Objective Caml

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Objective Caml
Paradigm multi-paradigm: imperative, functional, object-oriented
Developer INRIA
Stable release 3.11.2 (January 20, 2010; 368795490 ago)
Typing discipline static, strong, inferred
Dialects F#, JoCaml, MetaOCaml, OcamlP3l
Influenced by Caml Light, Standard ML
Influenced Scala, ATS
OS Cross-platform
License Q Public License (compiler)
LGPL (library)
Website http://caml.inria.fr/

Objective Caml, or OCaml (pronounced /oʊˈkæməl/ oh-KAM-əl) 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 a free 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.



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. This means the program author is not required to be overly familiar with pure functional language paradigm in order to use OCaml.

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 explicitly 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 runtime type and safety checks that burden the performance of dynamically typed languages, while still guaranteeing runtime safety (except when array bounds checking is turned off, or when certain type-unsafe features like serialization are used; these are rare enough that avoiding them is quite possible in practice).

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"[1], and benchmarks have shown that this is generally the case[2]. Some functions in the OCaml standard library are implemented with faster algorithms than equivalent functions in the standard libraries of other languages. For example, the implementation of set union in the OCaml standard library is asymptotically faster than the equivalent function in the standard libraries of imperative languages (e.g. C++, Java) because the OCaml implementation exploits the immutability of sets in order to reuse parts of input sets in the output (persistence).


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 to C primitives is provided, including language support for efficient numerical arrays in formats compatible with both C and FORTRAN. OCaml also supports the creation of libraries of OCaml functions that can be linked to a "main" program in C, so that one could distribute an OCaml library to C programmers who have no knowledge nor installation of OCaml.

The OCaml distribution contains:

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: IA-32, IA-64, AMD64, HP/PA; PowerPC, SPARC, Alpha, MIPS, and StrongARM.

OCaml bytecode and native code programs can be written in a multithreaded style, with preemptive context switching. However, because the garbage collector is not designed for concurrency, symmetric multiprocessing is not supported[3]. OCaml threads in the same process execute by time sharing only.

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:

$ ocamlc hello.ml -o hello

and executed:

$ ./hello
Hello world!

Summing a list of integers

Lists are one of the most fundamental datatypes in OCaml. The following code example defines a recursive function sum that accepts one argument xs. Function recursively iterates over a given list and provides a sum of integer elements. Match statement has similarities with C++ or Java languages' switch element.

let rec sum xs =
  match xs with
    | [] -> 0
    | x :: xs' -> x + sum xs'
 # sum [1;2;3;4;5];;
 - : int = 15

Another way is to use standard fold function which works with lists.

let sum xs =
    List.fold_left (+) 0 xs;;
 # sum [1;2;3;4;5];;
 - : int = 15


Ocaml lends itself to the concise expression of recursive algorithms. The following code example implements the quicksort algorithm to sort a list into increasing order.

 let rec quicksort = function
   | [] -> []
   | pivot :: rest ->
       let is_less x = x < pivot in
       let left, right = List.partition is_less rest in
       quicksort left @ [pivot] @ quicksort right

Birthday paradox

The following program 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).

 let year_size = 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)
         birthday_paradox prob' (people+1) ;;
 birthday_paradox 1.0 1;;

Church numerals

The following code defines a Church encoding of natural numbers, with successor (succ) and addition (add). A Church numeral n is a higher-order function that accepts a function f and a value x and applies f to x exactly n times. To convert a Church numeral from a functional value to a string, we pass it a function which prepends the string "S" to its input and the constant string "0".

let zero f x = x
let succ n f x = f (n f x)
let one = succ zero
let two = succ (succ zero)
let add n1 n2 f x = n1 f (n2 f x)
let to_string n = n (fun k -> "S" ^ k) "0"
let _ = to_string (add (succ two) two)

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 numeric 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 =

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 angle t = 10. *. t *. t in
   let render () =
     GlClear.clear [ `color ];
     GlMat.load_identity ();
     GlMat.rotate ~angle: (angle (Sys.time ())) ~z:1. ();
     GlDraw.begins `triangles;
     List.iter GlDraw.vertex2 [-1., -1.; 0., 1.; 1., -1.];
     GlDraw.ends ();
     Glut.swapBuffers () in
   GlMat.mode `modelview;
   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.

Fibonacci Sequence

The following code calculates the Fibonacci sequence of a number n inputed. It uses tail recursion and pattern matching.

let rec fib_aux (n, a, b) =
  match (n, a, b) with
  | (0, a, b) -> a
  | _ -> fib_aux (n - 1, a + b, a)
let fib n = fib_aux (n, 0, 1)

It is more likely written as:

let fib n =
  let rec fib_aux (n, a, b) =
    match (n, a, b) with
    | (0, a, b) -> a
    | _ -> fib_aux (n - 1, a + b, a)
  fib_aux (n, 0, 1)

Derived languages


MetaOCaml[4] 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>.
     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

  • AtomCaml provides a synchronization primitive for atomic (transactional) execution of code.
  • "Emily is a subset of OCaml that uses a design rule verifier to enforce object-capability [security] principles."
  • F# is a Microsoft .NET language based on OCaml.
  • Fresh OCaml facilitates the manipulation of names and binders.
  • GCaml adds extensional polymorphism to OCaml, thus allowing overloading and type-safe marshalling.
  • JoCaml integrates constructions for developing concurrent and distributed programs.
  • OCamlDuce extends OCaml with features such as XML expressions and regular-expression types.
  • OCamlP3l is a parallel programming system based on OCaml and the P3L language

Software written in OCaml

  • FFTW – a software library for computing discrete Fourier transforms. Several C routines have been generated by an OCaml program named genfft.
  • Unison – a file synchronization program to synchronize files between two directories.
  • Mldonkey – a peer to peer client based on the EDonkey network.
  • GeneWeb – free open source multi-platform genealogy software.
  • The haXe compiler – a free open source compiler for the haXe programming language.
  • Frama-c – a framework for C programs analysis.
  • Liquidsoap – Liquidsoap is the audio stream generator of the Savonet project, notably used for generating the stream of netradios. [1]
  • Coccinelle – Coccinelle is a program matching and transformation engine which provides the SmPL language (Semantic Patch Language) for specifying desired matches and transformations in C code. [2]

See also


  1. Linux Weekly News.
  2. The Computer Language Benchmarks Game.
  3. Xavier Leroy's "standard lecture" on threads
  4. MetaOCaml.

External links

cs:OCaml de:Objective CAML el:OCaml es:Ocaml fr:Objective Caml gl:Ocaml ko:Objective Caml it:Objective Caml ka:ობიექტური კამლი la:Ocaml nl:Ocaml ja:Objective Caml no:OCaml nn:OCaml pl:OCaml pt:OCaml ru:OCaml sl:Ocaml tg:OCaml tr:Ocaml uk:Objective Caml vi:OCaml zh:OCaml

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