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OCaml ( , formerly Objective Caml) is a general-purpose,
multi-paradigm programming language Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
which extends the Caml dialect of ML with
object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
features. OCaml was created in 1996 by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy, Ascánder Suárez, and others. The OCaml toolchain includes an interactive top-level interpreter, a
bytecode Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (norma ...
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
, an optimizing
native code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a ver ...
compiler, a reversible debugger, and a package manager (OPAM). OCaml was initially developed in the context of
automated theorem proving Automated theorem proving (also known as ATP or automated deduction) is a subfield of automated reasoning and mathematical logic dealing with proving mathematical theorems by computer programs. Automated reasoning over mathematical proof was a ...
, and has an outsize presence in static analysis and
formal methods In computer science, formal methods are mathematically rigorous techniques for the specification, development, and verification of software and hardware systems. The use of formal methods for software and hardware design is motivated by the exp ...
software. Beyond these areas, it has found serious use in systems programming,
web development Web development is the work involved in developing a website for the Internet (World Wide Web) or an intranet (a private network). Web development can range from developing a simple single static page of plain text to complex web applications ...
, and financial engineering, among other application domains. The acronym ''CAML'' originally stood for ''Categorical Abstract Machine Language'', but OCaml omits this
abstract machine An abstract machine is a computer science theoretical model that allows for a detailed and precise analysis of how a computer system functions. It is analogous to a mathematical function in that it receives inputs and produces outputs based on pr ...
. OCaml is a
free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...
project managed and principally maintained by the French Institute for Research in Computer Science and Automation (Inria). In the early 2000s, elements from OCaml were adopted by many languages, notably F# and Scala.


Philosophy

ML-derived languages are best known for their static
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
s and type-inferring compilers. OCaml unifies
functional Functional may refer to: * Movements in architecture: ** Functionalism (architecture) ** Form follows function * Functional group, combination of atoms within molecules * Medical conditions without currently visible organic basis: ** Functional sy ...
, imperative, and
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
under an ML-like type system. Thus, programmers need not be highly familiar with the pure functional language paradigm to use OCaml. By requiring the programmer to work within the constraints of its static type system, OCaml eliminates many of the type-related runtime problems associated with dynamically typed languages. Also, OCaml's type-inferring compiler greatly reduces the need for the manual type annotations that are required in most statically typed languages. For example, the
data type In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
s of variables and the
signature A signature (; from la, signare, "to sign") is a Handwriting, handwritten (and often Stylization, stylized) depiction of someone's name, nickname, or even a simple "X" or other mark that a person writes on documents as a proof of identity and ...
s of functions usually need not be declared explicitly, as they do in languages like
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
and C#, because they can be inferred from the operators and other functions that are applied to the variables and other values in the code. Effective use of OCaml's type system can require some sophistication on the part of a programmer, but this discipline is rewarded with reliable, high-performance software. OCaml is perhaps most distinguished from other languages with origins in academia by its emphasis on performance. Its static type system prevents runtime type mismatches 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 some type-unsafe features like
serialization In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
are used. These are rare enough that avoiding them is quite possible in practice. Aside from type-checking overhead,
functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions tha ...
languages are, in general, challenging to compile to efficient machine language code, due to issues such as the
funarg problem In computer science, the funarg problem ''(function argument problem)'' refers to the difficulty in implementing first-class functions (functions as first-class objects) in programming language implementations so as to use stack-based memory all ...
. Along with standard loop, register, and instruction optimizations, OCaml's optimizing compiler employs
static program analysis In computer science, static program analysis (or static analysis) is the analysis of computer programs performed without executing them, in contrast with dynamic program analysis, which is performed on programs during their execution. The term ...
methods to optimize value
boxing Boxing (also known as "Western boxing" or "pugilism") is a combat sport in which two people, usually wearing protective gloves and other protective equipment such as hand wraps and mouthguards, throw punches at each other for a predetermined ...
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 stated that "OCaml delivers at least 50% of the performance of a decent C compiler",Linux Weekly News
although a direct comparison is impossible. 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 in theory 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 to reuse parts of input sets in the output (see
persistent data structure In computing, a persistent data structure or not ephemeral data structure is a data structure that always preserves the previous version of itself when it is modified. Such data structures are effectively immutable, as their operations do not (v ...
).


History


Development of ML

Between the 1970s and 1980s, Robin Milner, a British computer scientist and
Turing Award The ACM A. M. Turing Award is an annual prize given by the Association for Computing Machinery (ACM) for contributions of lasting and major technical importance to computer science. It is generally recognized as the highest distinction in compu ...
winner, worked at the
University of Edinburgh The University of Edinburgh ( sco, University o Edinburgh, gd, Oilthigh Dhùn Èideann; abbreviated as ''Edin.'' in post-nominals) is a public research university based in Edinburgh, Scotland. Granted a royal charter by King James VI in 1 ...
's
Laboratory for Foundations of Computer Science The Laboratory for Foundations of Computer Science (LFCS) is a research institute within the School of Informatics at the University of Edinburgh, in Scotland. It was founded in 1987 and is a community of theoretical computer scientists with ...
. Milner and others were working on theorem provers, which were historically developed in languages such as
Lisp A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech. Types * A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispin ...
. Milner repeatedly ran into the issue that the theorem provers would attempt to claim a
proof Proof most often refers to: * Proof (truth), argument or sufficient evidence for the truth of a proposition * Alcohol proof, a measure of an alcoholic drink's strength Proof may also refer to: Mathematics and formal logic * Formal proof, a c ...
was valid by putting non-proofs together. As a result, he went on to develop the
meta language In logic and linguistics, a metalanguage is a language used to describe another language, often called the ''object language''. Expressions in a metalanguage are often distinguished from those in the object language by the use of italics, quo ...
for his Logic for Computable Functions, a language that would only allow the writer to construct valid proofs with its polymorphic type system. ML was turned into a
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs tha ...
in order to simplify using LCF on different machines, and, by the 1980s, was turned into a complete system of its own. ML would eventually serve as a basis for the creation of OCaml. In the early 1980s, there were some developments that prompted
INRIA The National Institute for Research in Digital Science and Technology (Inria) () is a French national research institution focusing on computer science and applied mathematics. It was created under the name ''Institut de recherche en informatiq ...
's Formel team to become interested in the ML language. Luca Cardelli, a research professor at
University of Oxford , mottoeng = The Lord is my light , established = , endowment = £6.1 billion (including colleges) (2019) , budget = £2.145 billion (2019–20) , chancellor ...
, used his Functional Abstract Machine to develop a faster implementation of ML, and Robin Milner proposed a new definition of ML in order to avoid divergence between various implementations. Simultaneously,
Pierre-Louis Curien Pierre-Louis or Pierre Louis is a given name and a surname. Notable people with the name include: Given name * Pierre-Louis Bentabole (1756–1798), revolutionary Frenchman * Pierre-Louis Billaudèle (1796–1869), priest from, and educated in, Fra ...
, a senior researcher at Paris Diderot University, developed a calculus of categorical combinators and linked it to
lambda calculus Lambda calculus (also written as ''λ''-calculus) is a formal system in mathematical logic for expressing computation based on function abstraction and application using variable binding and substitution. It is a universal model of computation th ...
, which led to the definition of the Categorical Abstract Machine (CAM). Guy Cousineau, a researcher at Paris Diderot University, recognized that this could be applied as a compilation technique for ML.


First implementation

Caml was initially designed and developed by INRIA's Formel team headed by Gérard Huet. The first implementation of Caml was created in 1987 and was further developed until 1992. Though it was spearheaded by Ascánder Suárez, Pierre Weis and Michel Mauny carried on with development after he left in 1988. Guy Cousineau is quoted recalling that his experience with programming language implementation was initially very limited, and that there were multiple inadequacies for which he is responsible. Despite this, he believes that "Ascander, Pierre and Michel did quite a nice piece of work.”


Caml Light

Between 1990 and 1991, Xavier Leroy designed a new implementation of Caml based on a bytecode interpreter written in C. In addition to this, Damien Doligez wrote a memory management system, also known as a sequential garbage collector, for this implementation. This new implementation, known as Caml Light, replaced the old Caml implementation and ran on small desktop machines. In the following years, libraries such as Michel Mauny's syntax manipulation tools appeared and helped promote the use of Caml in educational and research teams.


Caml Special Light

In 1995, Xavier Leroy released Caml Special Light, which was an improved version of Caml. An optimizing native-code compiler was added to the bytecode compiler, which greatly increased performance to comparable levels with mainstream languages such as C++. Additionally, Leroy designed a high-level module system inspired by the module system of Standard ML which provided powerful facilities for abstraction and parameterization and made larger-scale programs easier to construct.


Objective Caml

Didier Rémy and Jérôme Vouillon designed an expressive
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
for objects and classes, which was integrated within Caml Special Light. This led to the emergence of the Objective Caml language, first released in 1996 and subsequently renamed to OCaml in 2011. This object system notably supported many prevalent object-oriented idioms in a statically type-safe way, while those same idioms caused unsoundness or required runtime checks in languages such as C++ or
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
. In 2000, Jacques Garrigue extended Objective Caml with multiple new features such as polymorphic methods, variants, and labeled and optional arguments.


Ongoing development

Language improvements have been incrementally added for the last two decades in order to support the growing commercial and academic codebases in OCaml. The OCaml 4.0 release in 2012 added Generalized Algebraic Data Types (GADTs) and first-class modules in order to increase the flexibility of the language. The OCaml 5.0.0 release in 2022 is a complete rewrite of the language runtime, removing the global GC lock and adding effect handlers via delimited continuations. These changes enable support for shared-memory parallelism and color-blind concurrency respectively. OCaml's development continued within the Cristal team at INRIA until 2005, when it was succeeded by the Gallium team. Subsequently, Gallium was succeeded by the Cambium team in 2019.


Features

OCaml features a static
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progra ...
,
type inference Type inference refers to the automatic detection of the type of an expression in a formal language. These include programming languages and mathematical type systems, but also natural languages in some branches of computer science and linguistic ...
, parametric polymorphism, tail recursion,
pattern matching In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact: "either it will or will not be ...
, first class lexical closures, functors (parametric modules),
exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
, effect handling, and incremental generational automatic garbage collection. OCaml is 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 creating libraries of OCaml functions that can be linked to a ''main'' program in C, so that an OCaml library can be distributed to C programmers who have no knowledge or installation of OCaml. The OCaml distribution contains: *
Lexical analysis In computer science, lexical analysis, lexing or tokenization is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of ''lexical tokens'' ( strings with an assigned and thus identified ...
and
parsing Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar. The term ''parsing'' comes from ...
tools called ''ocamllex'' and ''ocamlyacc'' * Debugger that supports stepping backwards to investigate errors *
Documentation generator A documentation generator is a programming tool that generates software documentation intended for programmers ( API documentation) or end users (end-user guide), or both, from a set of source code files, and in some cases, binary files. Some gen ...
* Profiler – to measure performance * Many general-purpose
libraries A library is a collection of Document, materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or electronic media, digital access (soft copies) materials, and may be a ...
The native code compiler is available for many platforms, including
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
,
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
, and
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...
. Portability is achieved through native code generation support for major architectures:
IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnatio ...
,
X86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...
(AMD64), Power,
RISC-V RISC-V (pronounced "risk-five" where five refers to the number of generations of RISC architecture that were developed at the University of California, Berkeley since 1981) is an open standard instruction set architecture (ISA) based on est ...
, ARM, and ARM64. OCaml bytecode and native code programs can be written in a multithreaded style, with preemptive context switching. OCaml threads in the same domain execute by time sharing only. However, an OCaml program can contain several domains. There are several libraries for distributed computing such a
Functory
an


Development environment

Since 2011, many new tools and libraries have been contributed to the OCaml development environment: * Development tools *
opam
is a package manager for OCaml. *
Merlin
provides IDE-like functionality for multiple editors, including type throwback, go-to-definition, and auto-completion. *
Dune
is a composable build-system for OCaml. *
OCamlformat
is an auto-formatter for OCaml.
ocaml-lsp-server
is a Language Server Protocol for OCaml IDE integration. * Web sites: *
OCaml.org
is the primary site for the language. *
discuss.ocaml.org
is an instance of
Discourse Discourse is a generalization of the notion of a conversation to any form of communication. Discourse is a major topic in social theory, with work spanning fields such as sociology, anthropology, continental philosophy, and discourse analysis. ...
that serves as the primary discussion site for OCaml. * Alternate compilers for OCaml: *
js_of_ocaml
developed by the Ocsigen team, is an optimizing compiler from OCaml to
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
. *
BuckleScript
which also targets
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
, with a focus on producing readable, idiomatic JavaScript output. ** ocamlcc is a compiler from OCaml to C, to complement the native code compiler for unsupported platforms. ** OCamlJava, developed by INRIA, is a compiler from OCaml to the
Java virtual machine A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describe ...
(JVM). ** OCaPic, developed by Lip6, is an OCaml compiler for PIC microcontrollers.


Code examples

Snippets of OCaml code are most easily studied by entering them into the ''top-level REPL''. 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 An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
) and gives the result "7".


Hello World

The following program "hello.ml": print_endline "Hello World!" can be compiled into a bytecode executable: $ ocamlc hello.ml -o hello or compiled into an optimized native-code executable: $ ocamlopt hello.ml -o hello and executed: $ ./hello Hello World! $ The first argument to ocamlc, "hello.ml", specifies the source file to compile and the "-o hello" flag specifies the output file.


Option

The option type constructor in OCaml, similar to the Maybe monad in Haskell, augments a given data type to either return Some value of the given data type, or to return None. This is used to express that a value might or might not be present. # Some 42;; - : int option = Some 42 # None;; - : 'a option = None This is an example of a function that either extracts an int from an option, if there is one inside, and converts it into a string, or if not, returns an empty string: let extract o = match o with , Some i -> string_of_int i , None -> "";; # extract (Some 42);; - : string = "42" # extract None;; - : string = ""


Summing a list of integers

Lists are one of the fundamental datatypes in OCaml. The following code example defines a recursive function ''sum'' that accepts one argument, ''integers'', which is supposed to be a list of integers. Note the keyword rec which denotes that the function is recursive. The function recursively iterates over the given list of integers and provides a sum of the elements. The ''match'' statement has similarities to C's
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
element, though it is far more general. let rec sum integers = (* Keyword rec means 'recursive'. *) match integers with , [] -> 0 (* Yield 0 if integers is the empty list []. *) , first :: rest -> first + sum rest;; (* Recursive call if integers is a non- empty list; first is the first element of the list, and rest is a list of the rest of the elements, possibly []. *) # sum ;2;3;4;5; - : int = 15 Another way is to use standard fold function that works with lists. let sum integers = List.fold_left (fun accumulator x -> accumulator + x) 0 integers;; # sum ;2;3;4;5; - : int = 15 Since the
anonymous function In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed t ...
is simply the application of the + operator, this can be shortened to: let sum integers = List.fold_left (+) 0 integers Furthermore, one can omit the list argument by making use of a partial application: let sum = List.fold_left (+) 0


Quicksort

OCaml lends itself to concisely expressing recursive algorithms. The following code example implements an algorithm similar to
quicksort Quicksort is an efficient, general-purpose sorting algorithm. Quicksort was developed by British computer scientist Tony Hoare in 1959 and published in 1961, it is still a commonly used algorithm for sorting. Overall, it is slightly faster than ...
that sorts a list in increasing order. let rec qsort = function , [] -> [] , pivot :: rest -> let is_less x = x < pivot in let left, right = List.partition is_less rest in qsort left @ [pivot] @ qsort right Or using partial application of the >= operator. let rec qsort = function , [] -> [] , pivot :: rest -> let is_less = (>=) pivot in let left, right = List.partition is_less rest in qsort left @ [pivot] @ qsort right


Birthday problem

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 birthday problem, where for 1 person the probability is 365/365 (or 100%), for 2 it is 364/365, for 3 it is 364/365 × 363/365, 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) else birthday_paradox prob (people+1) ;; birthday_paradox 1.0 1


Church numerals

The following code defines a Church encoding of
natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called '' cardinal ...
s, with successor (succ) and addition (add). A Church numeral is a
higher-order function In mathematics and computer science, a higher-order function (HOF) is a function that does at least one of the following: * takes one or more functions as arguments (i.e. a procedural parameter, which is a parameter of a procedure that is itse ...
that accepts a function and a value and applies to exactly times. To convert a Church numeral from a functional value to a string, we pass it a function that prepends the string to its input and the constant string . 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 In computer science, arbitrary-precision arithmetic, also called bignum arithmetic, multiple-precision arithmetic, or sometimes infinite-precision arithmetic, indicates that calculations are performed on numbers whose digits of precision are li ...
. 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 (now superseded by the ZArith module) provides arbitrary-precision arithmetic and can be loaded into a running top-level using: # #use "topfind";; # #require "num";; # 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 = 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 renders a rotating triangle in 2D using
OpenGL OpenGL (Open Graphics Library) is a cross-language, cross-platform application programming interface (API) for rendering 2D and 3D vector graphics. The API is typically used to interact with a graphics processing unit (GPU), to achieve hardwa ...
: 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 or, more simply, using the ocamlfind build command $ ocamlfind opt simple.ml -package lablgl.glut -linkpkg -o simple and run: $ ./simple Far more sophisticated, high-performance 2D and 3D graphical programs can be developed in OCaml. Thanks to the use of OpenGL and OCaml, the resulting programs can be cross-platform, compiling without any changes on many major platforms.


Fibonacci sequence

The following code calculates the Fibonacci sequence of a number ''n'' inputted. It uses tail recursion and pattern matching. let fib n = let rec fib_aux m a b = match m with , 0 -> a , _ -> fib_aux (m - 1) b (a + b) in fib_aux n 0 1


Higher-order functions

Functions may take functions as input and return functions as result. For example, applying ''twice'' to a function ''f'' yields a function that applies ''f'' two times to its argument. let twice (f : 'a -> 'a) = fun (x : 'a) -> f (f x);; let inc (x : int) : int = x + 1;; let add2 = twice inc;; let inc_str (x : string) : string = x ^ " " ^ x;; let add_str = twice(inc_str);; # add2 98;; - : int = 100 # add_str "Test";; - : string = "Test Test Test Test" The function ''twice'' uses a type variable'' 'a'' to indicate that it can be applied to any function ''f'' mapping from a type'' 'a'' to itself, rather than only to ''int->int'' functions. In particular, ''twice'' can even be applied to itself. # let fourtimes f = (twice twice) f;; val fourtimes : ('a -> 'a) -> 'a -> 'a = # let add4 = fourtimes inc;; val add4 : int -> int = # add4 98;; - : int = 102


Derived languages


MetaOCaml

MetaOCaml is a multi-stage programming extension of OCaml enabling incremental compiling of new
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a ve ...
during runtime. Under some circumstances, significant speedups are possible using multistage 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 some power function is needed often, but the value of is known only at runtime, a two-stage power function can be used 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 is known at runtime, a specialized and very fast power function can be created: . .~(power 5 ..)>. 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
(2006) is a subset of OCaml 3.08 that uses a design rule verifier to enforce
object-capability model The object-capability model is a computer security model. A capability describes a transferable right to perform one (or more) operations on a given object. It can be obtained by the following combination: :* An unforgeable reference (in the sens ...
security" \n\n\nsecurity.txt is a proposed standard for websites' security information that is meant to allow security researchers to easily report security vulnerabilities. The standard prescribes a text file called \"security.txt\" in the well known locat ...
principles. * F# is a
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
language based on OCaml. * Fresh OCaml facilitates manipulating names and binders. * GCaml adds extensional polymorphism to OCaml, thus allowing overloading and type-safe marshalling. *
JoCaml JoCaml is an experimental functional programming language derived from OCaml. It integrates the primitives of the join-calculus to enable flexible, type-checked concurrent and distributed programming. The current version of JoCaml is a re-imple ...
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. *
Reason Reason is the capacity of consciously applying logic by drawing conclusions from new or existing information, with the aim of seeking the truth. It is closely associated with such characteristically human activities as philosophy, science, lang ...
is an alternative OCaml
syntax In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituenc ...
and toolchain for OCaml created at
Facebook Facebook is an online social media and social networking service owned by American company Meta Platforms. Founded in 2004 by Mark Zuckerberg with fellow Harvard College students and roommates Eduardo Saverin, Andrew McCollum, Dust ...
, which can compile to both native code and JavaScript. *
ReScript In legal terminology, a rescript is a document that is issued not on the initiative of the author, but in response (it literally means 'written back') to a specific demand made by its addressee. It does not apply to more general legislation. Over ...
is a rebranding and new language from the Reason/BuckleScript toolchain, which has different syntax and only compiles to JavaScript.


Software written in OCaml

* 0install, a multi-platform package manager. * CamlPDF, an OCaml library for reading, writing and modifying PDF files * Coccinelle, a utility for transforming the
source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the ...
of C programs. * Coq, a
formal proof In logic and mathematics, a formal proof or derivation is a finite sequence of sentences (called well-formed formulas in the case of a formal language), each of which is an axiom, an assumption, or follows from the preceding sentences in the seq ...
management system. * FFTW, a
library A library is a collection of materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or digital access (soft copies) materials, and may be a physical location or a vi ...
for computing
discrete Fourier transform In mathematics, the discrete Fourier transform (DFT) converts a finite sequence of equally-spaced samples of a function into a same-length sequence of equally-spaced samples of the discrete-time Fourier transform (DTFT), which is a comple ...
s. Several C routines have been generated by an OCaml program named . * The web version of
Facebook Messenger Messenger is a proprietary instant messaging app and platform developed by Meta Platforms. Originally developed as Facebook Chat in 2008, the company revamped its messaging service in 2010, released standalone iOS and Android apps in 2011, and ...
. * Flow, a static analyzer created at
Facebook Facebook is an online social media and social networking service owned by American company Meta Platforms. Founded in 2004 by Mark Zuckerberg with fellow Harvard College students and roommates Eduardo Saverin, Andrew McCollum, Dust ...
that infers and checks static types for
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
. * Owl Scientific Computing, a dedicated system for scientific and engineering computing. *
Frama-C Frama-C stands for ''Framework for Modular Analysis of C programs''. Frama-C is a set of interoperable program analyzers for C programs. Frama-C has been developed by the French Commissariat à l'Énergie Atomique et aux Énergies Alternativ ...
, a framework for analyzing C programs. *
GeneWeb GeneWeb is a free multi-platform genealogy software tool created and owned by Daniel de Rauglaudre of INRIA. GeneWeb is accessed by a Web browser, either off-line or as a server in a Web environment. It uses very efficient techniques of relation ...
, free and open-source multi-platform genealogy software. * The
Hack Hack may refer to: Arts, entertainment, and media Games * ''Hack'' (Unix video game), a 1984 roguelike video game * ''.hack'' (video game series), a series of video games by the multimedia franchise ''.hack'' Music * ''Hack'' (album), a 199 ...
programming language compiler, created at Facebook, extending PHP with static types. * The
Haxe Haxe is an open source high-level cross-platform programming language and compiler that can produce applications and source code, for many different computing platforms from one code-base. It is free and open-source software, released under the ...
programming language compiler. *
HOL Light HOL Light is a member of the HOL theorem prover family. Like the other members, it is a proof assistant for classical higher order logic. Compared with other HOL systems, HOL Light is intended to have relatively simple foundations. HOL Light is ...
, a formal proof assistant. * Infer, a static analyzer created at Facebook for
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, C, C++, and
Objective-C Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXT ...
, used to detect bugs in iOS and Android apps.
Lexifi Apropos
a system for modeling complex derivatives. *
MirageOS A unikernel is a specialised, Single address space operating system, single address space machine image constructed by using Operating system#Library, library operating systems. A developer selects, from a modular stack, the minimal set of librar ...
, a unikernel programming framework written in pure OCaml. *
MLdonkey MLDonkey is an open-source, multi-protocol, peer-to-peer file sharing application that runs as a back-end server application on many platforms. It can be controlled through a user interface provided by one of many separate front-ends, including ...
, a
peer-to-peer file sharing Peer-to-peer file sharing is the distribution and sharing of digital media using peer-to-peer (P2P) networking technology. P2P file sharing allows users to access media files such as books, music, movies, and games using a P2P software program t ...
application based on the EDonkey network. * Ocsigen, an OCaml
web framework A web framework (WF) or web application framework (WAF) is a software framework that is designed to support the development of web applications including web services, web resources, and web APIs. Web frameworks provide a standard way to build an ...
. * Opa, a free and open-source programming language for web development. * pyre-check, a type checker for Python created at Facebook.
Semgrep
a bug-finding tool supporting many programming languages. *
Tezos Tezos is an open-source blockchain that can execute peer-to-peer transactions and serve as a platform for deploying smart contracts. The native cryptocurrency for the Tezos blockchain is the tez (ISO 4217: XTZ; sign: ꜩ). The Tezos network ac ...
, a self-amending smart contract platform using XTZ as a native currency.
Unison
a file synchronization program to synchronize files between two directories. * The reference interpreter for
WebAssembly WebAssembly (sometimes abbreviated Wasm) defines a portable binary-code format and a corresponding text format for executable programs as well as software interfaces for facilitating interactions between such programs and their host environmen ...
, a low-level
bytecode Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (norma ...
intended for execution inside
web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used o ...
s. * Xen Cloud Platform (XCP), a turnkey
virtualization In computing, virtualization or virtualisation (sometimes abbreviated v12n, a numeronym) is the act of creating a virtual (rather than actual) version of something at the same abstraction level, including virtual computer hardware platforms, stor ...
solution for the Xen
hypervisor A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called ...
.


Users

Several dozen companies use OCaml to some degree. Notable examples include: * Bloomberg L.P., which create
BuckleScript
an OCaml compiler backend targeting JavaScript. *
Citrix Systems Citrix Systems, Inc. is an American multinational corporation, multinational cloud computing and virtualization technology company that provides server, application and desktop virtualization, networking, software as a service (SaaS), and cloud ...
, which uses OCaml in
XenServer Xen (pronounced ) is a type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently. It was originally developed by the University of Cambridge Computer Laboratory a ...
(rebranded as Citrix Hypervisor during 2018). *
Facebook Facebook is an online social media and social networking service owned by American company Meta Platforms. Founded in 2004 by Mark Zuckerberg with fellow Harvard College students and roommates Eduardo Saverin, Andrew McCollum, Dust ...
, which developed Flow, Hack, Infer, Pfff, and Reason in OCaml. * Jane Street Capital, a
proprietary trading Proprietary trading (also known as prop trading) occurs when a trader trades stocks, bonds, currencies, commodities, their derivatives, or other financial instruments with the firm's own money (instead of using depositors' money) in order to ma ...
firm, which adopted OCaml as its preferred language in its early days.


References


External links

*
OCaml manual

OCaml Package Manager

Real World OCaml
{{Authority control Articles with example code Articles with example OCaml code Cross-platform free software Extensible syntax programming languages Free compilers and interpreters Functional languages ML programming language family Object-oriented programming languages OCaml programming language family OCaml software Pattern matching programming languages Programming languages created in 1996 Statically typed programming languages