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What is OCaml?

Fast, Flexible and Functional Programming with OCaml, Gemma Gordon and Anil Madhavapeddy

OCaml is a general purpose programming language with an emphasis on expressiveness and safety. Developed for more than 20 years at Inria by a group of leading researchers, it has an advanced type system that helps catch your mistakes without getting in your way. It's used in environments where a single mistake can cost millions and speed matters, is supported by an active community, and has a rich set of libraries and development tools.
For all its power, OCaml is also pretty simple, which is one reason it's often used as a teaching language.

See “A History of OCaml” for an account of the origins of the language.


The OCaml language offers:

  • A powerful type system, equipped with parametric polymorphism and type inference. For instance, the type of a collection can be parameterized by the type of its elements. This allows defining some operations over a collection independently of the type of its elements: sorting an array is one example. Furthermore, type inference allows defining such operations without having to explicitly provide the type of their parameters and result.
  • User-definable algebraic data types and pattern-matching. New algebraic data types can be defined as combinations of records and sums. Functions that operate over such data structures can then be defined by pattern matching, a generalized form of the well-known switch statement, which offers a clean and elegant way of simultaneously examining and naming data.
  • Automatic memory management, thanks to a fast, unobtrusive, incremental garbage collector.
  • Separate compilation of standalone applications. Portable bytecode compilers allow creating stand-alone applications out of Caml Light or OCaml programs. A foreign function interface allows OCaml code to interoperate with C code when necessary. Interactive use of OCaml is also supported via a “read-evaluate-print” loop.

In addition, OCaml features:

  • A sophisticated module system, which allows organizing modules hierarchically and parameterizing a module over a number of other modules.
  • An expressive object-oriented layer, featuring multiple inheritance, parametric and virtual classes.
  • Efficient native code compilers. In addition to its bytecode compiler, OCaml offers a compiler that produces efficient machine code for many architectures.

A Widely Used Programming Language

The OCaml language was initially used to develop applications that involve symbolic computation: automatic theorem provers, compilers and interpreters, program analyzers, etc. It is now used to develop software in many other application areas. This is illustrated by our list of selected success stories.

The OCaml language is widely used for teaching programming. It is also used inside academic projects in Europe, Asia, and the Americas. Several large corporations develop significant industrial projects in OCaml: these include Dassault Systèmes, Microsoft, IBM, and CEA (Commissariat à l'Énergie Atomique).

The OCaml Consortium offers industrial and academic partners a formal framework for participating in the development, maintenance, and evolution of OCaml. Partners also enjoy very liberal licensing conditions regarding the OCaml software distribution.

Overview of the OCaml language and tools


OCaml is a safe language. Programs are verified by the compiler before they can be executed. This rules out many programming errors, such as, for instance, confusing an integer and a pointer, or accessing a nonexistent field in a record. More precisely, this protects the integrity of the data manipulated by an OCaml program.

Although OCaml is statically type-checked, it does not require that the types of function parameters, local variables, etc. be explicitly declared, contrary to, say, C or Java. Much of the necessary type information is automatically inferred by the compiler.


OCaml is a functional programming language: there is no restriction on the definition and use of functions. In other words, functions are ordinary values: a function can be passed as an argument to a function or returned by a function.

Imperative features

OCaml offers a full array of imperative features. In particular, variables, arrays, and record components can be declared as modifiable. Several varieties of loops are available.

Error recovery

OCaml features a general exception mechanism to signal and handle errors and exceptional situations.

Data Types

OCaml offers numerous built-in data types, including:

  • basic types: integers, floating point numbers, booleans, characters, strings.
  • more sophisticated data types: tuples, arrays, lists, sets, hash tables, queues, stacks, data streams.

Beyond these powerful built-in types, OCaml offers powerful means of defining new data types: records, enumerations, and sum types. Sum types are a simple and powerful generalization of enumerations. They allow bringing heterogeneous values together into a common type, thanks to the use of tags known as data constructors.

The types defined within a module can be made visible to the module's clients either concretely or abstractly, as the programmer desires. When a type is exported in a concrete manner, its full definition remains known: so, clients of the modules can build or examine values of this type. On the other hand, when a type is exported in an abstract manner, only its name is known outside the module. It then becomes impossible, from the outside, to create or inspect values of this type.

This mechanism provides fine-grained control over data encapsulation, which is mandatory for programming in the large.

Aptness to symbolic computation

OCaml features pattern matching, a generalization of the traditional case analysis construct. Pattern matching offers a clean and elegant way of simultaneously examining and naming data. The OCaml compiler takes advantage of this feature to perform several checks: superfluous as well as missing branches are detected and reported, which often allows pinpointing subtle errors. When no error is signaled, one can trust that no case has been overlooked.

Pattern matching brings unrivaled comfort and safety to programs that manipulate symbolic data.


OCaml features a polymorphic type system: some undetermined types can be represented by variables, which may later be instantiated at will. Thus, for instance, a single sorting function can be applied to lists of integers, lists of integer pairs, or lists of records, without requiring any code duplication.

Programming in the large

An OCaml program is made of compilation units that are dealt with separately by the compiler. This scheme is fully compatible with the use of traditional project management tools, such as the “make” utility. The module system is powerful and safe: every interaction between modules is statically typechecked. In OCaml, a module may contain submodules, which allows organizing modules hierarchically, and it is possible to parameterize a module over a number of other modules, or, in other words, to define functions from modules to modules.

Object-oriented programming

OCaml allows writing programs in an object-oriented style. In keeping with the language's philosophy, the object-oriented layer obeys the “strong typing” paradigm: thus, it is impossible to send a message to some object that cannot answer it. Such safety does not come at a cost in expressiveness: thanks to features such as multiple inheritance and parametric classes, the most complex design patterns can be expressed in a natural manner.

Automatic memory management and incremental garbage collection

OCaml features automatic memory management: allocation and deallocation of data structures is implicit, and dealt with by the compiler: there is no “new”, “malloc”, “delete”, or “free” operator. This makes programs much safer: memory corruption cannot occur.

Moreover, the memory manager is incremental: it runs in parallel with the application, so that garbage collection does not cause noticeable delays.

Evaluation regime

OCaml's evaluation strategy is strict. A lazy evaluation regime may be simulated, when required, via explicit suspensions. Thus, it is possible, for instance, to manipulate potentially infinite data streams.

Efficient compiler, efficient compiled code

OCaml offers two batch compilers: a bytecode compiler and a native code compiler. Both support separate compilation. Caml Light only offers a bytecode compiler.

The bytecode compilers generate small, portable executables. Furthermore, these compilers are extremely fast.

The native code compiler produces more efficient machine code, whose performance meets the highest standards of modern compilers.


OCaml provides an interactive “read-eval-print” loop, which is convenient both for learning and for rapid testing and debugging of programs. Indeed, there is no need to create a file, or to insert display instructions into programs, since results are automatically printed by the interactive system.

Debugging facilities

Several different methods are available to debug OCaml programs:

  • the interactive system offers an elementary yet fast and simple method to test (small) functions: one types various inputs directly into the interactive system and checks that the results are as expected.
  • for more complex cases, the interactive system provides a cheap means of following the computation, using the so-called function call tracing mechanism.
  • last, the symbolic replay debugger is an extremely powerful tool to follow the computation. It allows stopping the program at any time to scrutinize the value of variables and the stack of calling functions, and even going back into the past to resume execution at a particular point of interest.


OCaml offers parser generation tools analogous to “lex” and “yacc”. Furthermore, it offers a built-in type of data streams which eases the definition of LL(1) recursive descent parsers.


OCaml runs on a wide variety of platforms. Officially supported platforms are described here, and other platforms are supported by the community. For example, OCaml apps are available in the Apple app store; compiling to iOS is described here. It is also possible to compile to javascript with js_of_ocaml, enabling the creation of rich client-side applications directly with OCaml. A striking example of this is Try OCaml, the result of compiling the OCaml compiler itself with js_of_ocaml. Finally, ocamljava enables direct compilation of OCaml to Java bytecode, as well as providing other mechanisms for interoperating with Java.