OCaml

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. Thus, programmers need not be highly familiar with the pure functional language paradigm to use OCaml.

OCaml's static type system can help eliminate 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 need not be declared explicitly, as they do in Java. Nonetheless, effective use of OCaml's type system can require some sophistication on the part of a programmer.

OCaml is perhaps most distinguished from other languages with origins in academia, by its emphasis on performance. Firstly, 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 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. Along with standard loop, register, and instruction optimizations, OCaml's optimizing compiler employs static program analysis methods 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 stated that "OCaml delivers at least 50% of the performance of a decent C compiler", but 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).

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 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:

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 that supports stepping backwards to investigate errors

Documentation generator

Profiler – to measure performance

Many general-purpose libraries

The native code compiler is available for many platforms, including Unix, Microsoft Windows, and Apple macOS. Portability is achieved through native code generation support for major architectures: IA-32, X86-64 (AMD64), Power, SPARC, ARM, and ARM64.

OCaml bytecode and native code programs can be written in a multithreaded style, with preemptive context switching. However, because the garbage collector of the INRIA OCaml system (which is the only currently available full implementation of the language) is not designed for concurrency, symmetric multiprocessing is unsupported. OCaml threads in the same process execute by time sharing only. There are however several libraries for distributed computing such as Functory and ocamlnet/Plasma.

Development environment

Since 2011, many new tools and libraries have been contributed to the OCaml development environment:

OCaml Package Manager (OPAM), developed by OCamlPro, is now an easy way to install OCaml and many of its tools and libraries

Optimizing compilers for OCaml:

js_of_ocaml, developed by the Ocsigen team, is an optimizing compiler from OCaml to JavaScript, to create webapps in OCaml.

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 (JVM).

OCaPic, developed by Lip6, is a compiler from OCaml to PIC microcontroller.

Web sites:

OCaml.org is a website managed by the OCaml community.

Try-OCaml, developed by OCamlPro, is a website containing a complete OCaml REPL in a webpage.

Development tools

TypeRex is a set of open-source tools and libraries for OCaml, developed and maintained by OCamlPro.

Merlin is an auto-completion tool for editing OCaml code in Emacs and Vim.

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":

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! $

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 xs. (Note the keyword rec). The function recursively iterates over a given list and provides a sum of integer elements. The match statement has similarities to C's switch element, though it is far more general.

# sum [1;2;3;4;5];; - : int = 15

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

# sum [1;2;3;4;5];; - : int = 15

Quicksort

OCaml lends itself to concisely expressing recursive algorithms. The following code example implements an algorithm similar to quicksort that sorts a list in increasing order.

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 365/365 (or 100%), for 2 it is 364/365, for 3 it is 364/365 × 363/365, etc.) (answer = 23).

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 that prepends the string "S" to its input and the constant string "0".

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:

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

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

The cumbersome syntax for Num operations can be alleviated thanks to the camlp4 syntax extension called Delimited overloading:

Triangle (graphics)

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

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 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.

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.

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.

MetaOCaml

MetaOCaml is a multi-stage programming extension of OCaml enabling incremental compiling of new machine code 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 x -> x^n is needed often, but the value of n is known only at runtime, a two-stage power function can be used in MetaOCaml:

As soon as n is known at runtime, a specialized and very fast power function can be created:

The result is:

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 model security principles.

F# is a .NET Framework 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 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 it

MirageOS, a unikernel programming framework written in pure OCaml

Hack, a programming language extending PHP with static typing, created by Facebook in 2014. The compiler is written in OCaml.

Flow, a static analyzer for JavaScript created at Facebook that infers and verifies static types for JavaScript programs.

Infer, a static analyzer for Java, C, and Objective-C created at Facebook, which is used to detect bugs in iOS and Android apps.

0Install, a multi-platform package manager

Xen Cloud Platform (XCP), an open source toolstack for the Xen Virtual Machine Hypervisor

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, multi-platform genealogy software, free open source

Haxe compiler, a compiler for the language Haxe, free open source

Frama-C, a framework to analyze C programs

Coq, a formal proof management system

Ocsigen, web development framework

Opa, a programming language for web development, free open source

WebAssembly, an experimental, low-level scripting language for in-browser client-side scripting. Its WIP interpreter and parser in the specification repository on Github is written mostly in OCaml.

Commercial users

Several dozen companies use OCaml to some degree. Notable examples include:

Jane Street Capital, a proprietary trading firm, which adopted OCaml as its preferred language in its early days

Citrix Systems, which uses OCaml in XenServer, a component of one of its products

Facebook, which developed Hack, Flow, Infer, and Pfff

Ahrefs Site Explorer, which uses OCaml for its back-end framework: database management, web-crawler and web-page parser

Bloomberg L.P.