OCaml

Objective Caml

File:Caml.128x58.gif
Paradigm	multi-paradigm: imperative, functional, object-oriented
Developer	INRIA
Stable release	3.09.3 / September 15, 2006
OS	Cross-platform
License	Q Public License (compiler) LGPL (library)
Website	http://caml.inria.fr/
Dialects
JoCaml, Metaocaml, Ocamlp3l
Influenced by
Caml Light, Standard ML

Objective Caml (OCaml) is the main implementation of the Caml programming language, created by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy and others in 1996. OCaml is an open source project managed and principally maintained by INRIA.

OCaml extends the core Caml language with object-oriented constructs.

OCaml's toolset includes an interactive toplevel interpreter, a bytecode compiler, and an optimizing native code compiler. It has a large standard library that makes it useful for many of the same applications as Python or Perl, as well as robust modular and object-oriented programming constructs that make it applicable for large-scale software engineering.

OCaml is the successor to Caml Light. The acronym CAML originally stood for Categorical Abstract Machine Language, although OCaml abandons this abstract machine.

Philosophy

ML-derived languages are best known for their static type systems and type-inferring compilers. OCaml unifies functional, imperative, and object-oriented programming under an ML-like type system.

OCaml's static type system eliminates a large class of programmer errors that may cause problems at runtime. However, it also forces the programmer to conform to the constraints of the type system, which can require careful thought and close attention. A type-inferring compiler greatly reduces the need for manual type annotations (for example, the data type of variables and the signature of functions usually do not need to be expressly declared, as they do in Java). Nonetheless, effective use of OCaml's type system can require some sophistication on the part of the programmer.

OCaml is perhaps most distinguished from other languages with origins in academia by its emphasis on performance. Firstly, its static type system renders runtime type mismatches impossible, and thus obviates the need for runtime type and safety checks that burden the performance of dynamically typed languages, while still guaranteeing runtime safety.

Aside from type-checking overhead, functional programming languages are, in general, challenging to compile to efficient machine language code, due to issues such as the funarg problem. In addition to standard loop, register, and instruction optimizations, OCaml's optimizing compiler employs static program analysis techniques to optimize value boxing and closure allocation, helping to maximize the performance of the resulting code even if it makes extensive use of functional programming constructs.

Xavier Leroy has cautiously stated that "OCaml delivers at least 50% of the performance of a decent C compiler", and benchmarks have shown that this is generally the case.

Features

OCaml features: a static type system, type inference, parametric polymorphism, tail recursion, pattern matching, first class lexical closures, functors (parametric modules), exception handling, and incremental generational automatic garbage collection.

OCaml is particularly notable for extending ML-style type inference to an object system in a general purpose language. This permits structural subtyping, where object types are compatible if their method signatures are compatible, regardless of their declared inheritance; an unusual feature in statically-typed languages.

A foreign function interface for linking with C primitives is provided, including language support for efficient numerical arrays in formats compatible with both C and FORTRAN.

The OCaml distribution contains:

• An extensible parser and macro language named Camlp4, which permits the syntax of OCaml to be extended or even replaced
• Lexer and parser tools called ocamllex and ocamlyacc
• Debugger which supports stepping backwards to investigate errors
• Documentation generator
• Profiler — for measuring performance
• Numerous general purpose libraries

The native code compiler is available for many platforms, including Unix, Microsoft Windows, and Apple Mac OS X. Excellent portability is ensured through native code generation support for major architectures: IA32, AMD64, PowerPC, Sparc, IA64, Alpha, HP/PA, MIPS, and StrongARM.

OCaml bytecode and native code programs can be written in a multithreaded style. However, because the garbage collector is not designed for concurrency, multiple OCaml threads in the same process cannot run concurrently.

Code examples

Snippets of OCaml code are most easily studied by entering them into the "top-level". This is an interactive OCaml session that prints the inferred types of resulting or defined expressions. The OCaml top-level is started by simply executing the "ocaml" program:

  $ ocaml
       Objective Caml version 3.09.0

  #

Code can then be entered at the "#" prompt. For example, to calculate 1+2*3:

  # 1 + 2 * 3;;
  - : int = 7

OCaml infers the type of the expression to be "int" (a machine-precision integer) and gives the result "7".

Hello World

The following program "hello.ml":

print_endline "Hello world!";;

can be compiled to bytecode:

$ ocamlc hello.ml -o hello

and executed:

$ ./hello
Hello world!
$

Birthday paradox

OCaml may be used as a scripting language, as the following script calculates the smallest number of people in a room for whom the probability of completely unique birthdays is less than 50% (the so-called birthday paradox, where for 1 person the probability is obviously 100%, for 2 it is 364/365, etc.) (answer = 23).

On a Unix-like operating system, save it to a file, chmod to executable (chmod 0755 birthday.ml) and run it from the command line (./birthday.ml).

#!/usr/bin/ocamlrun ocaml

let year_size = float(365) ;;

let rec birthday_paradox prob people =
    let prob' = (year_size -. float(people)) /. year_size *. prob  in
    if prob' < 0.5 then
        Printf.printf "answer = %d\n" (people+1)
    else
        birthday_paradox prob' (people+1) ;;

birthday_paradox 1.0 1

Arbitrary-precision factorial function (libraries)

A variety of libraries are directly accessible from OCaml. For example, OCaml has a built-in library for arbitrary precision arithmetic. As the factorial function grows very rapidly, it quickly overflows machine-precision numbers (typically 32- or 64-bits). Thus, factorial is a suitable candidate for arbitrary-precision arithmetic.

In OCaml, the Num module provides arbitrary-precision arithmetic and can be loaded into a running top-level using:

  # #load "nums.cma";;
  # open Num;;

The factorial function may then be written using the arbitrary-precision numbers operators =/, */ and -/ :

  # let rec fact n =
      if n =/ Int 0 then Int 1 else n */ fact(n -/ Int 1);;
  val fact : Num.num -> Num.num = <fun>

This function can compute much larger factorials, such as 120!:

  # string_of_num (fact (Int 120));;
  - : string =
  "6689502913449127057588118054090372586752746333138029810295671352301633
  55724496298936687416527198498130815763789321409055253440858940812185989
  8481114389650005964960521256960000000000000000000000000000"

Triangle (graphics)

The following program "simple.ml" renders a rotating triangle in 2D using OpenGL:

  let _ =
    ignore( Glut.init Sys.argv );
    Glut.initDisplayMode ~double_buffer:true ();
    ignore (Glut.createWindow ~title:"OpenGL Demo");
    let render () =
      GlClear.clear [ `color ];
      GlMat.rotate ~angle:(Sys.time() *. 0.01) ~z:1. ();
      GlDraw.begins `triangles;
      List.iter GlDraw.vertex2 [-1., -1.; 0., 1.; 1., -1.];
      GlDraw.ends ();
      Glut.swapBuffers () in
    Glut.displayFunc ~cb:render;
    Glut.idleFunc ~cb:(Some Glut.postRedisplay);
    Glut.mainLoop ()

The LablGL bindings to OpenGL are required. The program may then be compiled to bytecode with:

  $ ocamlc -I +lablGL lablglut.cma lablgl.cma simple.ml -o simple

or to nativecode with:

  $ ocamlopt -I +lablGL lablglut.cmxa lablgl.cmxa simple.ml -o simple

and run:

  $ ./simple

Far more sophisticated, high-performance 2D and 3D graphical programs are easily developed in OCaml. Thanks to the use of OpenGL, the resulting programs are not only succinct and efficient but also cross-platform, compiling without any changes on all major platforms.

Derived languages

MetaOCaml

MetaOCaml is a multi-stage programming extension of OCaml enabling incremental compiling of new machine code during runtime. Under certain circumstances, significant speedups are possible using multi-stage programming, because more detailed information about the data to process is available at runtime than at the regular compile time, so the incremental compiler can optimize away many cases of condition checking etc.

As an example: if at compile time it is known that a certain power function x -> x^n is needed very frequently, but the value of n is known only at runtime, you can use a two-stage power function in MetaOCaml:

let rec power n x =
  if n = 0
  then .<1>.
  else
    if even n
    then sqr (power (n/2) x)
    else .<.~x *. ~(power (n-1) x)>.;;

As soon as you know n at runtime, you can create a specialized and very fast power function:

.<fun x -> .~(power 5 .<x>.)>.;;

The result is:

fun x_1 -> (x_1 *
    let y_3 = 
        let y_2 = (x_1 * 1)
        in (y_2 * y_2)
    in (y_3 * y_3))

The new function is automatically compiled.

Other derived languages

• Fresh OCaml which facilitate the manipulation of names and binders,
• JoCaml which integrates constructions for developing concurrent and distributed programs,
• GCaml adding extensional polymorphism to OCaml, thus allowing overloading and type-safe marshalling,
• HDCaml, a hardware design and verification language,
• AtomCaml provides a synchronization primitive for atomic (transactional) execution of code.
• OCamlDuce extends OCaml with features such as XML expressions and regular-expression types.

See also

• Categorical Abstract Machine Language, the language that preceded OCaml
• Standard ML, a related language
• F Sharp, an OCaml-like language for Microsoft .NET
• Extensible ML, a different kind of object-oriented extension to ML
• O'Haskell an object-oriented extension to Haskell, a different functional language

External links

• Caml language family official website
  • OCaml libraries
  • Books about Caml
• OCaml tutorial for C, C++, Java and Perl programmers
• A basic OCaml tutorial
• OCamIL, an OCaml compiler for Microsoft .NET
• Comparison of the speed of various languages including Ocaml
• LablGL and LablGTK OpenGL+ bindings (LablGL) and GTK+ bindings (LablGTK)
• Newest Ocaml Projects on Sourceforge
• OCaml code wiki on CodeCodex

Books

• The Objective Caml system by Xavier Leroy (with Damien Doligez, Jacques Garrigue, Didier Rémy and Jérôme Vouillon).
• OCaml for Scientists by Jon Harrop
• Developing Applications With Objective Caml by Emmanuel Chailloux, Pascal Manoury and Bruno Pagano.
• Introduction to the Objective Caml Programming Language by Jason Hickey.
• Practical OCaml by Joshua Smith.