directory. Directories added with -I are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory. If the given directory starts with +, it is taken relative to the standard library directory. For instance, -I +labltk adds the subdirectory labltk of the standard library to the search path. -impl filename Compile the file filename as an implementation file, even if its extension is not .ml. -intf filename Compile the file filename as an interface file, even if its extension is not .mli. -linkall Force all modules contained in libraries to be linked in. If this flag is not given, unreferenced modules are not linked in. When building a library (-a flag), setting the -linkall flag forces all subsequent links of programs involving that library to link all the modules contained in the library. -make-runtime Build a custom runtime system (in the file specified by option -o) incorporating the C object files and libraries given on the command line. This custom runtime system can be used later to execute bytecode executables produced with the ocamlc -use-runtime runtime-name option. See section 18.1.6 for more information. -noassert Turn assertion checking off: assertions are not compiled. This flag has no effect when linking already compiled files. -noautolink When linking .cma libraries, ignore -custom, -cclib and -ccopt options potentially contained in the libraries (if these options were given when building the libraries). This can be useful if a library contains incorrect specifications of C libraries or C options; in this case, during linking, set -noautolink and pass the correct C libraries and options on the command line. -nolabels Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict. -o exec-file Specify the name of the output file produced by the linker. The default output name is a.out, in keeping with the Unix tradition. If the -a option is given, specify the name of the library produced. If the -output-obj option is given, specify the name of the output file produced. -output-obj Cause the linker to produce a C object file instead of a bytecode executable file. This is useful to wrap Caml code as a C library, callable from any C program. See chapter 18, section 18.7.5. The name of the output object file is camlprog.o by default; it can be set with the -o option. -pack Build a bytecode object file (.cmo file) and its associated compiled interface (.cmi) that combines the object files given on the command line, making them appear as sub-modules of the output .cmo file. The name of the output .cmo file must be given with the -o option. For instance, << ocamlc -pack -o p.cmo a.cmo b.cmo c.cmo >> generates compiled files p.cmo and p.cmi describing a compilation unit having three sub-modules A, B and C, corresponding to the contents of the object files a.cmo, b.cmo and c.cmo. These contents can be referenced as P.A, P.B and P.C in the remainder of the program. -pp command Cause the compiler to call the given command as a preprocessor for each source file. The output of command is redirected to an intermediate file, which is compiled. If there are no compilation errors, the intermediate file is deleted afterwards. The name of this file is built from the basename of the source file with the extension .ppi for an interface (.mli) file and .ppo for an implementation (.ml) file. -principal Check information path during type-checking, to make sure that all types are derived in a principal way. When using labelled arguments and/or polymorphic methods, this flag is required to ensure future versions of the compiler will be able to infer types correctly, even if internal algorithms change. All programs accepted in -principal mode are also accepted in default mode with equivalent types, but different binary signatures, and this may slow down type checking; yet this is a good idea to use it once before publishing source code. -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. -thread Compile or link multithreaded programs, in combination with the threads library described in chapter 24. What this option actually does is select a special, thread-safe version of the standard library. -unsafe Turn bound checking off on array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds. -use-runtime runtime-name Generate a bytecode executable file that can be executed on the custom runtime system runtime-name, built earlier with ocamlc -make-runtime runtime-name. See section 18.1.6 for more information. -v Print the version number of the compiler and the location of the standard library directory, then exit. -verbose Print all external commands before they are executed, in particular invocations of the C compiler and linker in -custom mode. Useful to debug C library problems. -version Print the version number of the compiler in short form (e.g. 3.06), then exit. -w warning-list Enable or disable warnings according to the argument warning-list. The argument is a string of one or several characters, with the following meaning for each character: A/a enable/disable all warnings. C/c enable/disable warnings for suspicious comments. D/d enable/disable warnings for deprecated features. F/f enable/disable warnings for partially applied functions (i.e. f x; expr where the application f x has a function type). L/l enable/disable warnings for labels omitted in application. M/m enable/disable warnings for overriden methods. P/p enable/disable warnings for partial matches (missing cases in pattern matchings). S/s enable/disable warnings for statements that do not have type unit (e.g. expr1; expr2 when expr1 does not have type unit). U/u enable/disable warnings for unused (redundant) match cases. V/v enable/disable warnings for hidden instance variables. X/x enable/disable all other warnings. The default setting is -w Al (all warnings but labels enabled). -warn-error warning-list Turn the warnings indicated in the argument warning-list into errors. The compiler will stop on an error as soon as one of these warnings is emitted, instead of going on. The warning-list is a string of one or several characters, with the same meaning as for the -w option: an uppercase character turns the corresponding warning into an error, a lowercase character leaves it as a warning. The default setting is -warn-error a (all warnings are not treated as errors). -where Print the location of the standard library, then exit. 8.3 Modules and the file system *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= This short section is intended to clarify the relationship between the names of the modules corresponding to compilation units and the names of the files that contain their compiled interface and compiled implementation. The compiler always derives the module name by taking the capitalized base name of the source file (.ml or .mli file). That is, it strips the leading directory name, if any, as well as the .ml or .mli suffix; then, it set the first letter to uppercase, in order to comply with the requirement that module names must be capitalized. For instance, compiling the file mylib/misc.ml provides an implementation for the module named Misc. Other compilation units may refer to components defined in mylib/misc.ml under the names Misc.name; they can also do open Misc, then use unqualified names name. The .cmi and .cmo files produced by the compiler have the same base name as the source file. Hence, the compiled files always have their base name equal (modulo capitalization of the first letter) to the name of the module they describe (for .cmi files) or implement (for .cmo files). When the compiler encounters a reference to a free module identifier Mod, it looks in the search path for a file mod.cmi (note lowercasing of first letter) and loads the compiled interface contained in that file. As a consequence, renaming .cmi files is not advised: the name of a .cmi file must always correspond to the name of the compilation unit it implements. It is admissible to move them to another directory, if their base name is preserved, and the correct -I options are given to the compiler. The compiler will flag an error if it loads a .cmi file that has been renamed. Compiled bytecode files (.cmo files), on the other hand, can be freely renamed once created. That's because the linker never attempts to find by itself the .cmo file that implements a module with a given name: it relies instead on the user providing the list of .cmo files by hand. 8.4 Common errors *=*=*=*=*=*=*=*=*= This section describes and explains the most frequently encountered error messages. Cannot find file filename The named file could not be found in the current directory, nor in the directories of the search path. The filename is either a compiled interface file (.cmi file), or a compiled bytecode file (.cmo file). If filename has the format mod.cmi, this means you are trying to compile a file that references identifiers from module mod, but you have not yet compiled an interface for module mod. Fix: compile mod.mli or mod.ml first, to create the compiled interface mod.cmi. If filename has the format mod.cmo, this means you are trying to link a bytecode object file that does not exist yet. Fix: compile mod.ml first. If your program spans several directories, this error can also appear because you haven't specified the directories to look into. Fix: add the correct -I options to the command line. Corrupted compiled interface filename The compiler produces this error when it tries to read a compiled interface file (.cmi file) that has the wrong structure. This means something went wrong when this .cmi file was written: the disk was full, the compiler was interrupted in the middle of the file creation, and so on. This error can also appear if a .cmi file is modified after its creation by the compiler. Fix: remove the corrupted .cmi file, and rebuild it. This expression has type t_1, but is used with type t_2 This is by far the most common type error in programs. Type t_1 is the type inferred for the expression (the part of the program that is displayed in the error message), by looking at the expression itself. Type t_2 is the type expected by the context of the expression; it is deduced by looking at how the value of this expression is used in the rest of the program. If the two types t_1 and t_2 are not compatible, then the error above is produced. In some cases, it is hard to understand why the two types t_1 and t_2 are incompatible. For instance, the compiler can report that ``expression of type foo cannot be used with type foo'', and it really seems that the two types foo are compatible. This is not always true. Two type constructors can have the same name, but actually represent different types. This can happen if a type constructor is redefined. Example: << type foo = A | B let f = function A -> 0 | B -> 1 type foo = C | D f C >> This result in the error message ``expression C of type foo cannot be used with type foo''. The type of this expression, t, contains type variables that cannot be generalized Type variables ('a, 'b, ...) in a type t can be in either of two states: generalized (which means that the type t is valid for all possible instantiations of the variables) and not generalized (which means that the type t is valid only for one instantiation of the variables). In a let binding let name = expr, the type-checker normally generalizes as many type variables as possible in the type of expr. However, this leads to unsoundness (a well-typed program can crash) in conjunction with polymorphic mutable data structures. To avoid this, generalization is performed at let bindings only if the bound expression expr belongs to the class of ``syntactic values'', which includes constants, identifiers, functions, tuples of syntactic values, etc. In all other cases (for instance, expr is a function application), a polymorphic mutable could have been created and generalization is therefore turned off. Non-generalized type variables in a type cause no difficulties inside a given structure or compilation unit (the contents of a .ml file, or an interactive session), but they cannot be allowed inside signatures nor in compiled interfaces (.cmi file), because they could be used inconsistently later. Therefore, the compiler flags an error when a structure or compilation unit defines a value name whose type contains non-generalized type variables. There are two ways to fix this error: - Add a type constraint or a .mli file to give a monomorphic type (without type variables) to name. For instance, instead of writing << let sort_int_list = Sort.list (<) (* inferred type 'a list -> 'a list, with 'a not generalized *) >> write << let sort_int_list = (Sort.list (<) : int list -> int list);; >> - If you really need name to have a polymorphic type, turn its defining expression into a function by adding an extra parameter. For instance, instead of writing << let map_length = List.map Array.length (* inferred type 'a array list -> int list, with 'a not generalized *) >> write << let map_length lv = List.map Array.length lv >> Reference to undefined global mod This error appears when trying to link an incomplete or incorrectly ordered set of files. Either you have forgotten to provide an implementation for the compilation unit named mod on the command line (typically, the file named mod.cmo, or a library containing that file). Fix: add the missing .ml or .cmo file to the command line. Or, you have provided an implementation for the module named mod, but it comes too late on the command line: the implementation of mod must come before all bytecode object files that reference mod. Fix: change the order of .ml and .cmo files on the command line. Of course, you will always encounter this error if you have mutually recursive functions across modules. That is, function Mod1.f calls function Mod2.g, and function Mod2.g calls function Mod1.f. In this case, no matter what permutations you perform on the command line, the program will be rejected at link-time. Fixes: - Put f and g in the same module. - Parameterize one function by the other. That is, instead of having << mod1.ml: let f x = ... Mod2.g ... mod2.ml: let g y = ... Mod1.f ... >> define << mod1.ml: let f g x = ... g ... mod2.ml: let rec g y = ... Mod1.f g ... >> and link mod1.cmo before mod2.cmo. - Use a reference to hold one of the two functions, as in : << mod1.ml: let forward_g = ref((fun x -> failwith "forward_g") : ) let f x = ... !forward_g ... mod2.ml: let g y = ... Mod1.f ... let _ = Mod1.forward_g := g >> This will not work if g is a polymorphic function, however. The external function f is not available This error appears when trying to link code that calls external functions written in C. As explained in chapter 18, such code must be linked with C libraries that implement the required f C function. If the C libraries in question are not shared libraries (DLLs), the code must be linked in ``custom runtime'' mode. Fix: add the required C libraries to the command line, and possibly the -custom option. Chapter 9 The toplevel system (ocaml) **************************************** This chapter describes the toplevel system for Objective Caml, that permits interactive use of the Objective Caml system through a read-eval-print loop. In this mode, the system repeatedly reads Caml phrases from the input, then typechecks, compile and evaluate them, then prints the inferred type and result value, if any. The system prints a # (sharp) prompt before reading each phrase. Input to the toplevel can span several lines. It is terminated by ;; (a double-semicolon). The toplevel input consists in one or several toplevel phrases, with the following syntax: toplevel-input ::= { toplevel-phrase } ;; toplevel-phrase ::= definition | expr | # ident directive-argument definition ::= let [rec] let-binding { and let-binding } | external value-name : typexpr = external-declaration | type-definition | exception-definition | module module-name [ : module-type ] = module-expr | module type modtype-name = module-type | open module-path directive-argument ::= nothing | string-literal | integer-literal | value-path A phrase can consist of a definition, similar to those found in implementations of compilation units or in struct ... end module expressions. The definition can bind value names, type names, an exception, a module name, or a module type name. The toplevel system performs the bindings, then prints the types and values (if any) for the names thus defined. A phrase may also consist in a open directive (see section 6.11), or a value expression (section 6.7). Expressions are simply evaluated, without performing any bindings, and the value of the expression is printed. Finally, a phrase can also consist in a toplevel directive, starting with # (the sharp sign). These directives control the behavior of the toplevel; they are listed below in section 9.2. Unix: The toplevel system is started by the command ocaml, as follows: << ocaml options objects # interactive mode ocaml options objects scriptfile # script mode >> options are described below. objects are filenames ending in .cmo or .cma; they are loaded into the interpreter immediately after options are set. scriptfile is any file name not ending in .cmo or .cma. If no scriptfile is given on the command line, the toplevel system enters interactive mode: phrases are read on standard input, results are printed on standard output, errors on standard error. End-of-file on standard input terminates ocaml (see also the #quit directive in section 9.2). On start-up (before the first phrase is read), if the file .ocamlinit exists in the current directory, its contents are read as a sequence of Objective Caml phrases and executed as per the #use directive described in section 9.2. The evaluation outcode for each phrase are not displayed. The toplevel system does not perform line editing, but it can easily be used in conjunction with an external line editor such as fep; just run fep -emacs ocaml or fep -vi ocaml. Another option is to use ocaml under Gnu Emacs, which gives the full editing power of Emacs (see the subdirectory emacs of the Objective Caml distribution). At any point, the parsing, compilation or evaluation of the current phrase can be interrupted by pressing ctrl-C (or, more precisely, by sending the sigintr signal to the ocaml process). The toplevel then immediately returns to the # prompt. If scriptfile is given on the command-line to ocaml, the toplevel system enters script mode: the contents of the file are read as a sequence of Objective Caml phrases and executed, as per the #use directive (section 9.2). The outcome of the evaluation is not printed. On reaching the end of file, the ocaml command exits immediately. No commands are read from standard input. Sys.argv is transformed, ignoring all Objective Caml parameters, and starting with the script file name in Sys.argv.(0). In script mode, the first line of the script is ignored if it starts with #!. Thus, it is theoretically possible to make the script itself executable and put as first line #!/usr/local/bin/ocaml, thus calling the toplevel system automatically when the script is run. However, ocaml itself is a #! script on most installations of Objective Caml, and Unix kernels usually do not handle nested #! scripts. Windows: In addition to the text-only command ocaml.exe, which works exactly as under Unix (see above), a graphical user interface for the toplevel is available under the name ocamlwin.exe. It should be launched from the Windows file manager or program manager. The ``Terminal'' windows is split in two panes. Phrases are entered and edited in the bottom pane. The top pane displays a copy of the input phrases as they are processed by the Objective Caml toplevel, interspersed with the toplevel responses. The ``Return'' key sends the contents of the bottom pane to the Objective Caml toplevel. The ``Enter'' key inserts a newline without sending the contents of the Input window. (This can be configured with the ``Preferences'' menu item.) The contents of the input window can be edited at all times, with the standard Windows interface. An history of previously entered phrases is maintained and displayed in a separate window. To quit the Camlwin application, either select ``Quit'' from the ``File'' menu, or use the quit function described below. At any point, the parsing, compilation or evaluation of the current phrase can be interrupted by selecting the ``Interrupt Objective Caml'' menu item. This goes back to the # prompt. 9.1 Options *=*=*=*=*=*= The following command-line options are recognized by the ocaml command. -I directory Add the given directory to the list of directories searched for source and compiled files. By default, the current directory is searched first, then the standard library directory. Directories added with -I are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory. If the given directory starts with +, it is taken relative to the standard library directory. For instance, -I +labltk adds the subdirectory labltk of the standard library to the search path. Directories can also be added to the search path once the toplevel is running with the #directory directive (section 9.2). -nolabels Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict. -principal Check information path during type-checking, to make sure that all types are derived in a principal way. All programs accepted in -principal mode are also accepted in default mode with equivalent types. -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. -unsafe See the corresponding option for ocamlc, chapter 8. Turn bound checking off on array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds. -w warning-list Enable or disable warnings according to the argument warning-list. Unix: The following environment variables are also consulted: LC_CTYPE If set to iso_8859_1, accented characters (from the ISO Latin-1 character set) in string and character literals are printed as is; otherwise, they are printed as decimal escape sequences (\ddd). TERM When printing error messages, the toplevel system attempts to underline visually the location of the error. It consults the TERM variable to determines the type of output terminal and look up its capabilities in the terminal database. 9.2 Toplevel directives *=*=*=*=*=*=*=*=*=*=*=*= The following directives control the toplevel behavior, load files in memory, and trace program execution. Note: all directives start with a # (sharp) symbol. This # must be typed before the directive, and must not be confused with the # prompt displayed by the interactive loop. For instance, typing #quit;; will exit the toplevel loop, but typing quit;; will result in an ``unbound value quit'' error. #quit;; Exit the toplevel loop and terminate the ocaml command. #labels bool;; Ignore labels in function types if argument is false, or switch back to default behaviour (commuting style) if argument is true. #warnings "warning-list";; Enable or disable warnings according to the argument. #directory "dir-name";; Add the given directory to the list of directories searched for source and compiled files. #cd "dir-name";; Change the current working directory. #load "file-name";; Load in memory a bytecode object file (.cmo file) produced by the batch compiler ocamlc. #use "file-name";; Read, compile and execute source phrases from the given file. This is textual inclusion: phrases are processed just as if they were typed on standard input. The reading of the file stops at the first error encountered. #install_printer printer-name;; This directive registers the function named printer-name (a value path) as a printer for values whose types match the argument type of the function. That is, the toplevel loop will call printer-name when it has such a value to print. The printing function printer-name should have type Format.formatter -> t -> unit, where t is the type for the values to be printed, and should output its textual representation for the value of type t on the given formatter, using the functions provided by the Format library. For backward compatibility, printer-name can also have type t -> unit and should then output on the standard formatter, but this usage is deprecated. #remove_printer printer-name;; Remove the named function from the table of toplevel printers. #trace function-name;; After executing this directive, all calls to the function named function-name will be ``traced''. That is, the argument and the result are displayed for each call, as well as the exceptions escaping out of the function, raised either by the function itself or by another function it calls. If the function is curried, each argument is printed as it is passed to the function. #untrace function-name;; Stop tracing the given function. #untrace_all;; Stop tracing all functions traced so far. #print_depth n;; Limit the printing of values to a maximal depth of n. The parts of values whose depth exceeds n are printed as ... (ellipsis). #print_length n;; Limit the number of value nodes printed to at most n. Remaining parts of values are printed as ... (ellipsis). 9.3 The toplevel and the module system *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* Toplevel phrases can refer to identifiers defined in compilation units with the same mechanisms as for separately compiled units: either by using qualified names (Modulename.localname), or by using the open construct and unqualified names (see section 6.3). However, before referencing another compilation unit, an implementation of that unit must be present in memory. At start-up, the toplevel system contains implementations for all the modules in the the standard library. Implementations for user modules can be entered with the #load directive described above. Referencing a unit for which no implementation has been provided results in the error ``Reference to undefined global `...' ''. Note that entering open mod merely accesses the compiled interface (.cmi file) for mod, but does not load the implementation of mod, and does not cause any error if no implementation of mod has been loaded. The error ``reference to undefined global mod'' will occur only when executing a value or module definition that refers to mod. 9.4 Common errors *=*=*=*=*=*=*=*=*= This section describes and explains the most frequently encountered error messages. Cannot find file filename The named file could not be found in the current directory, nor in the directories of the search path. If filename has the format mod.cmi, this means you have referenced the compilation unit mod, but its compiled interface could not be found. Fix: compile mod.mli or mod.ml first, to create the compiled interface mod.cmi. If filename has the format mod.cmo, this means you are trying to load with #load a bytecode object file that does not exist yet. Fix: compile mod.ml first. If your program spans several directories, this error can also appear because you haven't specified the directories to look into. Fix: use the #directory directive to add the correct directories to the search path. This expression has type t_1, but is used with type t_2 See section 8.4. Reference to undefined global mod You have neglected to load in memory an implementation for a module with #load. See section 9.3 above. 9.5 Building custom toplevel systems: ocamlmktop *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The ocamlmktop command builds Objective Caml toplevels that contain user code preloaded at start-up. The ocamlmktop command takes as argument a set of .cmo and .cma files, and links them with the object files that implement the Objective Caml toplevel. The typical use is: << ocamlmktop -o mytoplevel foo.cmo bar.cmo gee.cmo >> This creates the bytecode file mytoplevel, containing the Objective Caml toplevel system, plus the code from the three .cmo files. This toplevel is directly executable and is started by: << ./mytoplevel >> This enters a regular toplevel loop, except that the code from foo.cmo, bar.cmo and gee.cmo is already loaded in memory, just as if you had typed: << #load "foo.cmo";; #load "bar.cmo";; #load "gee.cmo";; >> on entrance to the toplevel. The modules Foo, Bar and Gee are not opened, though; you still have to do << open Foo;; >> yourself, if this is what you wish. 9.6 Options *=*=*=*=*=*= The following command-line options are recognized by ocamlmktop. -cclib libname Pass the -llibname option to the C linker when linking in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8. -ccopt option Pass the given option to the C compiler and linker, when linking in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8. -custom Link in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8. -I directory Add the given directory to the list of directories searched for compiled object code files (.cmo and .cma). -o exec-file Specify the name of the toplevel file produced by the linker. The default is a.out. Chapter 10 The runtime system (ocamlrun) ******************************************* The ocamlrun command executes bytecode files produced by the linking phase of the ocamlc command. 10.1 Overview *=*=*=*=*=*=*= The ocamlrun command comprises three main parts: the bytecode interpreter, that actually executes bytecode files; the memory allocator and garbage collector; and a set of C functions that implement primitive operations such as input/output. The usage for ocamlrun is: << ocamlrun options bytecode-executable arg_1 ... arg_n >> The first non-option argument is taken to be the name of the file containing the executable bytecode. (That file is searched in the executable path as well as in the current directory.) The remaining arguments are passed to the Caml program, in the string array Sys.argv. Element 0 of this array is the name of the bytecode executable file; elements 1 to n are the remaining arguments arg_1 to arg_n. As mentioned in chapter 8, the bytecode executable files produced by the ocamlc command are self-executable, and manage to launch the ocamlrun command on themselves automatically. That is, assuming caml.out is a bytecode executable file, << caml.out arg_1 ... arg_n >> works exactly as << ocamlrun caml.out arg_1 ... arg_n >> Notice that it is not possible to pass options to ocamlrun when invoking caml.out directly. Windows: Under several versions of Windows, bytecode executable files are self-executable only if their name ends in .exe. It is recommended to always give .exe names to bytecode executables, e.g. compile with ocamlc -o myprog.exe ... rather than ocamlc -o myprog .... 10.2 Options *=*=*=*=*=*=* The following command-line options are recognized by ocamlrun. -b When the program aborts due to an uncaught exception, print a detailed ``back trace'' of the execution, showing where the exception was raised and which function calls were outstanding at this point. The back trace is printed only if the bytecode executable contains debugging information, i.e. was compiled and linked with the -g option to ocamlc set. This is equivalent to setting the b flag in the OCAMLRUNPARAM environment variable (see below). -I dir Search the directory dir for dynamically-loaded libraries, in addition to the standard search path (see section 10.3). -v Direct the memory manager to print some progress messages on standard error. This is equivalent to setting v=63 in the OCAMLRUNPARAM environment variable (see below). The following environment variables are also consulted: CAML_LD_LIBRARY_PATH Additional directories to search for dynamically-loaded libraries (see section 10.3). OCAMLLIB The directory containing the Objective Caml standard library. (If OCAMLLIB is not set, CAMLLIB will be used instead.) Used to locate the ld.conf configuration file for dynamic loading (see section 10.3). If not set, default to the library directory specified when compiling Objective Caml. OCAMLRUNPARAM Set the runtime system options and garbage collection parameters. (If OCAMLRUNPARAM is not set, CAMLRUNPARAM will be used instead.) This variable must be a sequence of parameter specifications. A parameter specification is an option letter followed by an = sign, a decimal number, and an optional multiplier. There are nine options, six of which correspond to the fields of the control record documented in section 20.10. b (backtrace) Trigger the printing of a stack backtrace when an uncaught exception aborts the program. This option takes no argument. p (parser trace) Turn on debugging support for ocamlyacc-generated parsers. When this option is on, the pushdown automaton that executes the parsers prints a trace of its actions. This option takes no argument. s (minor_heap_size) Size of the minor heap. i (major_heap_increment) Minimum size increment for the major heap. o (space_overhead) The major GC speed setting. O (max_overhead) The heap compaction trigger setting. v (verbose) What GC messages to print to stderr. This is a sum of values selected from the following: 1 (= 0b0000000001) Start of major GC cycle. 2 (= 0b0000000010) Minor collection and major GC slice. 4 (= 0b0000000100) Growing and shrinking of the heap. 8 (= 0b0000001000) Resizing of stacks and memory manager tables. 16 (= 0b0000010000) Heap compaction. 32 (= 0b0000100000) Change of GC parameters. 64 (= 0b0001000000) Computation of major GC slice size. 128 (= 0b0010000000) Calling of finalization functions 256 (= 0b0100000000) Startup messages (loading the bytecode executable file, resolving shared libraries). l (stack_limit) The limit (in words) of the stack size. h The initial size of the major heap (in words). The multiplier is k, M, or G, for multiplication by 2^10, 2^20, and 2^30 respectively. For example, on a 32-bit machine, under bash the command << export OCAMLRUNPARAM='b,s=256k,v=1' >> tells a subsequent ocamlrun to print backtraces for uncaught exceptions, set its initial minor heap size to 1 megabyte and print a message at the start of each major GC cycle. PATH List of directories searched to find the bytecode executable file. 10.3 Dynamic loading of shared libraries *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* On platforms that support dynamic loading, ocamlrun can link dynamically with C shared libraries (DLLs) providing additional C primitives beyond those provided by the standard runtime system. The names for these libraries are provided at link time as described in section 18.1.4), and recorded in the bytecode executable file; ocamlrun, then, locates these libraries and resolves references to their primitives when the bytecode executable program starts. The ocamlrun command searches shared libraries in the following directories, in the order indicated: 1. Directories specified on the ocamlrun command line with the -I option. 2. Directories specified in the CAML_LD_LIBRARY_PATH environment variable. 3. Directories specified at link-time via the -dllpath option to ocamlc. (These directories are recorded in the bytecode executable file.) 4. Directories specified in the file ld.conf. This file resides in the Objective Caml standard library directory, and lists directory names (one per line) to be searched. Typically, it contains only one line naming the stublibs subdirectory of the Objective Caml standard library directory. Users can add there the names of other directories containing frequently-used shared libraries; however, for consistency of installation, we recommend that shared libraries are installed directly in the system stublibs directory, rather than adding lines to the ld.conf file. 5. Default directories searched by the system dynamic loader. Under Unix, these generally include /lib and /usr/lib, plus the directories listed in the file /etc/ld.so.conf and the environment variable LD_LIBRARY_PATH. Under Windows, these include the Windows system directories, plus the directories listed in the PATH environment variable. 10.4 Common errors *=*=*=*=*=*=*=*=*=* This section describes and explains the most frequently encountered error messages. filename: no such file or directory If filename is the name of a self-executable bytecode file, this means that either that file does not exist, or that it failed to run the ocamlrun bytecode interpreter on itself. The second possibility indicates that Objective Caml has not been properly installed on your system. Cannot exec ocamlrun (When launching a self-executable bytecode file.) The ocamlrun could not be found in the executable path. Check that Objective Caml has been properly installed on your system. Cannot find the bytecode file The file that ocamlrun is trying to execute (e.g. the file given as first non-option argument to ocamlrun) either does not exist, or is not a valid executable bytecode file. Truncated bytecode file The file that ocamlrun is trying to execute is not a valid executable bytecode file. Probably it has been truncated or mangled since created. Erase and rebuild it. Uncaught exception The program being executed contains a ``stray'' exception. That is, it raises an exception at some point, and this exception is never caught. This causes immediate termination of the program. The name of the exception is printed, along with its string and integer arguments (arguments of more complex types are not correctly printed). To locate the context of the uncaught exception, compile the program with the -g option and either run it again under the ocamldebug debugger (see chapter 16), or run it with ocamlrun -b or with the OCAMLRUNPARAM environment variable set to b=1. Out of memory The program being executed requires more memory than available. Either the program builds excessively large data structures; or the program contains too many nested function calls, and the stack overflows. In some cases, your program is perfectly correct, it just requires more memory than your machine provides. In other cases, the ``out of memory'' message reveals an error in your program: non-terminating recursive function, allocation of an excessively large array or string, attempts to build an infinite list or other data structure, ... To help you diagnose this error, run your program with the -v option to ocamlrun, or with the OCAMLRUNPARAM environment variable set to v=63. If it displays lots of ``Growing stack...'' messages, this is probably a looping recursive function. If it displays lots of ``Growing heap...'' messages, with the heap size growing slowly, this is probably an attempt to construct a data structure with too many (infinitely many?) cells. If it displays few ``Growing heap...'' messages, but with a huge increment in the heap size, this is probably an attempt to build an excessively large array or string. Chapter 11 Native-code compilation (ocamlopt) ************************************************ This chapter describes the Objective Caml high-performance native-code compiler ocamlopt, which compiles Caml source files to native code object files and link these object files to produce standalone executables. The native-code compiler is only available on certain platforms. It produces code that runs faster than the bytecode produced by ocamlc, at the cost of increased compilation time and executable code size. Compatibility with the bytecode compiler is extremely high: the same source code should run identically when compiled with ocamlc and ocamlopt. It is not possible to mix native-code object files produced by ocamlc with bytecode object files produced by ocamlopt: a program must be compiled entirely with ocamlopt or entirely with ocamlc. Native-code object files produced by ocamlopt cannot be loaded in the toplevel system ocaml. 11.1 Overview of the compiler *=*=*=*=*=*=*=*=*=*=*=*=*=*=*= The ocamlopt command has a command-line interface very close to that of ocamlc. It accepts the same types of arguments: - Arguments ending in .mli are taken to be source files for compilation unit interfaces. Interfaces specify the names exported by compilation units: they declare value names with their types, define public data types, declare abstract data types, and so on. From the file x.mli, the ocamlopt compiler produces a compiled interface in the file x.cmi. The interface produced is identical to that produced by the bytecode compiler ocamlc. - Arguments ending in .ml are taken to be source files for compilation unit implementations. Implementations provide definitions for the names exported by the unit, and also contain expressions to be evaluated for their side-effects. From the file x.ml, the ocamlopt compiler produces two files: x.o, containing native object code, and x.cmx, containing extra information for linking and optimization of the clients of the unit. The compiled implementation should always be referred to under the name x.cmx (when given a .o file, ocamlopt assumes that it contains code compiled from C, not from Caml). The implementation is checked against the interface file x.mli (if it exists) as described in the manual for ocamlc (chapter 8). - Arguments ending in .cmx are taken to be compiled object code. These files are linked together, along with the object files obtained by compiling .ml arguments (if any), and the Caml standard library, to produce a native-code executable program. The order in which .cmx and .ml arguments are presented on the command line is relevant: compilation units are initialized in that order at run-time, and it is a link-time error to use a component of a unit before having initialized it. Hence, a given x.cmx file must come before all .cmx files that refer to the unit x. - Arguments ending in .cmxa are taken to be libraries of object code. Such a library packs in two files (lib.cmxa and lib.a) a set of object files (.cmx/.o files). Libraries are build with ocamlopt -a (see the description of the -a option below). The object files contained in the library are linked as regular .cmx files (see above), in the order specified when the library was built. The only difference is that if an object file contained in a library is not referenced anywhere in the program, then it is not linked in. - Arguments ending in .c are passed to the C compiler, which generates a .o object file. This object file is linked with the program. - Arguments ending in .o, .a or .so (.obj, .lib and .dll under Windows) are assumed to be C object files and libraries. They are linked with the program. The output of the linking phase is a regular Unix executable file. It does not need ocamlrun to run. 11.2 Options *=*=*=*=*=*=* The following command-line options are recognized by ocamlopt. -a Build a library (.cmxa/.a file) with the object files (.cmx/.o files) given on the command line, instead of linking them into an executable file. The name of the library can be set with the -o option. The default name is library.cmxa. If -cclib or -ccopt options are passed on the command line, these options are stored in the resulting .cmxa library. Then, linking with this library automatically adds back the -cclib and -ccopt options as if they had been provided on the command line, unless the -noautolink option is given. -c Compile only. Suppress the linking phase of the compilation. Source code files are turned into compiled files, but no executable file is produced. This option is useful to compile modules separately. -cc ccomp Use ccomp as the C linker called to build the final executable and as the C compiler for compiling .c source files. -cclib -llibname Pass the -llibname option to the linker. This causes the given C library to be linked with the program. -ccopt option Pass the given option to the C compiler and linker. For instance, -ccopt -Ldir causes the C linker to search for C libraries in directory dir. -compact Optimize the produced code for space rather than for time. This results in slightly smaller but slightly slower programs. The default is to optimize for speed. -i Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (.ml file). This can be useful to check the types inferred by the compiler. Also, since the output follows the syntax of interfaces, it can help in writing an explicit interface (.mli file) for a file: just redirect the standard output of the compiler to a .mli file, and edit that file to remove all declarations of unexported names. -I directory Add the given directory to the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmx), and libraries (.cmxa). By default, the current directory is searched first, then the standard library directory. Directories added with -I are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory. If the given directory starts with +, it is taken relative to the standard library directory. For instance, -I +labltk adds the subdirectory labltk of the standard library to the search path. -inline n Set aggressiveness of inlining to n, where n is a positive integer. Specifying -inline 0 prevents all functions from being inlined, except those whose body is smaller than the call site. Thus, inlining causes no expansion in code size. The default aggressiveness, -inline 1, allows slightly larger functions to be inlined, resulting in a slight expansion in code size. Higher values for the -inline option cause larger and larger functions to become candidate for inlining, but can result in a serious increase in code size. -linkall Forces all modules contained in libraries to be linked in. If this flag is not given, unreferenced modules are not linked in. When building a library (-a flag), setting the -linkall flag forces all subsequent links of programs involving that library to link all the modules contained in the library. -noassert Turn assertion checking off: assertions are not compiled. This flag has no effect when linking already compiled files. -noautolink When linking .cmxa libraries, ignore -cclib and -ccopt options potentially contained in the libraries (if these options were given when building the libraries). This can be useful if a library contains incorrect specifications of C libraries or C options; in this case, during linking, set -noautolink and pass the correct C libraries and options on the command line. -nolabels Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict. -o exec-file Specify the name of the output file produced by the linker. The default output name is a.out, in keeping with the Unix tradition. If the -a option is given, specify the name of the library produced. If the -output-obj option is given, specify the name of the output file produced. -output-obj Cause the linker to produce a C object file instead of an executable file. This is useful to wrap Caml code as a C library, callable from any C program. See chapter 18, section 18.7.5. The name of the output object file is camlprog.o by default; it can be set with the -o option. -p Generate extra code to write profile information when the program is executed. The profile information can then be examined with the analysis program gprof. (See chapter 17 for more information on profiling.) The -p option must be given both at compile-time and at link-time. Linking object files not compiled with -p is possible, but results in less precise profiling. Unix: See the Unix manual page for gprof(1) for more information about the profiles. Full support for gprof is only available for certain platforms (currently: Intel x86/Linux and Alpha/Digital Unix). On other platforms, the -p option will result in a less precise profile (no call graph information, only a time profile). Windows: The -p option does not work under Windows. -pack Build an object file (.cmx/.o file) and its associated compiled interface (.cmi) that combines the .cmx object files given on the command line, making them appear as sub-modules of the output .cmx file. The name of the output .cmx file must be given with the -o option. For instance, << ocamlopt -pack -o p.cmx a.cmx b.cmx c.cmx >> generates compiled files p.cmx, p.o and p.cmi describing a compilation unit having three sub-modules A, B and C, corresponding to the contents of the object files a.cmx, b.cmx and c.cmx. These contents can be referenced as P.A, P.B and P.C in the remainder of the program. Unix: The -pack option is available only under platforms that provide the GNU binutils tools nm and objcopy. -pp command Cause the compiler to call the given command as a preprocessor for each source file. The output of command is redirected to an intermediate file, which is compiled. If there are no compilation errors, the intermediate file is deleted afterwards. The name of this file is built from the basename of the source file with the extension .ppi for an interface (.mli) file and .ppo for an implementation (.ml) file. -principal Check information path during type-checking, to make sure that all types are derived in a principal way. All programs accepted in -principal mode are also accepted in default mode with equivalent types, but different binary signatures. -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. -S Keep the assembly code produced during the compilation. The assembly code for the source file x.ml is saved in the file x.s. -thread Compile or link multithreaded programs, in combination with the threads library described in chapter 24. What this option actually does is select a special, thread-safe version of the standard library. -unsafe Turn bound checking off on array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds. -v Print the version number of the compiler and the location of the standard library directory, then exit. -verbose Print all external commands before they are executed, in particular invocations of the assembler, C compiler, and linker. -version Print the version number of the compiler in short form (e.g. 3.06), then exit. -w warning-list Enable or disable warnings according to the argument warning-list. The argument is a string of one or several characters, with the following meaning for each character: A/a enable/disable all warnings. C/c enable/disable warnings for suspicious comments. D/d enable/disable warnings for deprecated features. F/f enable/disable warnings for partially applied functions (i.e. f x; expr where the application f x has a function type). L/l enable/disable warnings for labels omitted in application. M/m enable/disable warnings for overriden methods. P/p enable/disable warnings for partial matches (missing cases in pattern matchings). S/s enable/disable warnings for statements that do not have type unit (e.g. expr1; expr2 when expr1 does not have type unit). U/u enable/disable warnings for unused (redundant) match cases. V/v enable/disable warnings for hidden instance variables. X/x enable/disable all other warnings. The default setting is -w Al (all warnings but labels enabled). -warn-error warning-list Turn the warnings indicated in the argument warning-list into errors. The compiler will stop on an error as soon as one of these warnings is emitted, instead of going on. The warning-list is a string of one or several characters, with the same meaning as for the -w option: an uppercase character turns the corresponding warning into an error, a lowercase character leaves it as a warning. The default setting is -warn-error a (all warnings are not treated as errors). -where Print the location of the standard library. 11.3 Common errors *=*=*=*=*=*=*=*=*=* The error messages are almost identical to those of ocamlc. See section 8.4. 11.4 Compatibility with the bytecode compiler *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= This section lists the known incompatibilities between the bytecode compiler and the native-code compiler. Except on those points, the two compilers should generate code that behave identically. - The following operations abort the program (via an hardware trap or fatal Unix signal) instead of raising an exception: - integer division by zero, modulus by zero; - stack overflow; - on the Alpha processor only, floating-point operations involving infinite or denormalized numbers (all other processors supported by ocamlopt treat these numbers correctly, as per the IEEE 754 standard). In particular, notice that stack overflow caused by excessively deep recursion is reported by most Unix kernels as a ``segmentation violation'' signal. - Signals are detected only when the program performs an allocation in the heap. That is, if a signal is delivered while in a piece of code that does not allocate, its handler will not be called until the next heap allocation. The best way to avoid running into those incompatibilities is to never trap the Division_by_zero and Stack_overflow exceptions, thus also treating them as fatal errors with the bytecode compiler as well as with the native-code compiler. Often, it is feasible to test the divisor before performing the operation, instead of trapping the exception afterwards. Chapter 12 Lexer and parser generators (ocamllex, ocamlyacc) *************************************************************** This chapter describes two program generators: ocamllex, that produces a lexical analyzer from a set of regular expressions with associated semantic actions, and ocamlyacc, that produces a parser from a grammar with associated semantic actions. These program generators are very close to the well-known lex and yacc commands that can be found in most C programming environments. This chapter assumes a working knowledge of lex and yacc: while it describes the input syntax for ocamllex and ocamlyacc and the main differences with lex and yacc, it does not explain the basics of writing a lexer or parser description in lex and yacc. Readers unfamiliar with lex and yacc are referred to ``Compilers: principles, techniques, and tools'' by Aho, Sethi and Ullman (Addison-Wesley, 1986), or ``Lex & Yacc'', by Levine, Mason and Brown (O'Reilly, 1992). 12.1 Overview of ocamllex *=*=*=*=*=*=*=*=*=*=*=*=*= The ocamllex command produces a lexical analyzer from a set of regular expressions with attached semantic actions, in the style of lex. Assuming the input file is lexer.mll, executing << ocamllex lexer.mll >> produces Caml code for a lexical analyzer in file lexer.ml. This file defines one lexing function per entry point in the lexer definition. These functions have the same names as the entry points. Lexing functions take as argument a lexer buffer, and return the semantic attribute of the corresponding entry point. Lexer buffers are an abstract data type implemented in the standard library module Lexing. The functions Lexing.from_channel, Lexing.from_string and Lexing.from_function create lexer buffers that read from an input channel, a character string, or any reading function, respectively. (See the description of module Lexing in chapter 20.) When used in conjunction with a parser generated by ocamlyacc, the semantic actions compute a value belonging to the type token defined by the generated parsing module. (See the description of ocamlyacc below.) 12.2 Syntax of lexer definitions *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The format of lexer definitions is as follows: << { header } let ident = regexp ... rule entrypoint = parse regexp { action } | ... | regexp { action } and entrypoint = parse ... and ... { trailer } >> Comments are delimited by (* and *), as in Caml. 12.2.1 Header and trailer ========================== The header and trailer sections are arbitrary Caml text enclosed in curly braces. Either or both can be omitted. If present, the header text is copied as is at the beginning of the output file and the trailer text at the end. Typically, the header section contains the `open' directives required by the actions, and possibly some auxiliary functions used in the actions. 12.2.2 Naming regular expressions ================================== Between the header and the entry points, one can give names to frequently-occurring regular expressions. This is written let ident = regexp. In following regular expressions, the identifier ident can be used as shorthand for regexp. 12.2.3 Entry points ==================== The names of the entry points must be valid identifiers for Caml values (starting with a lowercase letter). Each entry point becomes a Caml function that takes one argument of type Lexing.lexbuf. Characters are read from the Lexing.lexbuf argument and matched against the regular expressions provided in the rule, until a prefix of the input matches one of the rule. The corresponding action is then evaluated and returned as the result of the function. If several regular expressions match a prefix of the input, the ``longest match'' rule applies: the regular expression that matches the longest prefix of the input is selected. In case of tie, the regular expression that occurs earlier in the rule is selected. 12.2.4 Regular expressions =========================== The regular expressions are in the style of lex, with a more Caml-like syntax. ' char ' A character constant, with the same syntax as Objective Caml character constants. Match the denoted character. _ (Underscore.) Match any character. eof Match the end of the lexer input. Note: On some systems, with interactive input, an end-of-file may be followed by more characters. However, ocamllex will not correctly handle regular expressions that contain eof followed by something else. " string " A string constant, with the same syntax as Objective Caml string constants. Match the corresponding sequence of characters. [ character-set ] Match any single character belonging to the given character set. Valid character sets are: single character constants ' c '; ranges of characters ' c_1 ' - ' c_2 ' (all characters between c_1 and c_2, inclusive); and the union of two or more character sets, denoted by concatenation. [ ^ character-set ] Match any single character not belonging to the given character set. regexp * (Repetition.) Match the concatenation of zero or more strings that match regexp. regexp + (Strict repetition.) Match the concatenation of one or more strings that match regexp. regexp ? (Option.) Match either the empty string, or a string matching regexp. regexp_1 | regexp_2 (Alternative.) Match any string that matches either regexp_1 or regexp_2 regexp_1 regexp_2 (Concatenation.) Match the concatenation of two strings, the first matching regexp_1, the second matching regexp_2. ( regexp ) Match the same strings as regexp. ident Reference the regular expression bound to ident by an earlier let ident = regexp definition. Concerning the precedences of operators, * and + have highest precedence, followed by ?, then concatenation, then | (alternation). 12.2.5 Actions =============== The actions are arbitrary Caml expressions. They are evaluated in a context where the identifier lexbuf is bound to the current lexer buffer. Some typical uses for lexbuf, in conjunction with the operations on lexer buffers provided by the Lexing standard library module, are listed below. Lexing.lexeme lexbuf Return the matched string. Lexing.lexeme_char lexbuf n Return the n^th character in the matched string. The first character corresponds to n = 0. Lexing.lexeme_start lexbuf Return the absolute position in the input text of the beginning of the matched string. The first character read from the input text has position 0. Lexing.lexeme_end lexbuf Return the absolute position in the input text of the end of the matched string. The first character read from the input text has position 0. entrypoint lexbuf (Where entrypoint is the name of another entry point in the same lexer definition.) Recursively call the lexer on the given entry point. Useful for lexing nested comments, for example. 12.2.6 Reserved identifiers ============================ All identifiers starting with __ocaml_lex are reserved for use by ocamllex; do not use any such identifier in your programs. 12.3 Overview of ocamlyacc *=*=*=*=*=*=*=*=*=*=*=*=*=* The ocamlyacc command produces a parser from a context-free grammar specification with attached semantic actions, in the style of yacc. Assuming the input file is grammar.mly, executing << ocamlyacc options grammar.mly >> produces Caml code for a parser in the file grammar.ml, and its interface in file grammar.mli. The generated module defines one parsing function per entry point in the grammar. These functions have the same names as the entry points. Parsing functions take as arguments a lexical analyzer (a function from lexer buffers to tokens) and a lexer buffer, and return the semantic attribute of the corresponding entry point. Lexical analyzer functions are usually generated from a lexer specification by the ocamllex program. Lexer buffers are an abstract data type implemented in the standard library module Lexing. Tokens are values from the concrete type token, defined in the interface file grammar.mli produced by ocamlyacc. 12.4 Syntax of grammar definitions *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* Grammar definitions have the following format: << %{ header %} declarations %% rules %% trailer >> Comments are enclosed between `/*' and `*/' (as in C) in the ``declarations'' and ``rules'' sections, and between `(*' and `*)' (as in Caml) in the ``header'' and ``trailer'' sections. 12.4.1 Header and trailer ========================== The header and the trailer sections are Caml code that is copied as is into file grammar.ml. Both sections are optional. The header goes at the beginning of the output file; it usually contains open directives and auxiliary functions required by the semantic actions of the rules. The trailer goes at the end of the output file. 12.4.2 Declarations ==================== Declarations are given one per line. They all start with a `%' sign. %token symbol ... symbol Declare the given symbols as tokens (terminal symbols). These symbols are added as constant constructors for the token concrete type. %token < type > symbol ... symbol Declare the given symbols as tokens with an attached attribute of the given type. These symbols are added as constructors with arguments of the given type for the token concrete type. The type part is an arbitrary Caml type expression, except that all type constructor names must be fully qualified (e.g. Modname.typename) for all types except standard built-in types, even if the proper `open' directives (e.g. `open Modname') were given in the header section. That's because the header is copied only to the .ml output file, but not to the .mli output file, while the type part of a `%token' declaration is copied to both. %start symbol ... symbol Declare the given symbols as entry points for the grammar. For each entry point, a parsing function with the same name is defined in the output module. Non-terminals that are not declared as entry points have no such parsing function. Start symbols must be given a type with the `%type' directive below. %type < type > symbol ... symbol Specify the type of the semantic attributes for the given symbols. This is mandatory for start symbols only. Other nonterminal symbols need not be given types by hand: these types will be inferred when running the output files through the Objective Caml compiler (unless the `-s' option is in effect). The type part is an arbitrary Caml type expression, except that all type constructor names must be fully qualified, as explained above for %token. %left symbol ... symbol %right symbol ... symbol %nonassoc symbol ... symbol Associate precedences and associativities to the given symbols. All symbols on the same line are given the same precedence. They have higher precedence than symbols declared before in a `%left', `%right' or `%nonassoc' line. They have lower precedence than symbols declared after in a `%left', `%right' or `%nonassoc' line. The symbols are declared to associate to the left (`%left'), to the right (`%right'), or to be non-associative (`%nonassoc'). The symbols are usually tokens. They can also be dummy nonterminals, for use with the `%prec' directive inside the rules. 12.4.3 Rules ============= The syntax for rules is as usual: << nonterminal : symbol ... symbol { semantic-action } | ... | symbol ... symbol { semantic-action } ; >> Rules can also contain the `%prec 'symbol directive in the right-hand side part, to override the default precedence and associativity of the rule with the precedence and associativity of the given symbol. Semantic actions are arbitrary Caml expressions, that are evaluated to produce the semantic attribute attached to the defined nonterminal. The semantic actions can access the semantic attributes of the symbols in the right-hand side of the rule with the `$' notation: `$1' is the attribute for the first (leftmost) symbol, `$2' is the attribute for the second symbol, etc. The rules may contain the special symbol error to indicate resynchronization points, as in yacc. Actions occurring in the middle of rules are not supported. Nonterminal symbols are like regular Caml symbols, except that they cannot end with ' (single quote). 12.4.4 Error handling ====================== Error recovery is supported as follows: when the parser reaches an error state (no grammar rules can apply), it calls a function named parse_error with the string "syntax error" as argument. The default parse_error function does nothing and returns, thus initiating error recovery (see below). The user can define a customized parse_error function in the header section of the grammar file. The parser also enters error recovery mode if one of the grammar actions raises the Parsing.Parse_error exception. In error recovery mode, the parser discards states from the stack until it reaches a place where the error token can be shifted. It then discards tokens from the input until it finds three successive tokens that can be accepted, and starts processing with the first of these. If no state can be uncovered where the error token can be shifted, then the parser aborts by raising the Parsing.Parse_error exception. Refer to documentation on yacc for more details and guidance in how to use error recovery. 12.5 Options *=*=*=*=*=*=* The ocamlyacc command recognizes the following options: -v Generate a description of the parsing tables and a report on conflicts resulting from ambiguities in the grammar. The description is put in file grammar.output. -bprefix Name the output files prefix.ml, prefix.mli, prefix.output, instead of the default naming convention. At run-time, the ocamlyacc-generated parser can be debugged by setting the p option in the OCAMLRUNPARAM environment variable (see section 10.2). This causes the pushdown automaton executing the parser to print a trace of its action (tokens shifted, rules reduced, etc). The trace mentions rule numbers and state numbers that can be interpreted by looking at the file grammar.output generated by ocamlyacc -v. 12.6 A complete example *=*=*=*=*=*=*=*=*=*=*=*= The all-time favorite: a desk calculator. This program reads arithmetic expressions on standard input, one per line, and prints their values. Here is the grammar definition: << /* File parser.mly */ %token INT %token PLUS MINUS TIMES DIV %token LPAREN RPAREN %token EOL %left PLUS MINUS /* lowest precedence */ %left TIMES DIV /* medium precedence */ %nonassoc UMINUS /* highest precedence */ %start main /* the entry point */ %type main %% main: expr EOL { $1 } ; expr: INT { $1 } | LPAREN expr RPAREN { $2 } | expr PLUS expr { $1 + $3 } | expr MINUS expr { $1 - $3 } | expr TIMES expr { $1 * $3 } | expr DIV expr { $1 / $3 } | MINUS expr %prec UMINUS { - $2 } ; >> Here is the definition for the corresponding lexer: << (* File lexer.mll *) { open Parser (* The type token is defined in parser.mli *) exception Eof } rule token = parse [' ' '\t'] { token lexbuf } (* skip blanks *) | ['\n' ] { EOL } | ['0'-'9']+ { INT(int_of_string(Lexing.lexeme lexbuf)) } | '+' { PLUS } | '-' { MINUS } | '*' { TIMES } | '/' { DIV } | '(' { LPAREN } | ')' { RPAREN } | eof { raise Eof } >> Here is the main program, that combines the parser with the lexer: << (* File calc.ml *) let _ = try let lexbuf = Lexing.from_channel stdin in while true do let result = Parser.main Lexer.token lexbuf in print_int result; print_newline(); flush stdout done with Lexer.Eof -> exit 0 >> To compile everything, execute: << ocamllex lexer.mll # generates lexer.ml ocamlyacc parser.mly # generates parser.ml and parser.mli ocamlc -c parser.mli ocamlc -c lexer.ml ocamlc -c parser.ml ocamlc -c calc.ml ocamlc -o calc lexer.cmo parser.cmo calc.cmo >> 12.7 Common errors *=*=*=*=*=*=*=*=*=* ocamllex: transition table overflow, automaton is too big The deterministic automata generated by ocamllex are limited to at most 32767 transitions. The message above indicates that your lexer definition is too complex and overflows this limit. This is commonly caused by lexer definitions that have separate rules for each of the alphabetic keywords of the language, as in the following example. << rule token = parse "keyword1" { KWD1 } | "keyword2" { KWD2 } | ... | "keyword100" { KWD100 } | ['A'-'Z' 'a'-'z'] ['A'-'Z' 'a'-'z' '0'-'9' '_'] * { IDENT(Lexing.lexeme lexbuf) } >> To keep the generated automata small, rewrite those definitions with only one general ``identifier'' rule, followed by a hashtable lookup to separate keywords from identifiers: << { let keyword_table = Hashtbl.create 53 let _ = List.iter (fun (kwd, tok) -> Hashtbl.add keyword_table kwd tok) [ "keyword1", KWD1; "keyword2", KWD2; ... "keyword100", KWD100 ] } rule token = parse ['A'-'Z' 'a'-'z'] ['A'-'Z' 'a'-'z' '0'-'9' '_'] * { let id = Lexing.lexeme lexbuf in try Hashtbl.find keyword_table s with Not_found -> IDENT s } >> Chapter 13 Dependency generator (ocamldep) ********************************************* The ocamldep command scans a set of Objective Caml source files (.ml and .mli files) for references to external compilation units, and outputs dependency lines in a format suitable for the make utility. This ensures that make will compile the source files in the correct order, and recompile those files that need to when a source file is modified. The typical usage is: << ocamldep options *.mli *.ml > .depend >> where *.mli *.ml expands to all source files in the current directory and .depend is the file that should contain the dependencies. (See below for a typical Makefile.) Dependencies are generated both for compiling with the bytecode compiler ocamlc and with the native-code compiler ocamlopt. 13.1 Options *=*=*=*=*=*=* The following command-line option is recognized by ocamldep. -I directory Add the given directory to the list of directories searched for source files. If a source file foo.ml mentions an external compilation unit Bar, a dependency on that unit's interface bar.cmi is generated only if the source for bar is found in the current directory or in one of the directories specified with -I. Otherwise, Bar is assumed to be a module from the standard library, and no dependencies are generated. For programs that span multiple directories, it is recommended to pass ocamldep the same -I options that are passed to the compiler. -native Generate dependencies for a pure native-code program (no bytecode version). When an implementation file (.ml file) has no explicit interface file (.mli file), ocamldep generates dependencies on the bytecode compiled file (.cmo file) to reflect interface changes. This can cause unnecessary bytecode recompilations for programs that are compiled to native-code only. The flag -native causes dependencies on native compiled files (.cmx) to be generated instead of on .cmo files. (This flag makes no difference if all source files have explicit .mli interface files.) 13.2 A typical Makefile *=*=*=*=*=*=*=*=*=*=*=*= Here is a template Makefile for a Objective Caml program. << OCAMLC=ocamlc OCAMLOPT=ocamlopt OCAMLDEP=ocamldep INCLUDES= # all relevant -I options here OCAMLFLAGS=$(INCLUDES) # add other options for ocamlc here OCAMLOPTFLAGS=$(INCLUDES) # add other options for ocamlopt here # prog1 should be compiled to bytecode, and is composed of three # units: mod1, mod2 and mod3. # The list of object files for prog1 PROG1_OBJS=mod1.cmo mod2.cmo mod3.cmo prog1: $(PROG1_OBJS) $(OCAMLC) -o prog1 $(OCAMLFLAGS) $(PROG1_OBJS) # prog2 should be compiled to native-code, and is composed of two # units: mod4 and mod5. # The list of object files for prog2 PROG2_OBJS=mod4.cmx mod5.cmx prog2: $(PROG2_OBJS) $(OCAMLOPT) -o prog2 $(OCAMLFLAGS) $(PROG2_OBJS) # Common rules .SUFFIXES: .ml .mli .cmo .cmi .cmx .ml.cmo: $(OCAMLC) $(OCAMLFLAGS) -c $< .mli.cmi: $(OCAMLC) $(OCAMLFLAGS) -c $< .ml.cmx: $(OCAMLOPT) $(OCAMLOPTFLAGS) -c $< # Clean up clean: rm -f prog1 prog2 rm -f *.cm[iox] # Dependencies depend: $(OCAMLDEP) $(INCLUDES) *.mli *.ml > .depend include .depend >> Chapter 14 The browser/editor (ocamlbrowser) *********************************************** This chapter describes OCamlBrowser, a source and compiled interface browser, written using LablTk. This is a useful companion to the programmer. Its functions are: - navigation through Objective Caml's modules (using compiled interfaces). - source editing, type-checking, and browsing. - integrated Objective Caml shell, running as a subprocess. 14.1 Invocation *=*=*=*=*=*=*=*= The browser is started by the command ocamlbrowser, as follows: << ocamlbrowser options >> The following command-line options are recognized by ocamlbrowser. -I directory Add the given directory to the list of directories searched for source and compiled files. By default, only the standard library directory is searched. The standard library can also be changed by setting the OCAMLLIB environment variable. -nolabels Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict. -oldui Old multi-window interface. The default is now more like Smalltalk's class browser. -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. -w warning-list Enable or disable warnings according to the argument warning-list. Most options can also be modified inside the application by the Modules - Path editor and Compiler - Preferences commands. They are inherited when you start a toplevel shell. 14.2 Viewer *=*=*=*=*=*= This is the first window you get when you start OCamlBrowser. It displays a search window, and the list of modules in the load path. At the top a row of menus. - File - Open and File - Editor give access to the editor. - File - Shell creates an Objective Caml subprocess in a shell. - View - Show all defs displays the signature of the currently selected module. - View - Search entry shows/hides the search entry just below the menu bar. - Modules - Path editor changes the load path. Modules - Reset cache rescans the load path and resets the module cache. Do it if you recompile some interface, or get confused about what is in the cache. - Modules - Search symbol allows to search a symbol either by its name, like the bottom line of the viewer, or, more interestingly, by its type. Exact type searches for a type with exactly the same information as the pattern (variables match only variables). Included type allows to give only partial information: the actual type may take more arguments and return more results, and variables in the pattern match anything. In both cases, argument and tuple order is irrelevant (1), and unlabeled arguments in the pattern match any label. - The Search entry just below the menu bar allows one to search for an identifier in all modules (wildcards ``?'' and ``*'' allowed). If you choose the type option, the search is done by type inclusion (cf. Search Symbol - Included type). - The Close all button is there to dismiss the windows created by the Detach button. By double-clicking on it you will quit the browser. 14.3 Module browsing *=*=*=*=*=*=*=*=*=*=* You select a module in the leftmost box by either cliking on it or pressing return when it is selected. Fast access is available in all boxes pressing the first few letter of the desired name. Double-clicking / double-return displays the whole signature for the module. Defined identifiers inside the module are displayed in a box to the right of the previous one. If you click on one, this will either display its contents in another box (if this is a sub-module) or display the signature for this identifier below. Signatures are clickable. Double clicking with the left mouse button on an identifier in a signature brings you to its signature. A single click on the right button pops up a menu displaying the type declaration for the selected identifier. Its title, when selectable, also brings you to its signature. At the bottom, a series of buttons, depending on the context. - Detach copies the currently displayed signature in a new window, to keep it. - Impl and Intf bring you to the implementation or interface of the currently displayed signature, if it is available. Control-S lets you search a string in the signature. 14.4 File editor *=*=*=*=*=*=*=*=* You can edit files with it, if you're not yet used to emacs. Otherwise you can use it as a browser, making occasional corrections. The Edit menu contains commands for jump (C-g), search (C-s), and sending the current phrase (or selection if some text is selected) to a sub-shell (M-x). For this last option, you may choose the shell via a dialog. Essential functions are in the Compiler menu. - Preferences opens a dialog to set internals of the editor and type-checker. - Lex adds colors according to lexical categories. - Typecheck verifies typing, and memorizes to let one see an expression's type by double-clicking on it. This is also valid for interfaces. If an error occurs, the part of the interface preceding the error is computed. After typechecking, pressing the right button pops up a menu giving the type of the pointed expression, and eventually allowing to follow some links. - Clear errors dismisses type-checker error messages and warnings. - Signature shows the signature of the current file (after type checking). 14.5 Shell *=*=*=*=*=* When you create a shell, a dialog is presented to you, letting you choose which command you want to run, and the title of the shell (to choose it in the Editor). The executed subshell is given the current load path. - File use a source file or load a bytecode file. You may also import the browser's path into the subprocess. - History M-p and M-n browse up and down. - Signal C-c interrupts, and you can also kill the subprocess. --------------------------------------- (1) To avoid combinatorial explosion of the search space, optional arguments in the actual type are ignored in the actual if (1) there are too many of them, and (2) they do not appear explicitly in the pattern. Chapter 15 The documentation generator (ocamldoc) **************************************************** This chapter describes OCamldoc, a tool that generates documentation from special comments embedded in source files. The comments used by OCamldoc are of the form (**...*) and follow the format described in section 15.2. OCamldoc can produce documentation in various formats: HTML, LaTeX, TeXinfo, Unix man pages, and dot dependency graphs. Moreover, users can add their own custom generators, as explained in section 15.3. In this chapter, we use the word element to refer to any of the following parts of an OCaml source file: a type declaration, a value, a module, an exception, a module type, a type constructor, a record field, a class, a class type, a class method, a class value or a class inheritance clause. 15.1 Usage *=*=*=*=*=* 15.1.1 Invocation ================== OCamldoc is invoked via the command ocamldoc, as follows: << ocamldoc options sourcefiles >> Options for choosing the output format -------------------------------------- The following options determine the format for the generated documentation. -html Generate documentation in HTML default format. The generated HTML pages are stored in the current directory, or in the directory specified with the -d option. You can customize the style of the generated pages by editing the generated style.css file, or by providing your own style sheet using option -css-style. -latex Generate documentation in LaTeX default format. The generated LaTeX document is saved in file ocamldoc.out, or in the file specified with the -o option. The document uses the style file ocamldoc.sty included in the OCamldoc distribution. You can change this file to customize the style of your LaTeX documentation. -texi Generate documentation in TeXinfo default format. The generated LaTeX document is saved in file ocamldoc.out, or in the file specified with the -o option. -man Generate documentation as a set of Unix man pages. The generated pages are stored in the current directory, or in the directory specified with the -d option. -dot Generate a dependency graph for the toplevel modules, in a format suitable for displaying and processing by dot. The dot tool is available from http://www.research.att.com/sw/tools/graphviz/. The textual representation of the graph is written to the file ocamldoc.out, or to the file specified with the -o option. Use dot ocamldoc.out to display it. -g file.cm[o,a] Dynamically load the given file, which defines a custom documentation generator. See section 15.4.1. This option is supported by the ocamldoc command, but not by its native-code version ocamldoc.opt. General options --------------- -d dir Generate files in directory dir, rather than in the current directory. -dump file Dump collected information into file. This information can be read with the -load option in a subsequent invocation of ocamldoc. -hide modules Hide the given complete module names in the generated documentation modules is a list of complete module names are separated by ',', without blanks. For instance: Pervasives,M2.M3. -inv-merge-ml-mli Inverse implementations and interfaces when merging. All elements in implementation files are kept, and the -m option indicates which parts of the comments in interface files are merged with the comments in implementation files. -keep-code Always keep the source code for values, methods and instance variables, when available. The source code is always kept when a .ml file is given, but is by default discarded when a .mli is given. This option allows to always keep the source code. -load file Load information from file, which has been produced by ocamldoc -dump. Several -load options can be given. -m flags Specify merge options between interfaces and implementations. (see section 15.1.2 for details). flags can be one or several of the following characters: d merge description a merge @author v merge @version l merge @see s merge @since o merge @deprecated p merge @param e merge @raise r merge @return A merge everything -no-custom-tags Do not allow custom @-tags (see section 15.2.5). -no-stop Keep elements placed after the (**/**) special comment (see section 15.2). -o file Output the generated documentation to file instead of ocamldoc.out. This option is meaningful only in conjunction with the -latex, -texi, or -dot options. -pp command Pipe sources through preprocessor command. -sort Sort the list of top-level modules before generating the documentation. -stars Remove blank characters until the first asterisk ('*') in each line of comments. -t title Use title as the title for the generated documentation. -v Verbose mode. Display progress information. -warn-error Treat warnings as errors. Type-checking options --------------------- OCamldoc calls the Objective Caml type-checker to obtain type informations. The following options impact the type-checking phase. They have the same meaning as for the ocamlc and ocamlopt commands. -I directory Add directory to the list of directories search for compiled interface files (.cmi files). -nolabels Ignore non-optional labels in types. -rectypes Allow arbitrary recursive types. (See the -rectypes option to ocamlc.) Options for generating HTML pages --------------------------------- The following options apply in conjunction with the -html option: -all-params Display the complete list of parameters for functions and methods. -css-style filename Use filename as the Cascading Style Sheet file. -colorize-code Colorize the OCaml code enclosed in [ ] and \{[ ]\}, using colors to emphasize keywords, etc. If the code fragments are not syntactically correct, no color is added. -index-only Generate only index files. Options for generating LaTeX files ---------------------------------- The following options apply in conjunction with the -latex option: -latex-value-prefix prefix Give a prefix to use for the labels of the values in the generated LaTeX document. The default prefix is the empty string. You can also use the options -latex-type-prefix, -latex-exception-prefix, -latex-module-prefix, -latex-module-type-prefix, -latex-class-prefix, -latex-class-type-prefix, -latex-attribute-prefix and -latex-method-prefix. These options are useful when you have, for example, a type and a value with the same name. If you do not specify prefixes, LaTeX will complain about multiply defined labels. -latextitle n,style Associate style number n to the given LaTeX sectioning command style, e.g. section or subsection. (LaTeX only.) This is useful when including the generated document in another LaTeX document, at a given sectioning level. The default association is 1 for section, 2 for subsection, 3 for subsubsection, 4 for paragraph and 5 for subparagraph. -noheader Suppress header in generated documentation. -notoc Do not generate a table of contents (LaTeX only). -notrailer Suppress trailer in generated documentation. -sepfiles Generate one .tex file per toplevel module, instead of the global ocamldoc.out file. Options for generating TeXinfo files ------------------------------------ The following options apply in conjunction with the -texi option: -esc8 Escape accented characters in Info files. -noindex Do not build index for Info files. Options for generating dot graphs --------------------------------- The following options apply in conjunction with the -dot option: -dot-colors colors Specify the colors to use in the generated dot code. When generating module dependencies, ocamldoc uses different colors for modules, depending on the directories in which they reside. When generating types dependencies, ocamldoc uses different colors for types, depending on the modules in which they are defined. colors is a list of color names separated by ',', as in Red,Blue,Green. The available colors are the ones supported by the dot tool. -dot-include-all Include all modules in the dot output, not only modules given on the command line or loaded with the -load option. -dot-reduce Perform a transitive reduction of the dependency graph before outputting the dot code. This can be useful if there are a lot of transitive dependencies that clutter the graph. -dot-types Output dot code describing the type dependency graph instead of the module dependency graph. Options for generating man files -------------------------------- The following options apply in conjunction with the -man option: -man-mini Generate man pages only for modules, module types, clases and class types, instead of pages for all elements. -man-suffix Set the suffix used for generated man filenames. Default is 'o', like in List.o. 15.1.2 Merging of module information ===================================== Information on a module can be extracted either from the .mli or .ml file, or both, depending on the files given on the command line. When both .mli and .ml files are given for the same module, information extracted from these files is merged according to the following rules: - Only elements (values, types, classes, ...) declared in the .mli file are kept. In other terms, definitions from the .ml file that are not exported in the .mli file are not documented. - Descriptions of elements and descriptions in @-tags are handled as follows. If a description for the same element or in the same @-tag of the same element is present in both files, then the description of the .ml file is concatenated to the one in the .mli file, if the corresponding -m flag is given on the command line. If a description is present in the .ml file and not in the .mli file, the .ml description is kept. In either case, all the information given in the .mli file is kept. 15.1.3 Coding rules ==================== The following rules must be respected in order to avoid name clashes resulting in cross-reference errors: - In a module, there must not be two modules, two module types or a module and a module type with the same name. - In a module, there must not be two classes, two class types or a class and a class type with the same name. - In a module, there must not be two values, two types, or two exceptions with the same name. - Values defined in tuple, as in let (x,y,z) = (1,2,3) are not kept by OCamldoc. - Avoid the following construction: << open Foo (* which has a module Bar with a value x *) module Foo = struct module Bar = struct let x = 1 end end let dummy = Bar.x >> In this case, OCamldoc will associate Bar.x to the x of module Foo defined just above, instead of to the Bar.x defined in the opened module Foo. 15.2 Syntax of documentation comments *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= Comments containing documentation material are called special comments and are written between (** and *). Special comments must start exactly with (**. Comments beginning with ( and more than two * are ignored. 15.2.1 Placement of documentation comments =========================================== OCamldoc can associate comments to some elements of the language encountered in the source files. The association is made according to the locations of comments with respect to the language elements. The locations of comments in .mli and .ml files are different. Comments in .mli files ---------------------- A special comment is associated to an element if it is placed before or after the element. A special comment before an element is associated to this element if : - There is no blank line or another special comment between the special comment and the element. However, a regular comment can occur between the special comment and the element. - The special comment is not already associated to the previous element. - The special comment is not the first one of a toplevel module. A special comment after an element is associated to this element if there is no blank line or comment between the special comment and the element. There are two exceptions: for type constructors and record fields in type definitions, the associated comment can only be placed after the constructor or field definition, without blank lines or other comments between them. The following sample interface file foo.mli illustrates the placement rules for comments in .mli files. << (** The first special comment of the file is the comment associated with the whole module.*) (** Special comments can be placed between elements and are kept by the OCamldoc tool, but are not associated to any element. @-tags in these comments are ignored.*) (*******************************************************************) (** Comments like the one above, with more than two asterisks, are ignored. *) (** The comment for function f. *) val f : int -> int -> int (** The continuation of the comment for function f. *) (** Comment for exception My_exception, even with a simple comment between the special comment and the exception.*) (* Hello, I'm a simple comment :-) *) exception My_exception of (int -> int) * int (** Comment for type weather *) type weather = | Rain of int (** The comment for construtor Rain *) | Sun (** The comment for constructor Sun *) (** Comment for type weather2 *) type weather2 = | Rain of int (** The comment for construtor Rain *) | Sun (** The comment for constructor Sun *) (** I can continue the comment for type weather2 here because there is already a comment associated to the last constructor.*) (** The comment for type my_record *) type my_record = { val foo : int ; (** Comment for field foo *) val bar : string ; (** Comment for field bar *) } (** Continuation of comment for type my_record *) (** Comment for foo *) val foo : string (** This comment is associated to foo and not to bar. *) val bar : string (** This comment is assciated to bar. *) (** The comment for class my_class *) class my_class : object (** A comment to describe inheritance from cl *) inherit cl (** The comment for attribute tutu *) val mutable tutu : string (** The comment for attribute toto. *) val toto : int (** This comment is not attached to titi since there is a blank line before titi, but is kept as a comment in the class. *) val titi : string (** Comment for method toto *) method toto : string (** Comment for method m *) method m : float -> int end (** The comment for the class type my_class_type *) class type my_class_type = object (** The comment for variable x. *) val mutable x : int (** The commend for method m. *) method m : int -> int end (** The comment for module Foo *) module Foo = struct (** The comment for x *) val x : int (** A special comment that is kept but not associated to any element *) end (** The comment for module type my_module_type. *) module type my_module_type = sig (** The comment for value x. *) val x : int (** The comment for module M. *) module M = struct (** The comment for value y. *) val y : int (* ... *) end end >> Comments in .ml files --------------------- A special comment is associated to an element if it is placed before the element and there is no blank line between the comment and the element. Meanwhile, there can be a simple comment between the special comment and the element. There are two exceptions, for type constructors and record fields in type definitions, whose associated comment must be placed after the constructor or field definition, without blank line between them. The following example of file toto.ml shows where to place comments in a .ml file. << (** The first special comment of the file is the comment associated to the whole module.*) (** The comment for function f *) let f x y = x + y (** This comment is not attached to any element since there is another special comment just before the next element. *) (** Comment for exception My_exception, even with a simple comment between the special comment and the exception.*) (* A simple comment. *) exception My_exception of (int -> int) * int (** Comment for type weather *) type weather = | Rain of int (** The comment for constructor Rain *) | Sun (** The comment for constructor Sun *) (** The comment for type my_record *) type my_record = { val foo : int ; (** Comment for field foo *) val bar : string ; (** Comment for field bar *) } (** The comment for class my_class *) class my_class = object (** A comment to describe inheritance from cl *) inherit cl (** The comment for the instance variable tutu *) val mutable tutu = "tutu" (** The comment for toto *) val toto = 1 val titi = "titi" (** Comment for method toto *) method toto = tutu ^ "!" (** Comment for method m *) method m (f : float) = 1 end (** The comment for class type my_class_type *) class type my_class_type = object (** The comment for the instance variable x. *) val mutable x : int (** The commend for method m. *) method m : int -> int end (** The comment for module Foo *) module Foo = struct (** The comment for x *) val x : int (** A special comment in the class, but not associated to any element. *) end (** The comment for module type my_module_type. *) module type my_module_type = sig (* Comment for value x. *) val x : int (* ... *) end >> 15.2.2 The Stop special comment ================================ The special comment (**/**) tells OCamldoc to discard elements placed after this comment, up to the end of the current class, class type, module or module type. For instance: << class type foo = object (** comment for method m *) method m : string (**/**) (** This method won't appear in the documentation *) method bar : int end (** This value appears in the documentation, since the Stop special comment in the class does not affect the parent module of the class.*) val foo : string (**/**) (** The value bar does not appear in the documentation.*) val bar : string (** The type t does not appear either. *) type t = string >> The -no-stop option to ocamldoc causes the Stop special comments to be ignored. 15.2.3 Syntax of documentation comments ======================================== The inside of documentation comments (**...*) consists of free-form text with optional formatting annotations, followed by optional tags giving more specific information about parameters, version, authors, ... The tags are distinguished by a leading @ character. Thus, a documentation comment has the following shape: << (** The comment begins with a description, which is text formatted according to the rules described in the next section. The description continues until the first non-escaped '@' character. @author Mr Smith @param x description for parameter x *) >> Some elements support only a subset of all @-tags. Tags that are not relevant to the documented element are simply ignored. For instance, all tags are ignored when documenting type constructors, record fields, and class inheritance clauses. Similarly, a @param tag on a class instance variable is ignored. At last, (**) is the empty documentation comment. 15.2.4 Text formatting ======================= Here is the BNF grammar for the simple markup language used to format text descriptions. text ::= (text_element)+ text_element ::= | {[0-9]+ text} format text as a section header; the integer following { indicates the sectioning level. | {[0-9]+:label text} same, but also associate the name label to the current point. This point can be referenced by its fully-qualified label in a {! command, just like any other element. | {b text} set text in bold. | {i text} set text in italic. | {e text} emphasize text. | {C text} center text. | {L text} left align text. | {R text} right align text. | {ul list} build a list. | {ol list} build an enumerated list. | {{:string}text} put a link to the given address (given as a string) on the given text. | [string] set the given string in source code style. | {[string]} set the given string in preformatted source code style. | {v string v} set the given string in verbatim style. | {% string %} take the given string as raw LaTeX code. | {!string} insert a reference to the element named string. string must be a fully qualified element name, for example Foo.Bar.t. | {^ text} set text in superscript. | {_ text} set text in subscript. | escaped_string typeset the given string as is; special characters ('{', '}', '[', ']' and '@') must be escaped by a '\' | blank_line force a new line. list ::= | ({- text})+ | ({li text})+ A shortcut syntax exists for lists and enumerated lists: << (** Here is a {b list} - item 1 - item 2 - item 3 The list is ended by the blank line.*) >> is equivalent to: << (** Here is a {b list} {ul {- item 1} {- item 2} {- item 3}} The list is ended by the blank line.*) >> The same shortcut is available for enumerated lists, using '+' instead of '-'. Note that only one list can be defined by this shortcut in nested lists. In the description of a value, type, exception, module, module type, class or class type, the first sentence is sometimes used in indexes, or when just a part of the description is needed. The first sentence is composed of the first characters of the description, until - the first dot followed by a blank, or - the first blank line outside of the following text formatting : {ul list}, {ol list}, [string], {[string]}, {v string v}, {% string%}, {!string}, {^ text}, {_ text}. 15.2.5 Documentation tags (@-tags) =================================== Predefined tags --------------- The folowing table gives the list of predefined @-tags, with their syntax and meaning. --------------------------------------------- |@author string|The author of the element. | | |One author by @author tag. | | |There may be several @author| | |tags for the same element. | --------------------------------------------- |@deprecated |The text should describe | |text |when the element was | | |deprecated, what to use as a| | |replacement, and possibly | | |the reason for deprecation. | --------------------------------------------- |@param id text|Associate the given | | |description (text) to the | | |given parameter name id. | | |This tag is used for | | |functions, methods, classes | | |and functors. | --------------------------------------------- |@raise Exc |Explain that the element may| |text |raise the exception Exc. | --------------------------------------------- |@return text |Describe the return value | | |and its possible values. | | |This tag is used for | | |functions and methods. | --------------------------------------------- |@see |Add a reference to the URL | |text |between '<' and '>' with | | |the given text as comment. | --------------------------------------------- |@see |Add a reference to the given| |'filename' |file name (written between | |text |single quotes), with the | | |given text as comment. | --------------------------------------------- |@see "document|Add a reference to the given| |name" text |document name (written | | |between double quotes), with| | |the given text as comment. | --------------------------------------------- |@since string |Indicates when the element | | |was introduced. | --------------------------------------------- |@version |The version number for the | |string |element. | --------------------------------------------- Custom tags ----------- You can use custom tags in the documentation comments, but they will have no effect if the generator used does not handle them. To use a custom tag, for example foo, just put @foo with some text in your comment, as in: << (** My comment to show you a custom tag. @foo this is the text argument to the [foo] custom tag. *) >> To handle custom tags, you need to define a custom generator, as explained in section 15.3.2. 15.3 Custom generators *=*=*=*=*=*=*=*=*=*=*=* OCamldoc operates in two steps: 1. analysis of the source files; 2. generation of documentation, through a documentation generator, which is an object of class Odoc_args.class_generator. Users can provide their own documentation generator to be used during step 2 instead of the default generators. All the information retrieved during the analysis step is available through the Odoc_info module, which gives access to all the types and functions representing the elements found in the given modules, with their associated description. The files you can used to define custom generators are installed in the ocamldoc sub-directory of the OCaml standard library. 15.3.1 The generator class =========================== A generator class is a class of type Odoc_args.doc_generator. It has only one method generator : Odoc_info.Module.t_module list -> unit This method will be called with the list of analysed and possibly merged Odoc_info.t_module structures. Of course the class can have other methods, but the object of this class must be coerced to Odoc_args.doc_generator before being passed to the function Odoc_args.set_doc_generator : Odoc_args.doc_generator -> unit which installs the new documentation generator. The following example shows how to define and install a new documentation generator. See the odoc_fhtml generator (in the Ocamldoc Hump) for a complete example. << class my_doc_gen = object (* ... *) method generate module_list = (* ... *) () (* ... *) end let my_generator = new my_doc_gen let _ = Odoc_args.set_doc_generator (my_generator :> Odoc_args.doc_generator) >> Note: The new class can inherit from Odoc_html.html, Odoc_latex.latex, Odoc_man.man, Odoc_texi.texi or Odoc_dot.dot, and redefine only some methods to benefit from the existing methods. 15.3.2 Handling custom tags ============================ Making a custom generator handle custom tags (see 15.2.5) is very simple. For HTML -------- Here is how to develop a HTML generator handling your custom tags. The class Odoc_html.html inherits from the class Odoc_html.info, containing a field tag_functions which is a list pairs composed of a custom tag (e.g. 'foo') and a function taking a text and returning HTML code (of type string). To handle a new tag bar, create a HTML generator class from the existing one and complete the tag_functions field: << class my_gen = object(self) inherit Odoc_html.html (** Return HTML code for the given text of a bar tag. *) method html_of_bar t = (* your code here *) initializer tag_functions <- ("bar", self#html_of_bar) :: tag_functions end >> Another method of the class Odoc_html.info will look for the function associated to a custom tag and apply it to the text given to the tag. If no function is associated to a custom tag, then the method prints a warning message on stderr. For other generators -------------------- As for the HTML custom generator, you can define a new LaTeX(resp. man) generator by inheriting from the class Odoc_latex.latex (resp. Odoc_man.man) and adding your own tag handler to the field tag_functions. 15.4 Adding command line options *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The command line analysis is performed after loading the module containing the documentation generator, thus allowing command line options to be added to the list of existing ones. Adding an option can be done with the function Odoc_args.add_option : string * Arg.spec * string -> unit Note: Existing command line options can be redefined using this function. 15.4.1 Compilation and usage ============================= Defining a custom generator class in one file --------------------------------------------- Let custom.ml be the file defining a new generator class. Compilation of custom.ml can be performed by the following command : ocamlc -I +ocamldoc -c custom.ml The file custom.cmo is created and can be used this way : ocamldoc -g custom.cmo other_options source_files It is important not to give the -html or any other option selecting a built in generator to ocamldoc, which would result in using this generator instead of the one you just loaded. Defining a custom generator class in several files -------------------------------------------------- It is possible to define a generator class in several modules, which are defined in several files file1.ml[i], file2.ml[i], ..., fileN.ml[i]. A .cma library file must be created, including all these files. The following commands create the custom.cma file from files file1.ml[i], ..., fileN.ml[i] : ocamlc -I +ocamldoc -c file1.ml[i] ocamlc -I +ocamldoc -c file2.ml[i] ... ocamlc -I +ocamldoc -c fileN.ml[i] ocamlc -o custom.cma -a file1.cmo file2.cmo ... fileN.cmo Then, the following command uses custom.cma as custom generator: ocamldoc -g custom.cma other_options source_files Again, it is important not to give the -html or any other option selecting a built in generator to ocamldoc, which would result in using this generator instead of the one you just loaded. Chapter 16 The debugger (ocamldebug) *************************************** This chapter describes the Objective Caml source-level replay debugger ocamldebug. Unix: The debugger is available on Unix systems that provides BSD sockets. Windows: The debugger is available under the Cygwin port of Objective Caml, but not under the native Win32 port. MacOS: The debugger is not available. 16.1 Compiling for debugging *=*=*=*=*=*=*=*=*=*=*=*=*=*=* Before the debugger can be used, the program must be compiled and linked with the -g option: all .cmo and .cma files that are part of the program should have been created with ocamlc -g, and they must be linked together with ocamlc -g. Compiling with -g entails no penalty on the running time of programs: object files and bytecode executable files are bigger and take longer to produce, but the executable files run at exactly the same speed as if they had been compiled without -g. 16.2 Invocation *=*=*=*=*=*=*=*= 16.2.1 Starting the debugger ============================= The Objective Caml debugger is invoked by running the program ocamldebug with the name of the bytecode executable file as first argument: << ocamldebug [options] program [arguments] >> The arguments following program are optional, and are passed as command-line arguments to the program being debugged. (See also the set arguments command.) The following command-line options are recognized: -I directory Add directory to the list of directories searched for source files and compiled files. (See also the directory command.) -s socket Use socket for communicating with the debugged program. See the description of the command set socket (section 16.8.6) for the format of socket. -c count Set the maximum number of simultaneously live checkpoints to count. -cd directory Run the debugger program from the working directory directory, instead of the current directory. (See also the cd command.) -emacs Tell the debugger it is executed under Emacs. (See section 16.10 for information on how to run the debugger under Emacs.) 16.2.2 Exiting the debugger ============================ The command quit exits the debugger. You can also exit the debugger by typing an end-of-file character (usually ctrl-D). Typing an interrupt character (usually ctrl-C) will not exit the debugger, but will terminate the action of any debugger command that is in progress and return to the debugger command level. 16.3 Commands *=*=*=*=*=*=*= A debugger command is a single line of input. It starts with a command name, which is followed by arguments depending on this name. Examples: << run goto 1000 set arguments arg1 arg2 >> A command name can be truncated as long as there is no ambiguity. For instance, go 1000 is understood as goto 1000, since there are no other commands whose name starts with go. For the most frequently used commands, ambiguous abbreviations are allowed. For instance, r stands for run even though there are others commands starting with r. You can test the validity of an abbreviation using the help command. If the previous command has been successful, a blank line (typing just RET) will repeat it. 16.3.1 Getting help ==================== The Objective Caml debugger has a simple on-line help system, which gives a brief description of each command and variable. help Print the list of commands. help command Give help about the command command. help set variable, help show variable Give help about the variable variable. The list of all debugger variables can be obtained with help set. help info topic Give help about topic. Use help info to get a list of known topics. 16.3.2 Accessing the debugger state ==================================== set variable value Set the debugger variable variable to the value value. show variable Print the value of the debugger variable variable. info subject Give information about the given subject. For instance, info breakpoints will print the list of all breakpoints. 16.4 Executing a program *=*=*=*=*=*=*=*=*=*=*=*=* 16.4.1 Events ============== Events are ``interesting'' locations in the source code, corresponding to the beginning or end of evaluation of ``interesting'' sub-expressions. Events are the unit of single-stepping (stepping goes to the next or previous event encountered in the program execution). Also, breakpoints can only be set at events. Thus, events play the role of line numbers in debuggers for conventional languages. During program execution, a counter is incremented at each event encountered. The value of this counter is referred as the current time. Thanks to reverse execution, it is possible to jump back and forth to any time of the execution. Here is where the debugger events (written §§) are located in the source code: - Following a function application: << (f arg)§§ >> - On entrance to a function: << fun x y z -> §§ ... >> - On each case of a pattern-matching definition (function, match...with construct, try...with construct): << function pat1 -> §§ expr1 | ... | patN -> §§ exprN >> - Between subexpressions of a sequence: << expr1; §§ expr2; §§ ...; §§ exprN >> - In the two branches of a conditional expression: << if cond then §§ expr1 else §§ expr2 >> - At the beginning of each iteration of a loop: << while cond do §§ body done for i = a to b do §§ body done >> Exceptions: A function application followed by a function return is replaced by the compiler by a jump (tail-call optimization). In this case, no event is put after the function application. 16.4.2 Starting the debugged program ===================================== The debugger starts executing the debugged program only when needed. This allows setting breapoints or assigning debugger variables before execution starts. There are several ways to start execution: run Run the program until a breakpoint is hit, or the program terminates. step 0 Load the program and stop on the first event. goto time Load the program and execute it until the given time. Useful when you already know approximately at what time the problem appears. Also useful to set breakpoints on function values that have not been computed at time 0 (see section 16.5). The execution of a program is affected by certain information it receives when the debugger starts it, such as the command-line arguments to the program and its working directory. The debugger provides commands to specify this information (set arguments and cd). These commands must be used before program execution starts. If you try to change the arguments or the working directory after starting your program, the debugger will kill the program (after asking for confirmation). 16.4.3 Running the program =========================== The following commands execute the program forward or backward, starting at the current time. The execution will stop either when specified by the command or when a breakpoint is encountered. run Execute the program forward from current time. Stops at next breakpoint or when the program terminates. reverse Execute the program backward from current time. Mostly useful to go to the last breakpoint encountered before the current time. step [count] Run the program and stop at the next event. With an argument, do it count times. backstep [count] Run the program backward and stop at the previous event. With an argument, do it count times. next [count] Run the program and stop at the next event, skipping over function calls. With an argument, do it count times. previous [count] Run the program backward and stop at the previous event, skipping over function calls. With an argument, do it count times. finish Run the program until the current function returns. start Run the program backward and stop at the first event before the current function invocation. 16.4.4 Time travel =================== You can jump directly to a given time, without stopping on breakpoints, using the goto command. As you move through the program, the debugger maintains an history of the successive times you stop at. The last command can be used to revisit these times: each last command moves one step back through the history. That is useful mainly to undo commands such as step and next. goto time Jump to the given time. last [count] Go back to the latest time recorded in the execution history. With an argument, do it count times. set history size Set the size of the execution history. 16.4.5 Killing the program =========================== kill Kill the program being executed. This command is mainly useful if you wish to recompile the program without leaving the debugger. 16.5 Breakpoints *=*=*=*=*=*=*=*=* A breakpoint causes the program to stop whenever a certain point in the program is reached. It can be set in several ways using the break command. Breakpoints are assigned numbers when set, for further reference. The most comfortable way to set breakpoints is through the Emacs interface (see section 16.10). break Set a breakpoint at the current position in the program execution. The current position must be on an event (i.e., neither at the beginning, nor at the end of the program). break function Set a breakpoint at the beginning of function. This works only when the functional value of the identifier function has been computed and assigned to the identifier. Hence this command cannot be used at the very beginning of the program execution, when all identifiers are still undefined; use goto time to advance execution until the functional value is available. break @ [module] line Set a breakpoint in module module (or in the current module if module is not given), at the first event of line line. break @ [module] line column Set a breakpoint in module module (or in the current module if module is not given), at the event closest to line line, column column. break @ [module] # character Set a breakpoint in module module at the event closest to character number character. break address Set a breakpoint at the code address address. delete [breakpoint-numbers] Delete the specified breakpoints. Without argument, all breakpoints are deleted (after asking for confirmation). info breakpoints Print the list of all breakpoints. 16.6 The call stack *=*=*=*=*=*=*=*=*=*= Each time the program performs a function application, it saves the location of the application (the return address) in a block of data called a stack frame. The frame also contains the local variables of the caller function. All the frames are allocated in a region of memory called the call stack. The command backtrace (or bt) displays parts of the call stack. At any time, one of the stack frames is ``selected'' by the debugger; several debugger commands refer implicitly to the selected frame. In particular, whenever you ask the debugger for the value of a local variable, the value is found in the selected frame. The commands frame, up and down select whichever frame you are interested in. When the program stops, the debugger automatically selects the currently executing frame and describes it briefly as the frame command does. frame Describe the currently selected stack frame. frame frame-number Select a stack frame by number and describe it. The frame currently executing when the program stopped has number 0; its caller has number 1; and so on up the call stack. backtrace [count], bt [count] Print the call stack. This is useful to see which sequence of function calls led to the currently executing frame. With a positive argument, print only the innermost count frames. With a negative argument, print only the outermost -count frames. up [count] Select and display the stack frame just ``above'' the selected frame, that is, the frame that called the selected frame. An argument says how many frames to go up. down [count] Select and display the stack frame just ``below'' the selected frame, that is, the frame that was called by the selected frame. An argument says how many frames to go down. 16.7 Examining variable values *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The debugger can print the current value of simple expressions. The expressions can involve program variables: all the identifiers that are in scope at the selected program point can be accessed. Expressions that can be printed are a subset of Objective Caml expressions, as described by the following grammar: expr ::= lowercase-ident | { capitalized-ident . } lowercase-ident | * | $ integer | expr . lowercase-ident | expr .( integer ) | expr .[ integer ] | ! expr | ( expr ) The first two cases refer to a value identifier, either unqualified or qualified by the path to the structure that define it. * refers to the result just computed (typically, the value of a function application), and is valid only if the selected event is an ``after'' event (typically, a function application). $ integer refer to a previously printed value. The remaining four forms select part of an expression: respectively, a record field, an array element, a string element, and the current contents of a reference. print variables Print the values of the given variables. print can be abbreviated as p. display variables Same as print, but limit the depth of printing to 1. Useful to browse large data structures without printing them in full. display can be abbreviated as d. When printing a complex expression, a name of the form $integer is automatically assigned to its value. Such names are also assigned to parts of the value that cannot be printed because the maximal printing depth is exceeded. Named values can be printed later on with the commands p $integer or d $integer. Named values are valid only as long as the program is stopped. They are forgotten as soon as the program resumes execution. set print_depth d Limit the printing of values to a maximal depth of d. set print_length l Limit the printing of values to at most l nodes printed. 16.8 Controlling the debugger *=*=*=*=*=*=*=*=*=*=*=*=*=*=*= 16.8.1 Setting the program name and arguments ============================================== set program file Set the program name to file. set arguments arguments Give arguments as command-line arguments for the program. A shell is used to pass the arguments to the debugged program. You can therefore use wildcards, shell variables, and file redirections inside the arguments. To debug programs that read from standard input, it is recommended to redirect their input from a file (using set arguments < input-file), otherwise input to the program and input to the debugger are not properly separated, and inputs are not properly replayed when running the program backwards. 16.8.2 How programs are loaded =============================== The loadingmode variable controls how the program is executed. set loadingmode direct The program is run directly by the debugger. This is the default mode. set loadingmode runtime The debugger execute the Objective Caml runtime camlrun on the program. Rarely useful; moreover it prevents the debugging of programs compiled in ``custom runtime'' mode. set loadingmode manual The user starts manually the program, when asked by the debugger. Allows remote debugging (see section 16.8.6). 16.8.3 Search path for files ============================= The debugger searches for source files and compiled interface files in a list of directories, the search path. The search path initially contains the current directory . and the standard library directory. The directory command adds directories to the path. Whenever the search path is modified, the debugger will clear any information it may have cached about the files. directory directorynames Add the given directories to the search path. These directories are added at the front, and will therefore be searched first. directory Reset the search path. This requires confirmation. 16.8.4 Working directory ========================= Each time a program is started in the debugger, it inherits its working directory from the current working directory of the debugger. This working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in the debugger with the cd command or the -cd command-line option. cd directory Set the working directory for camldebug to directory. pwd Print the working directory for camldebug. 16.8.5 Turning reverse execution on and off ============================================ In some cases, you may want to turn reverse execution off. This speeds up the program execution, and is also sometimes useful for interactive programs. Normally, the debugger takes checkpoints of the program state from time to time. That is, it makes a copy of the current state of the program (using the Unix system call fork). If the variable checkpoints is set to off, the debugger will not take any checkpoints. set checkpoints on/off Select whether the debugger makes checkpoints or not. 16.8.6 Communication between the debugger and the program ========================================================== The debugger communicate with the program being debugged through a Unix socket. You may need to change the socket name, for example if you need to run the debugger on a machine and your program on another. set socket socket Use socket for communication with the program. socket can be either a file name, or an Internet port specification host:port, where host is a host name or an Internet address in dot notation, and port is a port number on the host. On the debugged program side, the socket name is passed either by the -D command line option to camlrun, or through the CAML_DEBUG_SOCKET environment variable. 16.8.7 Fine-tuning the debugger ================================ Several variables enables to fine-tune the debugger. Reasonable defaults are provided, and you should normally not have to change them. set processcount count Set the maximum number of checkpoints to count. More checkpoints facilitate going far back in time, but use more memory and create more Unix processes. As checkpointing is quite expensive, it must not be done too often. On the other hand, backward execution is faster when checkpoints are taken more often. In particular, backward single-stepping is more responsive when many checkpoints have been taken just before the current time. To fine-tune the checkpointing strategy, the debugger does not take checkpoints at the same frequency for long displacements (e.g. run) and small ones (e.g. step). The two variables bigstep and smallstep contain the number of events between two checkpoints in each case. set bigstep count Set the number of events between two checkpoints for long displacements. set smallstep count Set the number of events between two checkpoints for small displacements. The following commands display information on checkpoints and events: info checkpoints Print a list of checkpoints. info events [module] Print the list of events in the given module (the current module, by default). 16.8.8 User-defined printers ============================= Just as in the toplevel system (section 9.2), the user can register functions for printing values of certain types. For technical reasons, the debugger cannot call printing functions that reside in the program being debugged. The code for the printing functions must therefore be loaded explicitly in the debugger. load_printer "file-name" Load in the debugger the indicated .cmo or .cma object file. The file is loaded in an environment consisting only of the Objective Caml standard library plus the definitions provided by object files previously loaded using load_printer. If this file depends on other object files not yet loaded, the debugger automatically loads them if it is able to find them in the search path. The loaded file does not have direct access to the modules of the program being debugged. install_printer printer-name Register the function named printer-name (a value path) as a printer for objects whose types match the argument type of the function. That is, the debugger will call printer-name when it has such an object to print. The printing function printer-name must use the Format library module to produce its output, otherwise its output will not be correctly located in the values printed by the toplevel loop. The value path printer-name must refer to one of the functions defined by the object files loaded using load_printer. It cannot reference the functions of the program being debugged. remove_printer printer-name Remove the named function from the table of value printers. 16.9 Miscellaneous commands *=*=*=*=*=*=*=*=*=*=*=*=*=*= list [module] [beginning] [end] List the source of module module, from line number beginning to line number end. By default, 20 lines of the current module are displayed, starting 10 lines before the current position. source filename Read debugger commands from the script filename. 16.10 Running the debugger under Emacs *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* The most user-friendly way to use the debugger is to run it under Emacs. See the file emacs/README in the distribution for information on how to load the Emacs Lisp files for Caml support. The Caml debugger is started under Emacs by the command M-x camldebug, with argument the name of the executable file progname to debug. Communication with the debugger takes place in an Emacs buffer named *camldebug-progname*. The editing and history facilities of Shell mode are available for interacting with the debugger. In addition, Emacs displays the source files containing the current event (the current position in the program execution) and highlights the location of the event. This display is updated synchronously with the debugger action. The following bindings for the most common debugger commands are available in the *camldebug-progname* buffer: C-c C-s (command step): execute the program one step forward. C-c C-k (command backstep): execute the program one step backward. C-c C-n (command next): execute the program one step forward, skipping over function calls. Middle mouse button (command display): display named value. $n under mouse cursor (support incremental browsing of large data structures). C-c C-p (command print): print value of identifier at point. C-c C-d (command display): display value of identifier at point. C-c C-r (command run): execute the program forward to next breakpoint. C-c C-v (command reverse): execute the program backward to latest breakpoint. C-c C-l (command last): go back one step in the command history. C-c C-t (command backtrace): display backtrace of function calls. C-c C-f (command finish): run forward till the current function returns. C-c < (command up): select the stack frame below the current frame. C-c > (command down): select the stack frame above the current frame. In all buffers in Caml editing mode, the following debugger commands are also available: C-x C-a C-b (command break): set a breakpoint at event closest to point C-x C-a C-p (command print): print value of identifier at point C-x C-a C-d (command display): display value of identifier at point Chapter 17 Profiling (ocamlprof) *********************************** This chapter describes how the execution of Objective Caml programs can be profiled, by recording how many times functions are called, branches of conditionals are taken, ... 17.1 Compiling for profiling *=*=*=*=*=*=*=*=*=*=*=*=*=*=* Before profiling an execution, the program must be compiled in profiling mode, using the ocamlcp front-end to the ocamlc compiler (see chapter 8). When compiling modules separately, ocamlcp must be used when compiling the modules (production of .cmo files), and can also be used (though this is not strictly necessary) when linking them together. Note If a module (.ml file) doesn't have a corresponding interface (.mli file), then compiling it with ocamlcp will produce object files (.cmi and .cmo) that are not compatible with the ones produced by ocamlc, which may lead to problems (if the .cmi or .cmo is still around) when switching between profiling and non-profiling compilations. To avoid this problem, you should always have a .mli file for each .ml file. Note To make sure your programs can be compiled in profiling mode, avoid using any identifier that begins with __ocaml_prof. The amount of profiling information can be controlled through the -p option to ocamlcp, followed by one or several letters indicating which parts of the program should be profiled: a all options f function calls : a count point is set at the beginning of function bodies i if ...then ...else ... : count points are set in both then branch and else branch l while, for loops: a count point is set at the beginning of the loop body m match branches: a count point is set at the beginning of the body of each branch t try ...with ... branches: a count point is set at the beginning of the body of each branch For instance, compiling with ocamlcp -p film profiles function calls, if...then...else..., loops and pattern matching. Calling ocamlcp without the -p option defaults to -p fm, meaning that only function calls and pattern matching are profiled. Note: Due to the implementation of streams and stream patterns as syntactic sugar, it is hard to predict what parts of stream expressions and patterns will be profiled by a given flag. To profile a program with streams, we recommend using ocamlcp -p a. 17.2 Profiling an execution *=*=*=*=*=*=*=*=*=*=*=*=*=*= Running a bytecode executable file that has been compiled with ocamlcp records the execution counts for the specified parts of the program and saves them in a file called ocamlprof.dump in the current directory. The ocamlprof.dump file is written only if the program terminates normally (by calling exit or by falling through). It is not written if the program terminates with an uncaught exception. If a compatible dump file already exists in the current directory, then the profiling information is accumulated in this dump file. This allows, for instance, the profiling of several executions of a program on different inputs. 17.3 Printing profiling information *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= The ocamlprof command produces a source listing of the program modules where execution counts have been inserted as comments. For instance, << ocamlprof foo.ml >> prints the source code for the foo module, with comments indicating how many times the functions in this module have been called. Naturally, this information is accurate only if the source file has not been modified since the profiling execution took place. The following options are recognized by ocamlprof: -f dumpfile Specifies an alternate dump file of profiling information -F string Specifies an additional string to be output with profiling information. By default, ocamlprof will annotate programs with comments of the form (* n *) where n is the counter value for a profiling point. With option -F s, the annotation will be (* sn *). 17.4 Time profiling *=*=*=*=*=*=*=*=*=*= Profiling with ocamlprof only records execution counts, not the actual time spent into each function. There is currently no way to perform time profiling on bytecode programs generated by ocamlc. Native-code programs generated by ocamlopt can be profiled for time and execution counts using the -p option and the standard Unix profiler gprof. Just add the -p option when compiling and linking the program: << ocamlopt -o myprog -p other-options files ./myprog gprof myprog >> Caml function names in the output of gprof have the following format: << Module-name_function-name_unique-number >> Other functions shown are either parts of the Caml run-time system or external C functions linked with the program. The output of gprof is described in the Unix manual page for gprof(1). It generally consists of two parts: a ``flat'' profile showing the time spent in each function and the number of invocation of each function, and a ``hierarchical'' profile based on the call graph. Currently, only the Intel x86/Linux and Alpha/Digital Unix ports of ocamlopt support the two profiles. On other platforms, gprof will report only the ``flat'' profile with just time information. When reading the output of gprof, keep in mind that the accumulated times computed by gprof are based on heuristics and may not be exact. Chapter 18 Interfacing C with Objective Caml *********************************************** This chapter describes how user-defined primitives, written in C, can be linked with Caml code and called from Caml functions. 18.1 Overview and compilation information *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= 18.1.1 Declaring primitives ============================ User primitives are declared in an implementation file or struct...end module expression using the external keyword: << external name : type = C-function-name >> This defines the value name name as a function with type type that executes by calling the given C function. For instance, here is how the input primitive is declared in the standard library module Pervasives: << external input : in_channel -> string -> int -> int -> int = "input" >> Primitives with several arguments are always curried. The C function does not necessarily have the same name as the ML function. External functions thus defined can be specified in interface files or sig...end signatures either as regular values << val name : type >> thus hiding their implementation as a C function, or explicitly as ``manifest'' external functions << external name : type = C-function-name >> The latter is slightly more efficient, as it allows clients of the module to call directly the C function instead of going through the corresponding Caml function. The arity (number of arguments) of a primitive is automatically determined from its Caml type in the external declaration, by counting the number of function arrows in the type. For instance, input above has arity 4, and the input C function is called with four arguments. Similarly, << external input2 : in_channel * string * int * int -> int = "input2" >> has arity 1, and the input2 C function receives one argument (which is a quadruple of Caml values). Type abbreviations are not expanded when determining the arity of a primitive. For instance, << type int_endo = int -> int external f : int_endo -> int_endo = "f" external g : (int -> int) -> (int -> int) = "f" >> f has arity 1, but g has arity 2. This allows a primitive to return a functional value (as in the f example above): just remember to name the functional return type in a type abbreviation. 18.1.2 Implementing primitives =============================== User primitives with arity n <= 5 are implemented by C functions that take n arguments of type value, and return a result of type value. The type value is the type of the representations for Caml values. It encodes objects of several base types (integers, floating-point numbers, strings, ...), as well as Caml data structures. The type value and the associated conversion functions and macros are described in details below. For instance, here is the declaration for the C function implementing the input primitive: << CAMLprim value input(value channel, value buffer, value offset, value length) { ... } >> When the primitive function is applied in a Caml program, the C function is called with the values of the expressions to which the primitive is applied as arguments. The value returned by the function is passed back to the Caml program as the result of the function application. User primitives with arity greater than 5 should be implemented by two C functions. The first function, to be used in conjunction with the bytecode compiler ocamlc, receives two arguments: a pointer to an array of Caml values (the values for the arguments), and an integer which is the number of arguments provided. The other function, to be used in conjunction with the native-code compiler ocamlopt, takes its arguments directly. For instance, here are the two C functions for the 7-argument primitive Nat.add_nat: << CAMLprim value add_nat_native(value nat1, value ofs1, value len1, value nat2, value ofs2, value len2, value carry_in) { ... } CAMLprim value add_nat_bytecode(value * argv, int argn) { return add_nat_native(argv[0], argv[1], argv[2], argv[3], argv[4], argv[5], argv[6]); } >> The names of the two C functions must be given in the primitive declaration, as follows: << external name : type = bytecode-C-function-name native-code-C-function-name >> For instance, in the case of add_nat, the declaration is: << external add_nat: nat -> int -> int -> nat -> int -> int -> int -> int = "add_nat_bytecode" "add_nat_native" >> Implementing a user primitive is actually two separate tasks: on the one hand, decoding the arguments to extract C values from the given Caml values, and encoding the return value as a Caml value; on the other hand, actually computing the result from the arguments. Except for very simple primitives, it is often preferable to have two distinct C functions to implement these two tasks. The first function actually implements the primitive, taking native C values as arguments and returning a native C value. The second function, often called the ``stub code'', is a simple wrapper around the first function that converts its arguments from Caml values to C values, call the first function, and convert the returned C value to Caml value. For instance, here is the stub code for the input primitive: << CAMLprim value input(value channel, value buffer, value offset, value length) { return Val_long(getblock((struct channel *) channel, &Byte(buffer, Long_val(offset)), Long_val(length))); } >> (Here, Val_long, Long_val and so on are conversion macros for the type value, that will be described later. The CAMLprim macro expands to the required compiler directives to ensure that the function following it is exported and accessible from Caml.) The hard work is performed by the function getblock, which is declared as: << long getblock(struct channel * channel, char * p, long n) { ... } >> To write C code that operates on Objective Caml values, the following include files are provided: ---------------------------------------------------- | Include file | Provides | ---------------------------------------------------- |caml/mlvalues.h|definition of the value type, and | | |conversion macros | |caml/alloc.h |allocation functions (to create | | |structured Caml objects) | |caml/memory.h |miscellaneous memory-related | | |functions and macros (for GC | | |interface, in-place modification | | |of structures, etc). | |caml/fail.h |functions for raising exceptions | | |(see section 18.4.5) | |caml/callback.h|callback from C to Caml (see | | |section 18.7). | |caml/custom.h |operations on custom blocks (see | | |section 18.9). | |caml/intext.h |operations for writing | | |user-defined serialization and | | |deserialization functions for | | |custom blocks (see section 18.9). | ---------------------------------------------------- These files reside in the caml/ subdirectory of the Objective Caml standard library directory (usually /usr/local/lib/ocaml). 18.1.3 Statically linking C code with Caml code ================================================ The Objective Caml runtime system comprises three main parts: the bytecode interpreter, the memory manager, and a set of C functions that implement the primitive operations. Some bytecode instructions are provided to call these C functions, designated by their offset in a table of functions (the table of primitives). In the default mode, the Caml linker produces bytecode for the standard runtime system, with a standard set of primitives. References to primitives that are not in this standard set result in the ``unavailable C primitive'' error. (Unless dynamic loading of C libraries is supported -- see section 18.1.4 below.) In the ``custom runtime'' mode, the Caml linker scans the object files and determines the set of required primitives. Then, it builds a suitable runtime system, by calling the native code linker with: - the table of the required primitives; - a library that provides the bytecode interpreter, the memory manager, and the standard primitives; - libraries and object code files (.o files) mentioned on the command line for the Caml linker, that provide implementations for the user's primitives. This builds a runtime system with the required primitives. The Caml linker generates bytecode for this custom runtime system. The bytecode is appended to the end of the custom runtime system, so that it will be automatically executed when the output file (custom runtime + bytecode) is launched. To link in ``custom runtime'' mode, execute the ocamlc command with: - the -custom option; - the names of the desired Caml object files (.cmo and .cma files) ; - the names of the C object files and libraries (.o and .a files) that implement the required primitives. Under Unix and Windows, a library named libname.a residing in one of the standard library directories can also be specified as -cclib -lname. If you are using the native-code compiler ocamlopt, the -custom flag is not needed, as the final linking phase of ocamlopt always builds a standalone executable. To build a mixed Caml/C executable, execute the ocamlopt command with: - the names of the desired Caml native object files (.cmx and .cmxa files); - the names of the C object files and libraries (.o, .a, .so or .dll files) that implement the required primitives. Starting with OCaml 3.00, it is possible to record the -custom option as well as the names of C libraries in a Caml library file .cma or .cmxa. For instance, consider a Caml library mylib.cma, built from the Caml object files a.cmo and b.cmo, which reference C code in libmylib.a. If the library is built as follows: << ocamlc -a -o mylib.cma -custom a.cmo b.cmo -cclib -lmylib >> users of the library can simply link with mylib.cma: << ocamlc -o myprog mylib.cma ... >> and the system will automatically add the -custom and -cclib -lmylib options, achieving the same effect as << ocamlc -o myprog -custom a.cmo b.cmo ... -cclib -lmylib >> The alternative, of course, is to build the library without extra options: << ocamlc -a -o mylib.cma a.cmo b.cmo >> and then ask users to provide the -custom and -cclib -lmylib options themselves at link-time: << ocamlc -o myprog -custom mylib.cma ... -cclib -lmylib >> The former alternative is more convenient for the final users of the library, however. 18.1.4 Dynamically linking C code with Caml code ================================================= Starting with OCaml 3.03, an alternative to static linking of C code using the -custom code is provided. In this mode, the Caml linker generates a pure bytecode executable (no embedded custom runtime system) that simply records the names of dynamically-loaded libraries containing the C code. The standard Caml runtime system ocamlrun then loads dynamically these libraries, and resolves references to the required primitives, before executing the bytecode. This facility is currently supported and known to work well under Linux and Windows (the native Windows port). It is supported, but not fully tested yet, under FreeBSD, Tru64, Solaris and Irix. It is not supported yet under other Unixes, Cygwin for Windows, and MacOS. To dynamically link C code with Caml code, the C code must first be compiled into a shared library (under Unix) or DLL (under Windows). This involves 1- compiling the C files with appropriate C compiler flags for producing position-independent code, and 2- building a shared library from the resulting object files. The resulting shared library or DLL file must be installed in a place where ocamlrun can find it later at program start-up time (see section 10.3). Finally (step 3), execute the ocamlc command with - the names of the desired Caml object files (.cmo and .cma files) ; - the names of the C shared libraries (.so or .dll files) that implement the required primitives. Under Unix and Windows, a library named dllname.so (respectively, .dll) residing in one of the standard library directories can also be specified as -dllib -lname. Do not set the -custom flag, otherwise you're back to static linking as described in section 18.1.3. Under Unix, the ocamlmklib tool (see section 18.10) automates steps 2 and 3. As in the case of static linking, it is possible (and recommended) to record the names of C libraries in a Caml .cmo library archive. Consider again a Caml library mylib.cma, built from the Caml object files a.cmo and b.cmo, which reference C code in dllmylib.so. If the library is built as follows: << ocamlc -a -o mylib.cma a.cmo b.cmo -dllib -lmylib >> users of the library can simply link with mylib.cma: << ocamlc -o myprog mylib.cma ... >> and the system will automatically add the -dllib -lmylib option, achieving the same effect as << ocamlc -o myprog a.cmo b.cmo ... -dllib -lmylib >> Using this mechanism, users of the library mylib.cma do not need to known that it references C code, nor whether this C code must be statically linked (using -custom) or dynamically linked. 18.1.5 Choosing between static linking and dynamic linking =========================================================== After having described two different ways of linking C code with Caml code, we now review the pros and cons of each, to help developers of mixed Caml/C libraries decide. The main advantage of dynamic linking is that it preserves the platform-independence of bytecode executables. That is, the bytecode executable contains no machine code, and can therefore be compiled on platform A and executed on other platforms B, C, ..., as long as the required shared libraries are available on all these platforms. In contrast, executables generated by ocamlc -custom run only on the platform on which they were created, because they embark a custom-tailored runtime system specific to that platform. In addition, dynamic linking results in smaller executables. Another advantage of dynamic linking is that the final users of the library do not need to have a C compiler, C linker, and C runtime libraries installed on their machines. This is no big deal under Unix and Cygwin, but many Windows users are reluctant to install Microsoft Visual C just to be able to do ocamlc -custom. There are two drawbacks to dynamic linking. The first is that the resulting executable is not stand-alone: it requires the shared libraries, as well as ocamlrun, to be installed on the machine executing the code. If you wish to distribute a stand-alone executable, it is better to link it statically, using ocamlc -custom -ccopt -static or ocamlopt -ccopt -static. Dynamic linking also raises the ``DLL hell'' problem: some care must be taken to ensure that the right versions of the shared libraries are found at start-up time. The second drawback of dynamic linking is that it complicates the construction of the library. The C compiler and linker flags to compile to position-independent code and build a shared library vary wildly between different Unix systems. Also, dynamic linking is not supported on all Unix systems, requiring a fall-back case to static linking in the Makefile for the library. The ocamlmklib command (see section 18.10) tries to hide some of these system dependencies. In conclusion: dynamic linking is highly recommended under the native Windows port, because there are no portability problems and it is much more convenient for the end users. Under Unix, dynamic linking should be considered for mature, frequently used libraries because it enhances platform-independence of bytecode executables. For new or rarely-used libraries, static linking is much simpler to set up in a portable way. 18.1.6 Building standalone custom runtime systems ================================================== It is sometimes inconvenient to build a custom runtime system each time Caml code is linked with C libraries, like ocamlc -custom does. For one thing, the building of the runtime system is slow on some systems (that have bad linkers or slow remote file systems); for another thing, the platform-independence of bytecode files is lost, forcing to perform one ocamlc -custom link per platform of interest. An alternative to ocamlc -custom is to build separately a custom runtime system integrating the desired C libraries, then generate ``pure'' bytecode executables (not containing their own runtime system) that can run on this custom runtime. This is achieved by the -make_runtime and -use_runtime flags to ocamlc. For example, to build a custom runtime system integrating the C parts of the ``Unix'' and ``Threads'' libraries, do: << ocamlc -make-runtime -o /home/me/ocamlunixrun unix.cma threads.cma >> To generate a bytecode executable that runs on this runtime system, do: << ocamlc -use-runtime /home/me/ocamlunixrun -o myprog \ unix.cma threads.cma your .cmo and .cma files >> The bytecode executable myprog can then be launched as usual: myprog args or /home/me/ocamlunixrun myprog args. Notice that the bytecode libraries unix.cma and threads.cma must be given twice: when building the runtime system (so that ocamlc knows which C primitives are required) and also when building the bytecode executable (so that the bytecode from unix.cma and threads.cma is actually linked in). 18.2 The value type *=*=*=*=*=*=*=*=*=*= All Caml objects are represented by the C type value, defined in the include file caml/mlvalues.h, along with macros to manipulate values of that type. An object of type value is either: - an unboxed integer; - a pointer to a block inside the heap (such as the blocks allocated through one of the `alloc_*' functions below); - a pointer to an object outside the heap (e.g., a pointer to a block allocated by malloc, or to a C variable). 18.2.1 Integer values ====================== Integer values encode 31-bit signed integers (63-bit on 64-bit architectures). They are unboxed (unallocated). 18.2.2 Blocks ============== Blocks in the heap are garbage-collected, and therefore have strict structure constraints. Each block includes a header containing the size of the block (in words), and the tag of the block. The tag governs how the contents of the blocks are structured. A tag lower than No_scan_tag indicates a structured block, containing well-formed values, which is recursively traversed by the garbage collector. A tag greater than or equal to No_scan_tag indicates a raw block, whose contents are not scanned by the garbage collector. For the benefits of ad-hoc polymorphic primitives such as equality and structured input-output, structured and raw blocks are further classified according to their tags as follows: ------------------------------------------------- | Tag | Contents of the block | ------------------------------------------------- |0 to No_scan_tag-1|A structured block (an array| | |of Caml objects). Each field| | |is a value. | |Closure_tag |A closure representing a | | |functional value. The first | | |word is a pointer to a piece| | |of code, the remaining words| | |are value containing the | | |environment. | |String_tag |A character string. | |Double_tag |A double-precision | | |floating-point number. | |Double_array_tag |An array or record of | | |double-precision | | |floating-point numbers. | |Abstract_tag |A block representing an | | |abstract datatype. | |Custom_tag |A block representing an | | |abstract datatype with | | |user-defined finalization, | | |comparison, hashing, | | |serialization and | | |deserialization functions | | |atttached. | ------------------------------------------------- 18.2.3 Pointers outside the heap ================================= Any word-aligned pointer to an address outside the heap can be safely cast to and from the type value. This includes pointers returned by malloc, and pointers to C variables (of size at least one word) obtained with the `&' operator. Caution: if a pointer returned by malloc is cast to the type value and returned to Caml, explicit deallocation of the pointer using free is potentially dangerous, because the pointer may still be accessible from the Caml world. Worse, the memory space deallocated by free can later be reallocated as part of the Caml heap; the pointer, formerly pointing outside the Caml heap, now points inside the Caml heap, and this can confuse the garbage collector. To avoid these problems, it is preferable to wrap the pointer in a Caml block with tag Abstract_tag or Custom_tag. 18.3 Representation of Caml data types *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* This section describes how Caml data types are encoded in the value type. 18.3.1 Atomic types ==================== ------------------------------------------------ |Caml type| Encoding | ------------------------------------------------ |int |Unboxed integer values. | |char |Unboxed integer values (ASCII code).| |float |Blocks with tag Double_tag. | |string |Blocks with tag String_tag. | |int32 |Blocks with tag Custom_tag. | |int64 |Blocks with tag Custom_tag. | |nativeint|Blocks with tag Custom_tag. | ------------------------------------------------ 18.3.2 Tuples and records ========================== Tuples are represented by pointers to blocks, with tag 0. Records are also represented by zero-tagged blocks. The ordering of labels in the record type declaration determines the layout of the record fields: the value associated to the label declared first is stored in field 0 of the block, the value associated to the label declared next goes in field 1, and so on. As an optimization, records whose fields all have static type float are represented as arrays of floating-point numbers, with tag Double_array_tag. (See the section below on arrays.) 18.3.3 Arrays ============== Arrays of integers and pointers are represented like tuples, that is, as pointers to blocks tagged 0. They are accessed with the Field macro for reading and the modify function for writing. Arrays of floating-point numbers (type float array) have a special, unboxed, more efficient representation. These arrays are represented by pointers to blocks with tag Double_array_tag. They should be accessed with the Double_field and Store_double_field macros. 18.3.4 Concrete types ====================== Constructed terms are represented either by unboxed integers (for constant constructors) or by blocks whose tag encode the constructor (for non-constant constructors). The constant constructors and the non-constant constructors for a given concrete type are numbered separately, starting from 0, in the order in which they appear in the concrete type declaration. Constant constructors are represented by unboxed integers equal to the constructor number. Non-constant constructors declared with a n-tuple as argument are represented by a block of size n, tagged with the constructor number; the n fields contain the components of its tuple argument. Other non-constant constructors are represented by a block of size 1, tagged with the constructor number; the field 0 contains the value of the constructor argument. Example: ------------------------------------------ |Constructed term| Representation | ------------------------------------------ |() |Val_int(0) | |false |Val_int(0) | |true |Val_int(1) | |[] |Val_int(0) | |h::t |Block with size = 2 and| | |tag = 0; first field | | |contains h, second | | |field t | ------------------------------------------ As a convenience, caml/mlvalues.h defines the macros Val_unit, Val_false and Val_true to refer to (), false and true. 18.3.5 Objects =============== Objects are represented as zero-tagged blocks. The first field of the block refers to the object class and associated method suite, in a format that cannot easily be exploited from C. The remaining fields of the object contain the values of the instance variables of the object. Instance variables are stored in the order in which they appear in the class definition (taking inherited classes into account). 18.3.6 Variants ================ Like constructed terms, values of variant types are represented either as integers (for variants without arguments), or as blocks (for variants with an argument). Unlike constructed terms, variant constructors are not numbered starting from 0, but identified by a hash value (a Caml integer), as computed by the C function hash_variant (declared in ): the hash value for a variant constructor named, say, VConstr is hash_variant("VConstr"). The variant value `VConstr is represented by hash_variant("VConstr"). The variant value `VConstr(v) is represented by a block of size 2 and tag 0, with field number 0 containing hash_variant("VConstr") and field number 1 containing v. Unlike constructed values, variant values taking several arguments are not flattened. That is, `VConstr(v, v') is represented by a block of size 2, whose field number 1 contains the representation of the pair (v, v'), but not as a block of size 3 containing v and v' in fields 1 and 2. 18.4 Operations on values *=*=*=*=*=*=*=*=*=*=*=*=*= 18.4.1 Kind tests ================== - Is_long(v) is true if value v is an immediate integer, false otherwise - Is_block(v) is true if value v is a pointer to a block, and false if it is an immediate integer. 18.4.2 Operations on integers ============================== - Val_long(l) returns the value encoding the long int l. - Long_val(v) returns the long int encoded in value v. - Val_int(i) returns the value encoding the int i. - Int_val(v) returns the int encoded in value v. - Val_bool(x) returns the Caml boolean representing the truth value of the C integer x. - Bool_val(v) returns 0 if v is the Caml boolean false, 1 if v is true. - Val_true, Val_false represent the Caml booleans true and false. 18.4.3 Accessing blocks ======================== - Wosize_val(v) returns the size of the block v, in words, excluding the header. - Tag_val(v) returns the tag of the block v. - Field(v, n) returns the value contained in the n^th field of the structured block v. Fields are numbered from 0 to Wosize_val(v)-1. - Store_field(b, n, v) stores the value v in the field number n of value b, which must be a structured block. - Code_val(v) returns the code part of the closure v. - string_length(v) returns the length (number of characters) of the string v. - Byte(v, n) returns the n^th character of the string v, with type char. Characters are numbered from 0 to string_length(v)-1. - Byte_u(v, n) returns the n^th character of the string v, with type unsigned char. Characters are numbered from 0 to string_length(v)-1. - String_val(v) returns a pointer to the first byte of the string v, with type char *. This pointer is a valid C string: there is a null character after the last character in the string. However, Caml strings can contain embedded null characters, that will confuse the usual C functions over strings. - Double_val(v) returns the floating-point number contained in value v, with type double. - Double_field(v, n) returns the n^th element of the array of floating-point numbers v (a block tagged Double_array_tag). - Store_double_field(v, n, d) stores the double precision floating-point number d in the n^th element of the array of floating-point numbers v. - Data_custom_val(v) returns a pointer to the data part of the custom block v. This pointer has type void * and must be cast to the type of the data contained in the custom block. - Int32_val(v) returns the 32-bit integer contained in the int32 v. - Int64_val(v) returns the 64-bit integer contained in the int64 v. - Nativeint_val(v) returns the long integer contained in the nativeint v. The expressions Field(v, n), Byte(v, n) and Byte_u(v, n) are valid l-values. Hence, they can be assigned to, resulting in an in-place modification of value v. Assigning directly to Field(v, n) must be done with care to avoid confusing the garbage collector (see below). 18.4.4 Allocating blocks ========================= Simple interface ---------------- - Atom(t) returns an ``atom'' (zero-sized block) with tag t. Zero-sized blocks are preallocated outside of the heap. It is incorrect to try and allocate a zero-sized block using the functions below. For instance, Atom(0) represents the empty array. - alloc(n, t) returns a fresh block of size n with tag t. If t is less than No_scan_tag, then the fields of the block are initialized with a valid value in order to satisfy the GC constraints. - alloc_tuple(n) returns a fresh block of size n words, with tag 0. - alloc_string(n) returns a string value of length n characters. The string initially contains garbage. - copy_string(s) returns a string value containing a copy of the null-terminated C string s (a char *). - copy_double(d) returns a floating-point value initialized with the double d. - copy_int32(i), copy_int64(i) and copy_nativeint(i) return a value of Caml type int32, int64 and nativeint, respectively, initialized with the integer i. - alloc_array(f, a) allocates an array of values, calling function f over each element of the input array a to transform it into a value. The array a is an array of pointers terminated by the null pointer. The function f receives each pointer as argument, and returns a value. The zero-tagged block returned by alloc_array(f, a) is filled with the values returned by the successive calls to f. (This function must not be used to build an array of floating-point numbers.) - copy_string_array(p) allocates an array of strings, copied from the pointer to a string array p (a `char **'). Low-level interface ------------------- The following functions are slightly more efficient than alloc, but also much more difficult to use. From the standpoint of the allocation functions, blocks are divided according to their size as zero-sized blocks, small blocks (with size less than or equal to `Max_young_wosize'), and large blocks (with size greater than `Max_young_wosize'). The constant `Max_young_wosize' is declared in the include file mlvalues.h. It is guaranteed to be at least 64 (words), so that any block with constant size less than or equal to 64 can be assumed to be small. For blocks whose size is computed at run-time, the size must be compared against `Max_young_wosize' to determine the correct allocation procedure. - alloc_small(n, t) returns a fresh small block of size n <= Max_young_wosize words, with tag t. If this block is a structured block (i.e. if t < No_scan_tag), then the fields of the block (initially containing garbage) must be initialized with legal values (using direct assignment to the fields of the block) before the next allocation. - alloc_shr(n, t) returns a fresh block of size n, with tag t. The size of the block can be greater than `Max_young_wosize'. (It can also be smaller, but in this case it is more efficient to call alloc_small instead of alloc_shr.) If this block is a structured block (i.e. if t < No_scan_tag), then the fields of the block (initially containing garbage) must be initialized with legal values (using the initialize function described below) before the next allocation. 18.4.5 Raising exceptions ========================== Two functions are provided to raise two standard exceptions: - failwith(s), where s is a null-terminated C string (with type `char *'), raises exception Failure with argument s. - invalid_argument(s), where s is a null-terminated C string (with type `char *'), raises exception Invalid_argument with argument s. Raising arbitrary exceptions from C is more delicate: the exception identifier is dynamically allocated by the Caml program, and therefore must be communicated to the C function using the registration facility described below in section 18.7.3. Once the exception identifier is recovered in C, the following functions actually raise the exception: - raise_constant(id) raises the exception id with no argument; - raise_with_arg(id, v) raises the exception id with the Caml value v as argument; - raise_with_string(id, s), where s is a null-terminated C string, raises the exception id with a copy of the C string s as argument. 18.5 Living in harmony with the garbage collector *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= Unused blocks in the heap are automatically reclaimed by the garbage collector. This requires some cooperation from C code that manipulates heap-allocated blocks. 18.5.1 Simple interface ======================== All the macros described in this section are declared in the memory.h header file. Rule 1 A function that has parameters or local variables of type value must begin with a call to one of the CAMLparam macros and return with CAMLreturn or CAMLreturn0. There are six CAMLparam macros: CAMLparam0 to CAMLparam5, which take zero to five arguments respectively. If your function has fewer than 5 parameters of type value, use the corresponding macros with these parameters as arguments. If your function has more than 5 parameters of type value, use CAMLparam5 with five of these parameters, and use one or more calls to the CAMLxparam macros for the remaining parameters (CAMLxparam1 to CAMLxparam5). The macros CAMLreturn and CAMLreturn0 are used to replace the C keyword return. Every occurence of return x must be replaced by CAMLreturn (x), every occurence of return without argument must be replaced by CAMLreturn0. If your C function is a procedure (i.e. if it returns void), you must insert CAMLreturn0 at the end (to replace C's implicit return). Note: some C compilers give bogus warnings about unused variables caml__dummy_xxx at each use of CAMLparam and CAMLlocal. You should ignore them. Example: << void foo (value v1, value v2, value v3) { CAMLparam3 (v1, v2, v3); ... CAMLreturn0; } >> Note: if your function is a primitive with more than 5 arguments for use with the byte-code runtime, its arguments are not values and must not be declared (they have types value * and int). Rule 2 Local variables of type value must be declared with one of the CAMLlocal macros. Arrays of values are declared with CAMLlocalN. The macros CAMLlocal1 to CAMLlocal5 declare and initialize one to five local variables of type value. The variable names are given as arguments to the macros. CAMLlocalN(x, n) declares and initializes a local variable of type value [n]. You can use several calls to these macros if you have more than 5 local variables. You can also use them in nested C blocks within the function. Example: << value bar (value v1, value v2, value v3) { CAMLparam3 (v1, v2, v3); CAMLlocal1 (result); result = alloc (3, 0); ... CAMLreturn (result); } >> Rule 3 Assignments to the fields of structured blocks must be done with the Store_field macro (for normal blocks) or Store_double_field macro (for arrays and records of floating-point numbers). Other assignments must not use Store_field nor Store_double_field. Store_field (b, n, v) stores the value v in the field number n of value b, which must be a block (i.e. Is_block(b) must be true). Example: << value bar (value v1, value v2, value v3) { CAMLparam3 (v1, v2, v3); CAMLlocal1 (result); result = alloc (3, 0); Store_field (result, 0, v1); Store_field (result, 1, v2); Store_field (result, 2, v3); CAMLreturn (result); } >> Warning: The first argument of Store_field and Store_double_field must be a variable declared by CAMLparam* or a parameter declared by CAMLlocal* to ensure that a garbage collection triggered by the evaluation of the other arguments will not invalidate the first argument after it is computed. Rule 4 Global variables containing values must be registered with the garbage collector using the register_global_root function. Registration of a global variable v is achieved by calling register_global_root(&v) just before a valid value is stored in v for the first time. A registered global variable v can be un-registered by calling remove_global_root(&v). Note: The CAML macros use identifiers (local variables, type identifiers, structure tags) that start with caml__. Do not use any identifier starting with caml__ in your programs. 18.5.2 Low-level interface =========================== We now give the GC rules corresponding to the low-level allocation functions alloc_small and alloc_shr. You can ignore those rules if you stick to the simplified allocation function alloc. Rule 5 After a structured block (a block with tag less than No_scan_tag) is allocated with the low-level functions, all fields of this block must be filled with well-formed values before the next allocation operation. If the block has been allocated with alloc_small, filling is performed by direct assignment to the fields of the block: << Field(v, n) = v_n; >> If the block has been allocated with alloc_shr, filling is performed through the initialize function: << initialize(&Field(v, n), v_n); >> The next allocation can trigger a garbage collection. The garbage collector assumes that all structured blocks contain well-formed values. Newly created blocks contain random data, which generally do not represent well-formed values. If you really need to allocate before the fields can receive their final value, first initialize with a constant value (e.g. Val_unit), then allocate, then modify the fields with the correct value (see rule 6). Rule 6 Direct assignment to a field of a block, as in << Field(v, n) = w; >> is safe only if v is a block newly allocated by alloc_small; that is, if no allocation took place between the allocation of v and the assignment to the field. In all other cases, never assign directly. If the block has just been allocated by alloc_shr, use initialize to assign a value to a field for the first time: << initialize(&Field(v, n), w); >> Otherwise, you are updating a field that previously contained a well-formed value; then, call the modify function: << modify(&Field(v, n), w); >> To illustrate the rules above, here is a C function that builds and returns a list containing the two integers given as parameters. First, we write it using the simplified allocation functions: << value alloc_list_int(int i1, int i2) { CAMLparam0 (); CAMLlocal2 (result, r); r = alloc(2, 0); /* Allocate a cons cell */ Store_field(r, 0, Val_int(i2)); /* car = the integer i2 */ Store_field(r, 1, Val_int(0)); /* cdr = the empty list [] */ result = alloc(2, 0); /* Allocate the other cons cell */ Store_field(result, 0, Val_int(i1)); /* car = the integer i1 */ Store_field(result, 1, r); /* cdr = the first cons cell */ CAMLreturn (result); } >> Here, the registering of result is not strictly needed, because no allocation takes place after it gets its value, but it's easier and safer to simply register all the local variables that have type value. Here is the same function written using the low-level allocation functions. We notice that the cons cells are small blocks and can be allocated with alloc_small, and filled by direct assignments on their fields. << value alloc_list_int(int i1, int i2) { CAMLparam0 (); CAMLlocal2 (result, r); r = alloc_small(2, 0); /* Allocate a cons cell */ Field(r, 0) = Val_int(i2); /* car = the integer i2 */ Field(r, 1) = Val_int(0); /* cdr = the empty list [] */ result = alloc_small(2, 0); /* Allocate the other cons cell */ Field(result, 0) = Val_int(i1); /* car = the integer i1 */ Field(result, 1) = r; /* cdr = the first cons cell */ CAMLreturn (result); } >> In the two examples above, the list is built bottom-up. Here is an alternate way, that proceeds top-down. It is less efficient, but illustrates the use of modify. << value alloc_list_int(int i1, int i2) { CAMLparam0 (); CAMLlocal2 (tail, r); r = alloc_small(2, 0); /* Allocate a cons cell */ Field(r, 0) = Val_int(i1); /* car = the integer i1 */ Field(r, 1) = Val_int(0); /* A dummy value tail = alloc_small(2, 0); /* Allocate the other cons cell */ Field(tail, 0) = Val_int(i2); /* car = the integer i2 */ Field(tail, 1) = Val_int(0); /* cdr = the empty list [] */ modify(&Field(r, 1), tail); /* cdr of the result = tail */ return r; } >> It would be incorrect to perform Field(r, 1) = tail directly, because the allocation of tail has taken place since r was allocated. tail is not registered as a root because there is no allocation between the assignment where it takes its value and the modify statement that uses the value. 18.6 A complete example *=*=*=*=*=*=*=*=*=*=*=*= This section outlines how the functions from the Unix curses library can be made available to Objective Caml programs. First of all, here is the interface curses.mli that declares the curses primitives and data types: << type window (* The type "window" remains abstract *) external initscr: unit -> window = "curses_initscr" external endwin: unit -> unit = "curses_endwin" external refresh: unit -> unit = "curses_refresh" external wrefresh : window -> unit = "curses_wrefresh" external newwin: int -> int -> int -> int -> window = "curses_newwin" external mvwin: window -> int -> int -> unit = "curses_mvwin" external addch: char -> unit = "curses_addch" external mvwaddch: window -> int -> int -> char -> unit = "curses_mvwaddch" external addstr: string -> unit = "curses_addstr" external mvwaddstr: window -> int -> int -> string -> unit = "curses_mvwaddstr" (* lots more omitted *) >> To compile this interface: << ocamlc -c curses.mli >> To implement these functions, we just have to provide the stub code; the core functions are already implemented in the curses library. The stub code file, curses.o, looks like: << #include #include value curses_initscr(value unit) { CAMLparam1 (unit); CAMLreturn ((value) initscr()); /* OK to coerce directly from WINDOW * to value since that's a block created by malloc() */ } value curses_wrefresh(value win) { CAMLparam1 (win); wrefresh((WINDOW *) win); CAMLreturn (Val_unit); } value curses_newwin(value nlines, value ncols, value x0, value y0) { CAMLparam4 (nlines, ncols, x0, y0); CAMLreturn ((value) newwin(Int_val(nlines), Int_val(ncols), Int_val(x0), Int_val(y0))); } value curses_addch(value c) { CAMLparam1 (c); addch(Int_val(c)); /* Characters are encoded like integers */ CAMLreturn (Val_unit); } value curses_addstr(value s) { CAMLparam1 (s); addstr(String_val(s)); CAMLreturn (Val_unit); } /* This goes on for pages. */ >> The file curses.c can be compiled with: << cc -c -I/usr/local/lib/ocaml curses.c >> or, even simpler, << ocamlc -c curses.c >> (When passed a .c file, the ocamlc command simply calls the C compiler on that file, with the right -I option.) Now, here is a sample Caml program test.ml that uses the curses module: << open Curses let main_window = initscr () in let small_window = newwin 10 5 20 10 in mvwaddstr main_window 10 2 "Hello"; mvwaddstr small_window 4 3 "world"; refresh(); for i = 1 to 100000 do () done; endwin() >> To compile this program, run: << ocamlc -c test.ml >> Finally, to link everything together: << ocamlc -custom -o test test.cmo curses.o -cclib -lcurses >> (On some machines, you may need to put -cclib -ltermcap or -cclib -lcurses -cclib -ltermcap instead of -cclib -lcurses.) 18.7 Advanced topic: callbacks from C to Caml *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= So far, we have described how to call C functions from Caml. In this section, we show how C functions can call Caml functions, either as callbacks (Caml calls C which calls Caml), or because the main program is written in C. 18.7.1 Applying Caml closures from C ===================================== C functions can apply Caml functional values (closures) to Caml values. The following functions are provided to perform the applications: - callback(f, a) applies the functional value f to the value a and return the value returned by f. - callback2(f, a, b) applies the functional value f (which is assumed to be a curried Caml function with two arguments) to a and b. - callback3(f, a, b, c) applies the functional value f (a curried Caml function with three arguments) to a, b and c. - callbackN(f, n, args) applies the functional value f to the n arguments contained in the array of values args. If the function f does not return, but raises an exception that escapes the scope of the application, then this exception is propagated to the next enclosing Caml code, skipping over the C code. That is, if a Caml function f calls a C function g that calls back a Caml function h that raises a stray exception, then the execution of g is interrupted and the exception is propagated back into f. If the C code wishes to catch exceptions escaping the Caml function, it can use the functions callback_exn, callback2_exn, callback3_exn, callbackN_exn. These functions take the same arguments as their non-_exn counterparts, but catch escaping exceptions and return them to the C code. The return value v of the callback*_exn functions must be tested with the macro Is_exception_result(v). If the macro returns ``false'', no exception occured, and v is the value returned by the Caml function. If Is_exception_result(v) returns ``true'', an exception escaped, and its value (the exception descriptor) can be recovered using Extract_exception(v). 18.7.2 Registering Caml closures for use in C functions ======================================================== The main difficulty with the callback functions described above is obtaining a closure to the Caml function to be called. For this purpose, Objective Caml provides a simple registration mechanism, by which Caml code can register Caml functions under some global name, and then C code can retrieve the corresponding closure by this global name. On the Caml side, registration is performed by evaluating Callback.register n v. Here, n is the global name (an arbitrary string) and v the Caml value. For instance: << let f x = print_string "f is applied to "; print_int n; print_newline() let _ = Callback.register "test function" f >> On the C side, a pointer to the value registered under name n is obtained by calling caml_named_value(n). The returned pointer must then be dereferenced to recover the actual Caml value. If no value is registered under the name n, the null pointer is returned. For example, here is a C wrapper that calls the Caml function f above: << void call_caml_f(int arg) { callback(*caml_named_value("test function"), Val_int(arg)); } >> The pointer returned by caml_named_value is constant and can safely be cached in a C variable to avoid repeated name lookups. On the other hand, the value pointed to can change during garbage collection and must always be recomputed at the point of use. Here is a more efficient variant of call_caml_f above that calls caml_named_value only once: << void call_caml_f(int arg) { static value * closure_f = NULL; if (closure_f == NULL) { /* First time around, look up by name */ closure_f = caml_named_value("test function"); } callback(*closure_f, Val_int(arg)); } >> 18.7.3 Registering Caml exceptions for use in C functions ========================================================== The registration mechanism described above can also be used to communicate exception identifiers from Caml to C. The Caml code registers the exception by evaluating Callback.register_exception n exn, where n is an arbitrary name and exn is an exception value of the exception to register. For example: << exception Error of string let _ = Callback.register_exception "test exception" (Error "any string") >> The C code can then recover the exception identifier using caml_named_value and pass it as first argument to the functions raise_constant, raise_with_arg, and raise_with_string (described in section 18.4.5) to actually raise the exception. For example, here is a C function that raises the Error exception with the given argument: << void raise_error(char * msg) { raise_with_string(*caml_named_value("test exception"), msg); } >> 18.7.4 Main program in C ========================= In normal operation, a mixed Caml/C program starts by executing the Caml initialization code, which then may proceed to call C functions. We say that the main program is the Caml code. In some applications, it is desirable that the C code plays the role of the main program, calling Caml functions when needed. This can be achieved as follows: - The C part of the program must provide a main function, which will override the default main function provided by the Caml runtime system. Execution will start in the user-defined main function just like for a regular C program. - At some point, the C code must call caml_main(argv) to initialize the Caml code. The argv argument is a C array of strings (type char **) which represents the command-line arguments, as passed as second argument to main. The Caml array Sys.argv will be initialized from this parameter. For the bytecode compiler, argv[0] and argv[1] are also consulted to find the file containing the bytecode. - The call to caml_main initializes the Caml runtime system, loads the bytecode (in the case of the bytecode compiler), and executes the initialization code of the Caml program. Typically, this initialization code registers callback functions using Callback.register. Once the Caml initialization code is complete, control returns to the C code that called caml_main. - The C code can then invoke Caml functions using the callback mechanism (see section 18.7.1). 18.7.5 Embedding the Caml code in the C code ============================================= The bytecode compiler in custom runtime mode (ocamlc -custom) normally appends the bytecode to the executable file containing the custom runtime. This has two consequences. First, the final linking step must be performed by ocamlc. Second, the Caml runtime library must be able to find the name of the executable file from the command-line arguments. When using caml_main(argv) as in section 18.7.4, this means that argv[0] or argv[1] must contain the executable file name. An alternative is to embed the bytecode in the C code. The -output-obj option to ocamlc is provided for this purpose. It causes the ocamlc compiler to output a C object file (.o file) containing the bytecode for the Caml part of the program, as well as a caml_startup function. The C object file produced by ocamlc -output-obj can then be linked with C code using the standard C compiler, or stored in a C library. The caml_startup function must be called from the main C program in order to initialize the Caml runtime and execute the Caml initialization code. Just like caml_main, it takes one argv parameter containing the command-line parameters. Unlike caml_main, this argv parameter is used only to initialize Sys.argv, but not for finding the name of the executable file. The native-code compiler ocamlopt also supports the -output-obj option, causing it to output a C object file containing the native code for all Caml modules on the command-line, as well as the Caml startup code. Initialization is performed by calling caml_startup as in the case of the bytecode compiler. For the final linking phase, in addition to the object file produced by -output-obj, you will have to provide the Objective Caml runtime library (libcamlrun.a for bytecode, libasmrun.a for native-code), as well as all C libraries that are required by the Caml libraries used. For instance, assume the Caml part of your program uses the Unix library. With ocamlc, you should do: << ocamlc -output-obj -o camlcode.o unix.cma other .cmo and .cma files cc -o myprog C objects and libraries \ camlcode.o -L/usr/local/lib/ocaml -lunix -lcamlrun >> With ocamlopt, you should do: << ocamlopt -output-obj -o camlcode.o unix.cmxa other .cmx and .cmxa files cc -o myprog C objects and libraries \ camlcode.o -L/usr/local/lib/ocaml -lunix -lasmrun >> Warning: On some ports, special options are required on the final linking phase that links together the object file produced by the -output-obj option and the remainder of the program. Those options are shown in the configuration file config/Makefile generated during compilation of Objective Caml, as the variables BYTECCLINKOPTS (for object files produced by ocamlc -output-obj) and NATIVECCLINKOPTS (for object files produced by ocamlopt -output-obj). Currently, the only ports that require special attention are: - Alpha under Digital Unix / Tru64 Unix with gcc: object files produced by ocamlc -output-obj must be linked with the gcc options -Wl,-T,12000000 -Wl,-D,14000000. This is not necessary for object files produced by ocamlopt -output-obj. - Windows NT: the object file produced by Objective Caml have been compiled with the /MT flag, and therefore all other object files linked with it should also be compiled with /MT. 18.8 Advanced example with callbacks *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* This section illustrates the callback facilities described in section 18.7. We are going to package some Caml functions in such a way that they can be linked with C code and called from C just like any C functions. The Caml functions are defined in the following mod.ml Caml source: << (* File mod.ml -- some ``useful'' Caml functions *) let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2) let format_result n = Printf.sprintf "Result is: %d\n" n (* Export those two functions to C *) let _ = Callback.register "fib" fib let _ = Callback.register "format_result" format_result >> Here is the C stub code for calling these functions from C: << /* File modwrap.c -- wrappers around the Caml functions */ #include #include #include #include int fib(int n) { static value * fib_closure = NULL; if (fib_closure == NULL) fib_closure = caml_named_value("fib"); return Int_val(callback(*fib_closure, Val_int(n))); } char * format_result(int n) { static value * format_result_closure = NULL; if (format_result_closure == NULL) format_result_closure = caml_named_value("format_result"); return strdup(String_val(callback(*format_result_closure, Val_int(n)))); /* We copy the C string returned by String_val to the C heap so that it remains valid after garbage collection. */ } >> We now compile the Caml code to a C object file and put it in a C library along with the stub code in modwrap.c and the Caml runtime system: << ocamlc -custom -output-obj -o modcaml.o mod.ml ocamlc -c modwrap.c cp /usr/local/lib/ocaml/libcamlrun.a mod.a ar r mod.a modcaml.o modwrap.o >> (One can also use ocamlopt -output-obj instead of ocamlc -custom -output-obj. In this case, replace libcamlrun.a (the bytecode runtime library) by libasmrun.a (the native-code runtime library).) Now, we can use the two fonctions fib and format_result in any C program, just like regular C functions. Just remember to call caml_startup once before. << /* File main.c -- a sample client for the Caml functions */ #include int main(int argc, char ** argv) { int result; /* Initialize Caml code */ caml_startup(argv); /* Do some computation */ result = fib(10); printf("fib(10) = %s\n", format_result(result)); return 0; } >> To build the whole program, just invoke the C compiler as follows: << cc -o prog main.c mod.a -lcurses >> (On some machines, you may need to put -ltermcap or -lcurses -ltermcap instead of -lcurses.) 18.9 Advanced topic: custom blocks *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=* Blocks with tag Custom_tag contain both arbitrary user data and a pointer to a C struct, with type struct custom_operations, that associates user-provided finalization, comparison, hashing, serialization and deserialization functions to this block. 18.9.1 The struct custom_operations ==================================== The struct custom_operations is defined in and contains the following fields: - char *identifier A zero-terminated character string serving as an identifier for serialization and deserialization operations. - void (*finalize)(value v) The finalize field contains a pointer to a C function that is called when the block becomes unreachable and is about to be reclaimed. The block is passed as first argument to the function. The finalize field can also be NULL to indicate that no finalization function is associated with the block. IMPORTANT NOTE: the v parameter of this function is of type value, but it must not be declared using the CAMLparam macros. - int (*compare)(value v1, value v2) The compare field contains a pointer to a C function that is called whenever two custom blocks are compared using Caml's generic comparison operators (=, <>, <=, >=, <, > and compare). The C function should return 0 if the data contained in the two blocks are structurally equal, a negative integer if the data from the first block is less than the data from the second block, and a positive integer if the data from the first block is greater than the data from the second block. NOTE: You must use CAMLparam to declare v1 and v2 and CAMLreturn to return the result. The compare field can be set to custom_compare_default; this default comparison function simply raises Failure. - long (*hash)(value v) The hash field contains a pointer to a C function that is called whenever Caml's generic hash operator (see module Hashtbl) is applied to a custom block. The C function can return an arbitrary long integer representing the hash value of the data contained in the given custom block. The hash value must be compatible with the compare function, in the sense that two structurally equal data (that is, two custom blocks for which compare returns 0) must have the same hash value. NOTE: You must use CAMLparam to declare v and CAMLreturn to return the result. The hash field can be set to custom_hash_default, in which case the custom block is ignored during hash computation. - void (*serialize)(value v, unsigned long * wsize_32, unsigned long * wsize_64) The serialize field contains a pointer to a C function that is called whenever the custom block needs to be serialized (marshaled) using the Caml functions output_value or Marshal.to_.... For a custom block, those functions first write the identifier of the block (as given by the identifier field) to the output stream, then call the user-provided serialize function. That function is responsible for writing the data contained in the custom block, using the serialize_... functions defined in and listed below. The user-provided serialize function must then store in its wsize_32 and wsize_64 parameters the sizes in bytes of the data part of the custom block on a 32-bit architecture and on a 64-bit architecture, respectively. NOTE: You must use CAMLparam to declare v and CAMLreturn to return the result. The serialize field can be set to custom_serialize_default, in which case the Failure exception is raised when attempting to serialize the custom block. - unsigned long (*deserialize)(void * dst) The deserialize field contains a pointer to a C function that is called whenever a custom block with identifier identifier needs to be deserialized (un-marshaled) using the Caml functions input_value or Marshal.from_.... This user-provided function is responsible for reading back the data written by the serialize operation, using the deserialize_... functions defined in and listed below. It must then rebuild the data part of the custom block and store it at the pointer given as the dst argument. Finally, it returns the size in bytes of the data part of the custom block. This size must be identical to the wsize_32 result of the serialize operation if the architecture is 32 bits, or wsize_64 if the architecture is 64 bits. The deserialize field can be set to custom_deserialize_default to indicate that deserialization is not supported. In this case, do not register the struct custom_operations with the deserializer using register_custom_operations (see below). 18.9.2 Allocating custom blocks ================================ Custom blocks must be allocated via the alloc_custom function. alloc_custom(ops, size, used, max) returns a fresh custom block, with room for size bytes of user data, and whose associated operations are given by ops (a pointer to a struct custom_operations, usually statically allocated as a C global variable). The two parameters used and max are used to control the speed of garbage collection when the finalized object contains pointers to out-of-heap resources. Generally speaking, the Caml incremental major collector adjusts its speed relative to the allocation rate of the program. The faster the program allocates, the harder the GC works in order to reclaim quickly unreachable blocks and avoid having large amount of ``floating garbage'' (unreferenced objects that the GC has not yet collected). Normally, the allocation rate is measured by counting the in-heap size of allocated blocks. However, it often happens that finalized objects contain pointers to out-of-heap memory blocks and other resources (such as file descriptors, X Windows bitmaps, etc.). For those blocks, the in-heap size of blocks is not a good measure of the quantity of resources allocated by the program. The two arguments used and max give the GC an idea of how much out-of-heap resources are consumed by the finalized block being allocated: you give the amount of resources allocated to this object as parameter used, and the maximum amount that you want to see in floating garbage as parameter max. The units are arbitrary: the GC cares only about the ratio used / max. For instance, if you are allocating a finalized block holding an X Windows bitmap of w by h pixels, and you'd rather not have more than 1 mega-pixels of unreclaimed bitmaps, specify used = w * h and max = 1000000. Another way to describe the effect of the used and max parameters is in terms of full GC cycles. If you allocate many custom blocks with used / max = 1 / N, the GC will then do one full cycle (examining every object in the heap and calling finalization functions on those that are unreachable) every N allocations. For instance, if used = 1 and max = 1000, the GC will do one full cycle at least every 1000 allocations of custom blocks. If your finalized blocks contain no pointers to out-of-heap resources, or if the previous discussion made little sense to you, just take used = 0 and max = 1. But if you later find that the finalization functions are not called ``often enough'', consider increasing the used / max ratio. 18.9.3 Accessing custom blocks =============================== The data part of a custom block v can be accessed via the pointer Data_custom_val(v). This pointer has type void * and should be cast to the actual type of the data stored in the custom block. The contents of custom blocks are not scanned by the garbage collector, and must therefore not contain any pointer inside the Caml heap. In other terms, never store a Caml value in a custom block, and do not use Field, Store_field nor modify to access the data part of a custom block. Conversely, any C data structure (not containing heap pointers) can be stored in a custom block. 18.9.4 Writing custom serialization and deserialization functions ================================================================== The following functions, defined in , are provided to write and read back the contents of custom blocks in a portable way. Those functions handle endianness conversions when e.g. data is written on a little-endian machine and read back on a big-endian machine. -------------------------------------------------- | Function | Action | -------------------------------------------------- |serialize_int_1 |Write a 1-byte integer |serialize_int_2 |Write a 2-byte integer |serialize_int_4 |Write a 4-byte integer |serialize_int_8 |Write a 8-byte integer |serialize_float_4 |Write a 4-byte float |serialize_float_8 |Write a 8-byte float |serialize_block_1 |Write an array of 1-byte | |quantities |serialize_block_2 |Write an array of 2-byte | |quantities |serialize_block_4 |Write an array of 4-byte | |quantities |serialize_block_8 |Write an array of 8-byte | |quantities |deserialize_uint_1 |Read an unsigned 1-byte | |integer |deserialize_sint_1 |Read a signed 1-byte integer | | |deserialize_uint_2 |Read an unsigned 2-byte | |integer |deserialize_sint_2 |Read a signed 2-byte integer | | |deserialize_uint_4 |Read an unsigned 4-byte | |integer |deserialize_sint_4 |Read a signed 4-byte integer | | |deserialize_uint_8 |Read an unsigned 8-byte | |integer |deserialize_sint_8 |Read a signed 8-byte integer | | |deserialize_float_4|Read a 4-byte float |deserialize_float_8|Read an 8-byte float |deserialize_block_1|Read an array of 1-byte | |quantities |deserialize_block_2|Read an array of 2-byte | |quantities |deserialize_block_4|Read an array of 4-byte | |quantities |deserialize_block_8|Read an array of 8-byte | |quantities |deserialize_error |Signal an error during | |deserialization; input_value | |or Marshal.from_... raise a | |Failure exception after | |cleaning up their internal | |data structures -------------------------------------------------- Serialization functions are attached to the custom blocks to which they apply. Obviously, deserialization functions cannot be attached this way, since the custom block does not exist yet when deserialization begins! Thus, the struct custom_operations that contain deserialization functions must be registered with the deserializer in advance, using the register_custom_operations function declared in . Deserialization proceeds by reading the identifier off the input stream, allocating a custom block of the size specified in the input stream, searching the registered struct custom_operation blocks for one with the same identifier, and calling its deserialize function to fill the data part of the custom block. 18.9.5 Choosing identifiers ============================ Identifiers in struct custom_operations must be chosen carefully, since they must identify uniquely the data structure for serialization and deserialization operations. In particular, consider including a version number in the identifier; this way, the format of the data can be changed later, yet backward-compatible deserialisation functions can be provided. Identifiers starting with _ (an underscore character) are reserved for the Objective Caml runtime system; do not use them for your custom data. We recommend to use a URL (http://mymachine.mydomain.com/mylibrary/version-number) or a Java-style package name (com.mydomain.mymachine.mylibrary.version-number) as identifiers, to minimize the risk of identifier collision. 18.9.6 Finalized blocks ======================== Custom blocks generalize the finalized blocks that were present in Objective Caml prior to version 3.00. For backward compatibility, the format of custom blocks is compatible with that of finalized blocks, and the alloc_final function is still available to allocate a custom block with a given finalization function, but default comparison, hashing and serialization functions. alloc_final(n, f, used, max) returns a fresh custom block of size n words, with finalization function f. The first word is reserved for storing the custom operations; the other n-1 words are available for your data. The two parameters used and max are used to control the speed of garbage collection, as described for alloc_custom. 18.10 Building mixed C/Caml libraries: ocamlmklib *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= The ocamlmklib command facilitates the construction of libraries containing both Caml code and C code, and usable both in static linking and dynamic linking modes. Windows: This command is available only under Cygwin, but not for the native Win32 port. MacOS: This command is not available. The ocamlmklib command takes three kinds of arguments: - Caml source files and object files (.cmo, .cmx, .ml) comprising the Caml part of the library; - C object files (.o, .a) comprising the C part of the library; - Support libraries for the C part (-llib). It generates the following outputs: - A Caml bytecode library .cma incorporating the .cmo and .ml Caml files given as arguments, and automatically referencing the C library generated with the C object files. - A Caml native-code library .cmxa incorporating the .cmx and .ml Caml files given as arguments, and automatically referencing the C library generated with the C object files. - If dynamic linking is supported on the target platform, a .so shared library built from the C object files given as arguments, and automatically referencing the support libraries. - A C static library .a built from the C object files. In addition, the following options are recognized: -cclib, -ccopt, -I, -linkall These options are passed as is to ocamlc or ocamlopt. See the documentation of these commands. -pthread, -rpath, -R, -Wl,-rpath, -Wl,-R These options are passed as is to the C compiler. Refer to the documentation of the C compiler. -custom Force the construction of a statically linked library only, even if dynamic linking is supported. -failsafe Fall back to building a statically linked library if a problem occurs while building the shared library (e.g. some of the support libraries are not available as shared libraries). -Ldir Add dir to the search path for support libraries (-llib). -ocamlc cmd Use cmd instead of ocamlc to call the bytecode compiler. -ocamlopt cmd Use cmd instead of ocamlopt to call the native-code compiler. -o output Set the name of the generated Caml library. ocamlmklib will generate output.cma and/or output.cmxa. If not specified, defaults to a. -oc outputc Set the name of the generated C library. ocamlmklib will generate liboutputc.so (if shared libraries are supported) and liboutputc.a. If not specified, defaults to the output name given with -o. Example Consider a Caml interface to the standard libz C library for reading and writing compressed files. Assume this library resides in /usr/local/zlib. This interface is composed of a Caml part zip.cmo/zip.cmx and a C part zipstubs.o containing the stub code around the libz entry points. The following command builds the Caml libraries zip.cma and zip.cmxa, as well as the companion C libraries dllzip.so and libzip.a: << ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib >> If shared libraries are supported, this performs the following commands: << ocamlc -a -o zip.cma zip.cmo -dllib -lzip \ -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \ -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib gcc -shared -o dllzip.so zipstubs.o -lz -L/usr/local/zlib ar rc libzip.a zipstubs.o >> If shared libraries are not supported, the following commands are performed instead: << ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \ -cclib -lz -ccopt -L/usr/local/zlib ocamlopt -a -o zip.cmxa zip.cmx -lzip \ -cclib -lz -ccopt -L/usr/local/zlib ar rc libzip.a zipstubs.o >> Instead of building simultaneously the bytecode library, the native-code library and the C libraries, ocamlmklib can be called three times to build each separately. Thus, << ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib >> builds the bytecode library zip.cma, and << ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib >> builds the native-code library zip.cmxa, and << ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib >> builds the C libraries dllzip.so and libzip.a. Notice that the support libraries (-lz) and the corresponding options (-L/usr/local/zlib) must be given on all three invocations of ocamlmklib, because they are needed at different times depending on whether shared libraries are supported. Part: IV ******** The Objective Caml library ************************** Chapter 19 The core library ****************************** This chapter describes the Objective Caml core library, which is composed of declarations for built-in types and exceptions, plus the module Pervasives that provides basic operations on these built-in types. The Pervasives module is special in two ways: - It is automatically linked with the user's object code files by the ocamlc command (chapter 8). - It is automatically ``opened'' when a compilation starts, or when the toplevel system is launched. Hence, it is possible to use unqualified identifiers to refer to the functions provided by the Pervasives module, without adding a open Pervasives directive. Conventions *=*=*=*=*=* The declarations of the built-in types and the components of module Pervasives are printed one by one in typewriter font, followed by a short comment. All library modules and the components they provide are indexed at the end of this report. 19.1 Built-in types and predefined exceptions *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= The following built-in types and predefined exceptions are always defined in the compilation environment, but are not part of any module. As a consequence, they can only be referred by their short names. Built-in types ============== << type int >> The type of integer numbers. << type char >> The type of characters. << type string >> The type of character strings. << type float >> The type of floating-point numbers. << type bool = false | true >> The type of booleans (truth values). << type unit = () >> The type of the unit value. << type exn >> The type of exception values. << type 'a array >> The type of arrays whose elements have type 'a. << type 'a list = [] | :: of 'a * 'a list >> The type of lists whose elements have type 'a. << type 'a option = None | Some of 'a >> The type of optional values of type 'a. << type ('a, 'b, 'c) format >> The type of format strings. 'a is the type of the parameters of the format, 'c is the result type for the printf-style function, and 'b is the type of the first argument given to \%a and \%t printing functions (see module Printf[20.24]). << type 'a lazy_t >> This type is used to implement the Lazy[20.15] module. It should not be used directly. Predefined exceptions ===================== << exception Match_failure of (string * int * int) >> Exception raised when none of the cases of a pattern-matching apply. The arguments are the location of the pattern-matching in the source code (file name, position of first character, position of last character). << exception Assert_failure of (string * int * int) >> Exception raised when an assertion fails. The arguments are the location of the pattern-matching in the source code (file name, position of first character, position of last character). << exception Invalid_argument of string >> Exception raised by library functions to signal that the given arguments do not make sense. << exception Failure of string >> Exception raised by library functions to signal that they are undefined on the given arguments. << exception Not_found >> Exception raised by search functions when the desired object could not be found. << exception Out_of_memory >> Exception raised by the garbage collector when there is insufficient memory to complete the computation. << exception Stack_overflow >> Exception raised by the bytecode interpreter when the evaluation stack reaches its maximal size. This often indicates infinite or excessively deep recursion in the user's program. (Not fully implemented by the native-code compiler; see section 11.4.) << exception Sys_error of string >> Exception raised by the input/output functions to report an operating system error. << exception End_of_file >> Exception raised by input functions to signal that the end of file has been reached. << exception Division_by_zero >> Exception raised by division and remainder operations when their second argument is null. (Not fully implemented by the native-code compiler; see section 11.4.) << exception Sys_blocked_io >> A special case of Sys_error raised when no I/O is possible on a non-blocking I/O channel. 19.2 Module Pervasives : The initially opened module. *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*= This module provides the basic operations over the built-in types (numbers, booleans, strings, exceptions, references, lists, arrays, input-output channels, ...) This module is automatically opened at the beginning of each compilation. All components of this module can therefore be referred by their short name, without prefixing them by Pervasives. Exceptions ========== << val raise : exn -> 'a >> Raise the given exception value << val invalid_arg : string -> 'a >> Raise exception Invalid_argument with the given string. << val failwith : string -> 'a >> Raise exception Failure with the given string. << exception Exit >> The Exit exception is not raised by any library function. It is provided for use in your programs. Comparisons =========== << val (=) : 'a -> 'a -> bool >> e1 = e2 tests for structural equality of e1 and e2. Mutable structures (e.g. references and arrays) are equal if and only if their current contents are structurally equal, even if the two mutable objects are not the same physical object. Equality between functional values may raise Invalid_argument. Equality between cyclic data structures may not terminate. << val (<>) : 'a -> 'a -> bool >> Negation of Pervasives.(