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61-479: OCaml ( / oʊ ˈ k æ m əl / oh- KAM -əl , formerly Objective Caml ) is a general-purpose , high-level , multi-paradigm programming language which extends the Caml dialect of ML with object-oriented 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 ,

122-509: 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" . A variety of libraries are directly accessible from OCaml. For example, OCaml has

183-600: A bytecode compiler , an optimizing native code compiler, a reversible debugger , and a package manager ( OPAM ) together with a composable build system for OCaml ( Dune ). OCaml was initially developed in the context of automated theorem proving , and is used in static analysis and formal methods software. Beyond these areas, it has found use in systems programming , web development , and specific financial utilities, among other application domains. The acronym CAML originally stood for Categorical Abstract Machine Language , but OCaml omits this abstract machine . OCaml

244-655: A static type system , type inference , parametric polymorphism , tail recursion , pattern matching , first class lexical closures , functors (parametric modules) , exception handling , 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

305-459: A string , or if not, returns an empty string: 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

366-561: 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 (now superseded by the ZArith module) provides arbitrary-precision arithmetic and can be loaded into a running top-level using: The factorial function may then be written using

427-480: A different behavior in the case of mutable variables, because the state will no longer be shared between closures. But if it is known that the variables are constant, then this approach will be equivalent. The ML languages take this approach, since variables in those languages are bound to values—i.e. variables cannot be changed. Java also takes this approach with respect to anonymous classes (and lambdas since Java 8), in that it only allows one to refer to variables in

488-427: A function's state when that function is not actually executing. However, because, by definition, the existence of a downwards funarg is contained in the execution of the function that creates it, the stack frame for the function can usually still be stored on the stack. Nonetheless, the existence of downwards funargs implies a tree structure of closures and stack frames that can complicate human and machine reasoning about

549-437: A general purpose programming language.  For example, COBOL , Fortran , and Lisp were created as DSLs (for business processing, numeric computation, and symbolic processing), but became GPL's over time. Inversely, a language may be designed for general use but only applied in a specific area in practice. A programming language that is well suited for a problem, whether it be general-purpose language or DSL, should minimize

610-773: A general-purpose language with an appropriate library of data types and functions for the domain may be used instead. While DSLs are usually smaller than GPL in that they offer a smaller range of notations of abstractions, some DSLs actually contain an entire GPL as a sublanguage. In these instances, the DSLs are able to offer domain-specific expressive power along with the expressive power of GPL. General Purpose programming languages are all Turing complete , meaning that they can theoretically solve any computational problem. Domain-specific languages are often similarly Turing complete but are not exclusively so. General-purpose programming languages are more commonly used by programmers. According to

671-405: A hybrid approach in which the activation records for a function are allocated from the stack if the compiler is able to deduce, through static program analysis , that the function creates no upwards funargs. Otherwise, the activation records are allocated from the heap. Another solution is to simply copy the value of the variables into the closure at the time the closure is created. This will cause

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732-420: A language that would only allow the writer to construct valid proofs with its polymorphic type system. ML was turned into a compiler 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 's Formel team to become interested in

793-413: A list in increasing order. Or using partial application of the >= operator. 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). The following code defines

854-441: 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. In 1995, Xavier Leroy released Caml Special Light, which

915-424: A parameter to another function call. When one function calls another during a typical program's execution, the local state of the caller (including parameters and local variables ) must be preserved in order for execution to proceed after the callee returns. In most compiled programs, this local state is stored on the call stack in a data structure called a stack frame or activation record . This stack frame

976-607: A result, though it was first used by its creators to rewrite the kernel of the Unix operating system, it was easily adapted for use in application development, embedded systems (e.g., microprocessor programming), video games (e.g., Doom ), and so on. Today, C remains one of the most popular and widely-used programming languages. Conceived as an extension to C, C++ introduced object-oriented features, as well as other conveniences like references, operator overloading, and default arguments. Like C, C++'s generality allowed it to be used in

1037-465: A study, C , Python , and Java were the most commonly used programming languages in 2021.  One argument in favor of using general-purpose programming languages over domain-specific languages is that more people will be familiar with these languages, overcoming the need to learn a new language. Additionally, for many tasks (e.g., statistical analysis, machine learning, etc.) there are libraries that are extensively tested and optimized. Theoretically,

1098-459: A wide range of areas. While its C++'s core area of application is in systems programming (because of C++'s ability to grant access to low-level architecture), it has been used extensively to build desktop applications, video games, databases, financial systems, and much more. Major software and finance companies, such as Microsoft , Apple , Bloomberg , and Morgan Stanley , still widely use C++ in their internal and external applications. Python

1159-654: Is a free and open-source software 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 . 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

1220-472: Is available for many platforms, including Unix , Microsoft Windows , and Apple macOS . Portability is achieved through native code generation support for major architectures: The bytecode compiler supports operation on any 32- or 64-bit architecture when native code generation is not available, requiring only a C compiler. OCaml bytecode and native code programs can be written in a multithreaded style, with preemptive context switching. OCaml threads in

1281-405: 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. Although OCaml does not have a macro system as an indivisible part of

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1342-406: Is pushed, or allocated, as prelude to calling another function, and is popped, or deallocated, when the other function returns to the function that did the call. The upwards funarg problem arises when the calling function refers to the called/exited function's state after that function has returned. Therefore, the stack frame containing the called function's state variables must not be deallocated when

1403-444: 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.” 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

1464-411: 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

1525-554: 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

1586-500: The Pascal programming language allows functions to be passed as arguments but not returned as results; thus implementations of Pascal are required to address the downwards funarg problem but not the upwards one. The Modula-2 and Oberon programming languages (descendants of Pascal) allow functions both as parameters and return values, but the assigned function may not be a nested function. The C programming language historically avoids

1647-489: The University of Edinburgh 's Laboratory for Foundations of Computer Science . Milner and others were working on theorem provers , which were historically developed in languages such as Lisp . Milner repeatedly ran into the issue that the theorem provers would attempt to claim a proof was valid by putting non-proofs together. As a result, he went on to develop the meta language for his Logic for Computable Functions ,

1708-400: The data types of variables and the signatures of functions usually need not be declared explicitly, as they do in languages like Java 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

1769-420: 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 allocation of the functions. The difficulty only arises if the body of a nested function refers directly (i.e., not by argument passing) to identifiers defined in the environment in which

1830-401: The "#" prompt. For example, to calculate 1+2*3: OCaml infers the type of the expression to be "int" (a machine-precision integer ) and gives the result "7". The following program "hello.ml": can be compiled into a bytecode executable: or compiled into an optimized native-code executable: and executed: The first argument to ocamlc, "hello.ml", specifies the source file to compile and

1891-535: The "-o hello" flag specifies the output file. The option type constructor in OCaml, similar to the Maybe type 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. This is an example of a function that either extracts an int from an option, if there is one inside, and converts it into

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1952-451: The 1960s: GPSS and Simula for discrete event simulation; MAD , BASIC , Logo , and Pascal for teaching programming; C for systems programming; JOSS and APL\360 for interactive programming. The distinction between general-purpose programming languages and domain-specific programming languages is not always clear. A programming language may be created for a specific task, but used beyond that original domain and thus be considered

2013-604: 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. As of 2023, there are 23 core developers of the compiler distribution from a variety of organizations and 41 developers for the broader OCaml tooling and packaging ecosystem. In 2023, the OCaml compiler was recognised with ACM SIGPLAN's Programming Languages Software Award . OCaml features

2074-550: The ML language. Luca Cardelli , a research professor at University of Oxford , used his functional abstract machine to develop a faster implementation of ML, and Robin Milner proposed a new definition of ML to avoid divergence between various implementations. Simultaneously, Pierre-Louis Curien, a senior researcher at Paris Diderot University , developed a calculus of categorical combinators and linked it to lambda calculus , which led to

2135-485: The arbitrary-precision numeric operators =/ , */ and -/  : This function can compute much larger factorials, such as 120!: The following program renders a rotating triangle in 2D using OpenGL : The LablGL bindings to OpenGL are required. The program may then be compiled to bytecode with: General-purpose programming language Early programming languages were designed for scientific computing (numerical calculations) or commercial data processing, as

2196-517: The definition of the categorical abstract machine (CAM). Guy Cousineau, a researcher at Paris Diderot University, recognized that this could be applied as a compiling method for ML. 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

2257-526: The elements. The match statement has similarities to C 's switch element, though it is far more general. Another way is to use standard fold function that works with lists. Since the anonymous function is simply the application of the + operator, this can be shortened to: Furthermore, one can omit the list argument by making use of a partial application : OCaml lends itself to concisely expressing recursive algorithms. The following code example implements an algorithm similar to quicksort that sorts

2318-523: The enclosing scope that are effectively final (i.e. constant). Some languages allow the programmer to explicitly choose between the two behaviors. PHP 5.3's anonymous functions require one to specify which variables to include in the closure using the use () clause; if the variable is listed by reference, it includes a reference to the original variable; otherwise, it passes the value. In Apple's Blocks anonymous functions, captured local variables are by default captured by value; if one wants to share

2379-410: The function is defined, but not in the environment of the function call. A standard resolution is either to forbid such references or to create closures . There are two subtly different versions of the funarg problem. The upwards funarg problem arises from returning (or otherwise transmitting "upwards") a function from a function call. The downwards funarg problem arises from passing a function as

2440-422: The function returns, violating the stack-based function call paradigm . One solution to the upwards funarg problem is to simply allocate all activation records from the heap instead of the stack and rely on some form of garbage collection or reference counting to deallocate them when they are no longer needed. Managing activation records on the heap has historically been perceived to be less efficient than on

2501-414: The functions f and g (or pointers to them) as internal state. The problem in this case exists if the compose function allocates the parameter variables f and g on the stack. When compose returns, the stack frame containing f and g is discarded. When the internal function λx attempts to access g , it will access a discarded memory area. A downwards funarg may also refer to

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2562-456: 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 can exploit the immutability of sets to reuse parts of input sets in the output (see persistent data structure ). Between the 1970s and 1980s, Robin Milner , a British computer scientist and Turing Award winner, worked at

2623-618: The language (metaprogramming), i.e. built-in support for preprocessing, the OCaml platform does officially support a library for writing such preprocessors . These can be of two types: one that works at the source code level (as in C), and one that works on the Abstract Syntax Tree level. The latter, which is called PPX, acronym for Pre-Processor eXtension, is the recommended one. The OCaml distribution contains: The native code compiler

2684-547: The last two decades 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 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

2745-438: The level of detail required while still being expressive enough in the problem domain. As the name suggests, general-purpose language is "general" in that it cannot provide support for domain-specific notation while DSLs can be designed in diverse problem domains to handle this problem. General-purpose languages are preferred to DSLs when an application domain is not well understood enough to warrant its own language. In this case,

2806-423: The main difficulty of the funarg problem by not allowing function definitions to be nested; because the environment of every function is the same, containing just the statically allocated global variables and functions, a pointer to a function's code describes the function completely. Apple has proposed and implemented a closure syntax for C that solves the upwards funarg problem by dynamically moving closures from

2867-425: 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", 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,

2928-624: The presence of these libraries should bridge the gap between general-purpose and domain-specific languages. An empirical study in 2010 sought to measure problem-solving and productivity between GPLs and DSLs by giving users problems who were familiar with the GPL ( C# ) and unfamiliar with the DSL ( XAML ). Ultimately, users of this specific domain-specific language performed better by a factor of 15%, even though they were more familiar with GPL, warranting further research. The predecessor to C , B ,

2989-410: The program state. The downwards funarg problem complicates the efficient compilation of tail calls and code written in continuation-passing style . In these special cases, the intent of the programmer is (usually) that the function run in limited stack space, so the "faster" behavior may actually be undesirable. Historically, the upwards funarg problem has proven to be more difficult. For example,

3050-411: 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,

3111-400: The same domain execute by time sharing only. However, an OCaml program can contain several domains. 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: Code can then be entered at

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3172-478: The stack (although this is partially contradicted ) and has been perceived to impose significant implementation complexity. Most functions in typical programs (less so for programs in functional programming languages ) do not create upwards funargs, adding to concerns about potential overhead associated with their implementation. Furthermore, this approach is genuinely difficult in languages that do not support garbage collection. Some efficiency-minded compilers employ

3233-514: The stack to the heap as necessary. The Java programming language deals with it by requiring that context used by nested functions in anonymous inner and local classes be declared final , and context used by lambda expressions be effectively final. C# and D have lambdas (closures) that encapsulate a function pointer and related variables. In functional languages , functions are first-class values that can be passed anywhere. Thus, implementations of Scheme or Standard ML must address both

3294-476: The state between closures or between the closure and the outside scope, the variable must be declared with the __block modifier, in which case that variable is allocated on the heap. The following Haskell -like pseudocode defines function composition : λ is the operator for constructing a new function, which in this case has one argument, x , and returns the result of first applying g to x , then applying f to that. This λ function carries

3355-518: 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++ . Also, 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 build. Didier Rémy and Jérôme Vouillon designed an expressive type system for objects and classes, which

3416-476: Was computer hardware. Scientific languages such as Fortran and Algol supported floating-point calculations and multidimensional arrays, while business languages such as COBOL supported fixed-field file formats and data records . Much less widely used were specialized languages such as IPL-V and LISP for symbolic list processing ; COMIT for string manipulation; APT for numerically controlled machines . Systems programming requiring pointer manipulation

3477-508: Was conceived as a language that emphasized code readability and extensibility. The former allowed non-software engineers to easily learn and write computer programs, while the latter allowed domain specialists to easily create libraries suited to their own use cases. For these reasons, Python has been used across a wide range of domains. Below are some of the areas where Python is used: The following are some general-purpose programming languages: Funarg problem In computer science ,

3538-467: Was developed largely for a specific purpose: systems programming . By contrast, C has found use in a variety of computational domains, such as operating systems , device drivers , application software , and embedded systems . C is suitable for use in a variety of areas because of its generality. It provides economy of expression, flow control, data structures, and a rich set of operators, but does not constrain its users to use it in any one context. As

3599-720: 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 . In 2000, Jacques Garrigue extended Objective Caml with multiple new features such as polymorphic methods, variants, and labeled and optional arguments. Language improvements have been incrementally added for

3660-406: Was typically done in assembly language , though JOVIAL was used for some military applications. IBM 's System/360 , announced in 1964, was designed as a unified hardware architecture supporting both scientific and commercial applications, and IBM developed PL/I for it as a single, general-purpose language that supported scientific, commercial, and systems programming. Indeed, a subset of PL/I

3721-475: Was used as the standard systems programming language for the Multics operating system. Since PL/I, the distinction between scientific and commercial programming languages has diminished, with most languages supporting the basic features required by both, and much of the special file format handling delegated to specialized database management systems . Many specialized languages were also developed starting in

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