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Haskell () is a general-purpose,
statically-typed 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 ...
, purely functional
programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...
with
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 linguistics ...
and
lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...
. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming language features such as
type class In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and ...
es, which enable
type-safe In computer science, type safety and type soundness are the extent to which a programming language discourages or prevents type errors. Type safety is sometimes alternatively considered to be a property of facilities of a computer language; that is ...
operator overloading In computer programming, operator overloading, sometimes termed ''operator ad hoc polymorphism'', is a specific case of polymorphism, where different operators have different implementations depending on their arguments. Operator overloading is ...
, and monadic IO. Haskell's main implementation is the
Glasgow Haskell Compiler The Glasgow Haskell Compiler (GHC) is an open-source native code compiler for the functional programming language Haskell. It provides a cross-platform environment for the writing and testing of Haskell code and it supports numerous extensions, ...
(GHC). It is named after
logic Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from premises ...
ian
Haskell Curry Haskell Brooks Curry (; September 12, 1900 – September 1, 1982) was an American mathematician and logician. Curry is best known for his work in combinatory logic. While the initial concept of combinatory logic was based on a single paper by ...
. Haskell's
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy Philosophy (f ...
are historically based on those of the Miranda programming language, which served to focus the efforts of the initial Haskell working group. The last formal specification of the language was made in July 2010, while the development of GHC continues to expand Haskell via language extensions. Haskell is used in academia and industry. , Haskell was the 28th most popular programming language by
Google search Google Search (also known simply as Google) is a search engine provided by Google. Handling more than 3.5 billion searches per day, it has a 92% share of the global search engine market. It is also the most-visited website in the world. The ...
es for tutorials, and made up less than 1% of active users on the
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continuous ...
source code repository.


History

Following the release of Miranda by Research Software Ltd. in 1985, interest in lazy functional languages grew. By 1987, more than a dozen non- strict, purely functional programming languages existed. Miranda was the most widely used, but it was
proprietary software Proprietary software is software that is deemed within the free and open-source software to be non-free because its creator, publisher, or other rightsholder or rightsholder partner exercises a legal monopoly afforded by modern copyright and int ...
. At the conference on Functional Programming Languages and Computer Architecture (FPCA '87) in
Portland, Oregon Portland (, ) is a port city in the Pacific Northwest and the largest city in the U.S. state of Oregon. Situated at the confluence of the Willamette and Columbia rivers, Portland is the county seat of Multnomah County, the most populous co ...
, there was a strong consensus that a committee be formed to define an
open standard An open standard is a standard that is openly accessible and usable by anyone. It is also a prerequisite to use open license, non-discrimination and extensibility. Typically, anybody can participate in the development. There is no single definition ...
for such languages. The committee's purpose was to consolidate existing
functional languages 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 that m ...
into a common one to serve as a basis for future research in functional-language design.


Haskell 1.0 to 1.4

Haskell was developed by a committee, attempting to bring together off the shelf solutions where possible.
Type classes In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and ...
, which enable
type-safe In computer science, type safety and type soundness are the extent to which a programming language discourages or prevents type errors. Type safety is sometimes alternatively considered to be a property of facilities of a computer language; that is ...
operator overloading In computer programming, operator overloading, sometimes termed ''operator ad hoc polymorphism'', is a specific case of polymorphism, where different operators have different implementations depending on their arguments. Operator overloading is ...
, were first proposed by
Philip Wadler Philip Lee Wadler (born April 8, 1956) is an American computer scientist known for his contributions to programming language design and type theory. He is the chair of Theoretical Computer Science at the Laboratory for Foundations of Computer S ...
and Stephen Blott to address the ad-hoc handling of equality types and arithmetic overloading in languages at the time. In early versions of Haskell up until and including version 1.2, user interaction and IO (input and output) were handled by both streams based and continuation based mechanisms which were widely considered unsatisfactory. In version 1.3, monadic IO was introduced, along with the generalisation of type classes to higher kinds (type constructors). Along with "do notation", which provides syntactic sugar for the Monad type class, this gave Haskell an effect system that maintained referential transparency and was convenient. Other notable changes in early versions were the approach to the 'seq' function, which creates a data dependency between values, and is used in lazy languages to avoid excessive memory consumption; with it moving from a type class to a standard function to make refactoring more practical. The first version of Haskell ("Haskell 1.0") was defined in 1990. The committee's efforts resulted in a series of language definitions (1.0, 1.1, 1.2, 1.3, 1.4).


Haskell 98

In late 1997, the series culminated in ''Haskell 98'', intended to specify a stable, minimal, portable version of the language and an accompanying standard
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 vir ...
for teaching, and as a base for future extensions. The committee expressly welcomed creating extensions and variants of Haskell 98 via adding and incorporating experimental features. In February 1999, the Haskell 98 language standard was originally published as ''The Haskell 98 Report''. In January 2003, a revised version was published as ''Haskell 98 Language and Libraries: The Revised Report''. The language continues to evolve rapidly, with the
Glasgow Haskell Compiler The Glasgow Haskell Compiler (GHC) is an open-source native code compiler for the functional programming language Haskell. It provides a cross-platform environment for the writing and testing of Haskell code and it supports numerous extensions, ...
(GHC) implementation representing the current ''de facto'' standard.


Haskell 2010

In early 2006, the process of defining a successor to the Haskell 98 standard, informally named ''Haskell Prime'', began. This was intended to be an ongoing incremental process to revise the language definition, producing a new revision up to once per year. The first revision, named Haskell 2010, was announced in November 2009 and published in July 2010. Haskell 2010 is an incremental update to the language, mostly incorporating several well-used and uncontroversial features previously enabled via compiler-specific flags. * Hierarchical module names. Module names are allowed to consist of dot-separated sequences of capitalized identifiers, rather than only one such identifier. This lets modules be named in a hierarchical manner (e.g., Data.List instead of List), although technically modules are still in a single monolithic namespace. This extension was specified in an addendum to Haskell 98 and was in practice universally used. * The
foreign function interface A foreign function interface (FFI) is a mechanism by which a program written in one programming language can call routines or make use of services written in another. Naming The term comes from the specification for Common Lisp, which explicit ...
(FFI) allows bindings to other programming languages. Only bindings to C are specified in the Report, but the design allows for other language bindings. To support this, data type declarations were permitted to contain no constructors, enabling robust nonce types for foreign data that could not be constructed in Haskell. This extension was also previously specified in an Addendum to the Haskell 98 Report and widely used. * So-called ''n''+''k'' patterns (definitions of the form fact (n+1) = (n+1) * fact n) were no longer allowed. This
syntactic sugar In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
had misleading semantics, in which the code looked like it used the (+) operator, but in fact desugared to code using (-) and (>=). * The rules of
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 linguistics ...
were relaxed to allow more programs to type check. * Some
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 ( constituency) ...
issues (changes in the formal grammar) were fixed: pattern guards were added, allowing pattern matching within guards; resolution of operator fixity was specified in a simpler way that reflected actual practice; an edge case in the interaction of the language's
lexical syntax 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 ...
of operators and comments was addressed, and the interaction of do-notation and if-then-else was tweaked to eliminate unexpected syntax errors. * The LANGUAGE pragma was specified. By 2010, dozens of extensions to the language were in wide use, and GHC (among other compilers) provided the LANGUAGE pragma to specify individual extensions with a list of identifiers. Haskell 2010 compilers are required to support the Haskell2010 extension and are encouraged to support several others, which correspond to extensions added in Haskell 2010.


Future standards

The next formal specification had been planned for 2020. On 29 October 2021, with GHC version 9.2.1, the GHC2021 extension was released. While this is not a formal language spec, it combines a number of stable, widely-used GHC extensions to Haskell 2010. GHC 2020 Team (29 October 2021
GHC 9.2.1
released


Features

Haskell features
lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...
,
lambda expressions Lambda expression may refer to: *Lambda expression in computer programming, also called an anonymous function In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a f ...
,
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 ...
,
list comprehension A list comprehension is a Syntax of programming languages, syntactic construct available in some programming languages for creating a list based on existing list (computing), lists. It follows the form of the mathematical ''set-builder notation'' ( ...
,
type class In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and ...
es and
type polymorphism In programming language theory and type theory, polymorphism is the provision of a single interface to entities of different types or the use of a single symbol to represent multiple different types.: "Polymorphic types are types whose operatio ...
. It is a
purely functional language In computer science, purely functional programming usually designates a programming paradigm—a style of building the structure and elements of computer programs—that treats all computation as the evaluation of mathematical functions. Program ...
, which means that functions generally have no
side effects In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...
. A distinct construct exists to represent side effects,
orthogonal In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
to the type of functions. A pure function can return a side effect that is subsequently executed, modeling the impure functions of other languages. Haskell has a strong,
static Static may refer to: Places *Static Nunatak, a nunatak in Antarctica United States * Static, Kentucky and Tennessee *Static Peak, a mountain in Wyoming **Static Peak Divide, a mountain pass near the peak Science and technology Physics *Static el ...
type system based on Hindley–Milner type inference. Its principal innovation in this area is type classes, originally conceived as a principled way to add overloading to the language, but since finding many more uses. The construct that represents side-effects is an example of a
monad Monad may refer to: Philosophy * Monad (philosophy), a term meaning "unit" **Monism, the concept of "one essence" in the metaphysical and theological theory ** Monad (Gnosticism), the most primal aspect of God in Gnosticism * ''Great Monad'', an ...
: a general framework which can model various computations such as error handling,
nondeterminism Nondeterminism or nondeterministic may refer to: Computer science *Nondeterministic programming *Nondeterministic algorithm *Nondeterministic model of computation **Nondeterministic finite automaton **Nondeterministic Turing machine *Indeterminacy ...
,
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 Lati ...
and
software transactional memory In computer science, software transactional memory (STM) is a concurrency control mechanism analogous to database transactions for controlling access to shared memory in concurrent computing. It is an alternative to lock-based synchronization. STM ...
. They are defined as ordinary datatypes, but Haskell provides some
syntactic sugar In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express. It makes the language "sweeter" for human use: things can be expressed more clearly, more concisely, or in an ...
for their use. Haskell has an open, published specification, and multiple implementations exist. Its main implementation, the
Glasgow Haskell Compiler The Glasgow Haskell Compiler (GHC) is an open-source native code compiler for the functional programming language Haskell. It provides a cross-platform environment for the writing and testing of Haskell code and it supports numerous extensions, ...
(GHC), is both an interpreter and native-code
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 that ...
that runs on most platforms. GHC is noted for its rich type system incorporating recent innovations such as
generalized algebraic data type In functional programming, a generalized algebraic data type (GADT, also first-class phantom type, guarded recursive datatype, or equality-qualified type) is a generalization of parametric algebraic data types. Overview In a GADT, the product co ...
s and type families.
The Computer Language Benchmarks Game The Computer Language Benchmarks Game (formerly called The Great Computer Language Shootout) is a free software project for comparing how a given subset of simple algorithms can be implemented in various popular programming languages. The project ...
also highlights its high-performance implementation of concurrency and parallelism.Computer Language Benchmarks Game
/ref> An active, growing community exists around the language, and more than 5,400 third-party open-source libraries and tools are available in the online package repository ''Hackage''.


Code examples

A
"Hello, World!" program A "Hello, World!" program is generally a computer program that ignores any input and outputs or displays a message similar to "Hello, World!". A small piece of code in most general-purpose programming languages, this program is used to illustra ...
in Haskell (only the last line is strictly necessary): module Main (main) where -- not needed in interpreter, is the default in a module file main :: IO () -- the compiler can infer this type definition main = putStrLn "Hello, World!" The
factorial In mathematics, the factorial of a non-negative denoted is the product of all positive integers less than or equal The factorial also equals the product of n with the next smaller factorial: \begin n! &= n \times (n-1) \times (n-2) \t ...
function in Haskell, defined in a few different ways (the first line is the
type annotation In computer science, a type signature or type annotation defines the inputs and outputs for a function, subroutine or method. A type signature includes the number, types, and order of the arguments contained by a function. A type signature is ty ...
, which is optional and is the same for each implementation): factorial :: (Integral a) => a -> a -- Using recursion (with the "ifthenelse" expression) factorial n = if n < 2 then 1 else n * factorial (n - 1) -- Using recursion (with pattern matching) factorial 0 = 1 factorial n = n * factorial (n - 1) -- Using recursion (with guards) factorial n , n < 2 = 1 , otherwise = n * factorial (n - 1) -- Using a list and the "product" function factorial n = product ..n -- Using fold (implements "product") factorial n = foldl (*) 1 ..n -- Point-free style factorial = foldr (*) 1 . enumFromTo 1 As the ''Integer'' type has
arbitrary-precision 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 ...
, this code will compute values such as factorial 100000 (a 456,574-digit number), with no loss of precision. An implementation of an algorithm similar to
quick sort 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 ...
over lists, where the first element is taken as the pivot: -- Type annotation (optional, same for each implementation) quickSort :: Ord a => -> -- Using list comprehensions quickSort [] = [] -- The empty list is already sorted quickSort (x:xs) = quickSort [a , a <- xs, a < x] -- Sort the left part of the list ++ [x] ++ -- Insert pivot between two sorted parts quickSort a <- xs, a >= x -- Sort the right part of the list -- Using filter quickSort [] = [] quickSort (x:xs) = quickSort (filter (=x) xs)


Implementations

All listed implementations are distributed under
open source license An open-source license is a type of license for computer software and other products that allows the source code, blueprint or design to be used, modified and/or shared under defined terms and conditions. This allows end users and commercial compan ...
s."Implementations"
at the Haskell Wiki
Implementations that fully or nearly comply with the Haskell 98 standard, include: * The
Glasgow Haskell Compiler The Glasgow Haskell Compiler (GHC) is an open-source native code compiler for the functional programming language Haskell. It provides a cross-platform environment for the writing and testing of Haskell code and it supports numerous extensions, ...
(GHC) compiles to native code on many different processor architectures, and to
ANSI C ANSI C, ISO C, and Standard C are successive standards for the C programming language published by the American National Standards Institute (ANSI) and ISO/IEC JTC 1/SC 22/WG 14 of the International Organization for Standardization (ISO) and the ...
, via one of two
intermediate language An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive to further processing, such as optimization and translation. A "good ...
s:
C-- C-- (pronounced ''C minus minus'') is a C (programming language), C-like programming language. Its creators, functional programming researchers Simon Peyton Jones and Norman Ramsey, designed it to be generated mainly by compilers for very high-l ...
, or in more recent versions,
LLVM LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate represen ...
(formerly Low Level Virtual Machine) bitcode. GHC has become the ''de facto'' standard Haskell dialect. There are libraries (e.g., bindings to
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 ...
) that work only with GHC. GHC was also distributed with the
Haskell platform The Haskell Platform is a collection of software packages, tools and libraries that create a common platform for using and developing applications in Haskell. With the Haskell Platform, Haskell follows the same principle as Python: "Batteries inc ...
. * Jhc, a Haskell compiler written by John Meacham, emphasizes speed and efficiency of generated programs and exploring new program transformations. ** Ajhc is a fork of Jhc. * The Utrecht Haskell Compiler (UHC) is a Haskell implementation from
Utrecht University Utrecht University (UU; nl, Universiteit Utrecht, formerly ''Rijksuniversiteit Utrecht'') is a public research university in Utrecht, Netherlands. Established , it is one of the oldest universities in the Netherlands. In 2018, it had an enrollme ...
. It supports almost all Haskell 98 features plus many experimental extensions. It is implemented using
attribute grammar An attribute grammar is a formal way to supplement a formal grammar with semantic information processing. Semantic information is stored in attributes associated with terminal and nonterminal symbols of the grammar. The values of attributes are resu ...
s and is currently used mostly for research on generated type systems and language extensions. Implementations no longer actively maintained include: * The Haskell User's Gofer System ( Hugs) is 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 ...
interpreter. It was once one of the implementations used most widely, alongside the GHC compiler, but has now been mostly replaced by GHCi. It also comes with a graphics library. * HBC is an early implementation supporting Haskell 1.4. It was implemented by
Lennart Augustsson Lennart Augustsson is a Swedish computer scientist. He was previously a lecturer at the Computing Science Department at Chalmers University of Technology. His research field is functional programming and implementations of functional languages. ...
in, and based on,
Lazy ML Lazy is the adjective for laziness, a lack of desire to expend effort. It may also refer to: Music Groups and musicians * Lazy (band), a Japanese rock band * Lazy Lester, American blues harmonica player Leslie Johnson (1933–2018) * Lazy Bill ...
. It has not been actively developed for some time. * nhc98 is a bytecode compiler focusing on minimizing memory use. ** The York Haskell Compiler (
Yhc The York Haskell Compiler (Yhc) is a no longer maintained open source bytecode compiler for the functional programming language Haskell; it primarily targets the Haskell '98 standard. It is one of the four main Haskell compilers (behind GHC, Hu ...
) was a fork of nhc98, with the goals of being simpler, more portable and efficient, and integrating support for Hat, the Haskell tracer. It also had a
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 Website, websites use JavaScript on the Client (computing), client side ...
backend, allowing users to run Haskell programs in
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 on ...
s. Implementations not fully Haskell 98 compliant, and using a variant Haskell language, include: * Eta and Frege are dialects of Haskell targeting 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 describes ...
. *
Gofer A gofer, go-fer or gopher is an employee who specializes in the delivery of special items to their superior(s). Examples of these special items include a cup of coffee, a tool, a tailored suit, or a car. Outside of the business world, the term ...
was an educational dialect of Haskell, with a feature called ''constructor classes'', developed by Mark Jones. It was supplanted by Hugs (Haskell User's Gofer System). * Helium, a newer dialect of Haskell. The focus is on making learning easier via clearer error messages. It currently lacks full support for type classes, rendering it incompatible with many Haskell programs.


Notable applications

* The proof assistant
Agda Agda may refer to: * Agda (programming language), the programming language and theorem prover * Agda (Golgafrinchan), the character in ''The Hitchhiker's Guide to the Galaxy'' by Douglas Adams * Liten Agda, the heroine of a Swedish legend * Agda M ...
is written in Haskell. *
Cabal A cabal is a group of people who are united in some close design, usually to promote their private views or interests in an ideology, a state, or another community, often by intrigue and usually unbeknownst to those who are outside their group. Th ...
is a tool for
building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fun ...
and packaging Haskell libraries and programs. *
Darcs Darcs is a distributed version control system created by David Roundy. Key features include the ability to choose which changes to accept from other repositories, interaction with either other local (on-disk) repositories or remote repositories via ...
is a
revision control system Revision Control System (RCS) is an early implementation of a version control system (VCS). It is a set of UNIX commands that allow multiple users to develop and maintain program code or documents. With RCS, users can make their own revisions of ...
written in Haskell, with several innovative features, such as more precise control of patches to apply. * GHC is also often a testbed for advanced functional programming features and optimizations in other programming languages. * Git-annex is a tool to manage (big) data files under
Git Git () is a distributed version control system: tracking changes in any set of files, usually used for coordinating work among programmers collaboratively developing source code during software development. Its goals include speed, data in ...
version control. It also provides a distributed file synchronization system (git-annex assistant). *
Linspire Linspire (formerly Lindows) is a commercial operating system based on Debian and Ubuntu and currently owned by PC/OpenSystems LLC. It had been owned by Linspire. Inc. from 2001 to 2008, and then by Xandros from 2008 to 2017. On July 1, 2008, Li ...
Linux chose Haskell for system tools development. *
Pandoc Pandoc is a free-software document converter, widely used as a writing tool (especially by scholars)- - - and as a basis for publishing workflows. It was created by John MacFarlane, a philosophy professor at the University of California, Berke ...
is a tool to convert one markup format into another. *
Pugs The Pug is a breed of dog originally from China, with physically distinctive features of a wrinkly, short-muzzled face and curled tail. The breed has a fine, glossy coat that comes in a variety of colors, most often light brown (fawn) or bla ...
is 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 that ...
and interpreter for the Raku
programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...
(formerly Perl 6). *
TidalCycles TidalCycles (also known as "Tidal") is a live coding environment designed for musical improvisation and composition. In particular, it is a domain-specific language embedded in Haskell, focused on the generation and manipulation of audible or visu ...
is a domain special language for live coding musical pattern, embedded in Haskell. *
Xmonad xmonad is a dynamic window manager (tiling) for the X Window System, noted for being written in the functional programming language Haskell. Window manager Begun in March 2007, version 0.1 was announced in April 2007 as 500 lines of Haskell ...
is a
window manager A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface. Most window managers are designed to help provide a desktop environment. They work in conjunction ...
for the
X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wit ...
, written fully in Haskell.


Industry

*
Bluespec Bluespec, Inc. is a semiconductor tool design company co-founded by Professor Arvind (computer scientist), Arvind of MIT in June 2003. Arvind had previously founded Sandburst in 2000, which specialized in producing chips for 10 Gigabit Ethernet, 10 ...
SystemVerilog (BSV) is a language for semiconductor design that is an extension of Haskell. Also, Bluespec, Inc.'s tools are implemented in Haskell. *
Cryptol Cryptol is a domain-specific programming language for cryptography developed by the Portland, Oregon based software development firm, Galois, Inc. The language was originally developed for use by the United States National Security Agency. The lang ...
, a language and toolchain for developing and verifying
cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
algorithms, is implemented in Haskell. *
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, Dustin M ...
implements its anti-spam programs in Haskell, maintaining the underlying data access library as
open-source software Open-source software (OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose. Op ...
. *The Cardano blockchain platform is implemented in Haskell. *
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continuous ...
implemented
Semantic Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
, an open-source library for analysis, diffing, and interpretation of untrusted source code, in Haskell. *
Standard Chartered Standard Chartered plc is a multinational bank with operations in consumer, corporate and institutional banking, and treasury services. Despite being headquartered in the United Kingdom, it does not conduct retail banking in the UK, and around 9 ...
's financial modelling language Mu is syntactic Haskell running on a strict runtime. *
seL4 L4 is a family of second-generation microkernels, used to implement a variety of types of operating systems (OS), though mostly for Unix-like, ''Portable Operating System Interface'' (POSIX) compliant types. L4, like its predecessor microkernel ...
, the first formally verified
microkernel In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, ...
, A formal proof of functional correctness was completed in 2009. used Haskell as a prototyping language for the OS developer. At the same time, the Haskell code defined an executable specification with which to reason, for automatic translation by the theorem-proving tool. The Haskell code thus served as an intermediate prototype before final C refinement. *
Target Target may refer to: Physical items * Shooting target, used in marksmanship training and various shooting sports ** Bullseye (target), the goal one for which one aims in many of these sports ** Aiming point, in field artillery, fi ...
stores' supply chain optimization software is written in Haskell.


Web

Notable
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 and ...
s written for Haskell include: * IHP *
Yesod Yesod (Hebrew: יְסוֹד ''Yəsōḏ'', Tiberian: ''Yăsōḏ'', "foundation") is a sephirah or node in the kabbalistic Tree of Life, a system of Jewish philosophy. Yesod, located near the base of the Tree, is the sephirah below Hod and ...
*
Servant A domestic worker or domestic servant is a person who works within the scope of a residence. The term "domestic service" applies to the equivalent occupational category. In traditional English contexts, such a person was said to be "in service ...
*
Snap Snap or SNAP may refer to: Arts and entertainment * ''Snap'', the original release title for the 2013 film ''Enter the Dangerous Mind'' *''Snap'' (TV series), a CITV programme * ''The Stanly News and Press'', a newspaper in Albemarle, North Carol ...


Criticism

Jan-Willem Maessen, in 2002, and
Simon Peyton Jones Simon Peyton Jones (born 18 January 1958) is a British computer scientist who researches the implementation and applications of functional programming languages, particularly lazy functional programming. Education Peyton Jones graduated fr ...
, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it. In addition to purely practical considerations such as improved performance, they note that lazy evaluation makes it more difficult for programmers to reason about the performance of their code (particularly its space use). Bastiaan Heeren, Daan Leijen, and Arjan van IJzendoorn in 2003 also observed some stumbling blocks for Haskell learners: "The subtle syntax and sophisticated type system of Haskell are a double edged sword – highly appreciated by experienced programmers but also a source of frustration among beginners, since the generality of Haskell often leads to cryptic error messages." To address these, researchers from Utrecht University developed an advanced interpreter called
Helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, which improved the user-friendliness of error messages by limiting the generality of some Haskell features, and in particular removing support for type classes. Ben Lippmeier designed Disciple as a strict-by-default (lazy by explicit annotation) dialect of Haskell with a type-and-effect system, to address Haskell's difficulties in reasoning about lazy evaluation and in using traditional data structures such as mutable arrays. He argues (p. 20) that "destructive update furnishes the programmer with two important and powerful tools ... a set of efficient array-like data structures for managing collections of objects, and ... the ability to broadcast a new value to all parts of a program with minimal burden on the programmer."
Robert Harper Robert or Bob Harper may refer to: * Robert Almer Harper (1862–1946), American botanist * Robert Goodloe Harper (1765–1825), US senator from Maryland * Robert Harper (fl. 1734–1761), founder of Harpers Ferry, West Virginia * Robert Harper (a ...
, one of the authors of Standard ML, has given his reasons for not using Haskell to teach introductory programming. Among these are the difficulty of reasoning about resource use with non-strict evaluation, that lazy evaluation complicates the definition of datatypes and inductive reasoning, and the "inferiority" of Haskell's (old) class system compared to ML's module system. Haskell's build tool,
Cabal A cabal is a group of people who are united in some close design, usually to promote their private views or interests in an ideology, a state, or another community, often by intrigue and usually unbeknownst to those who are outside their group. Th ...
, has historically been criticized for poorly handling multiple versions of the same library, a problem known as "Cabal hell". The Stackage server and
Stack Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...
build tool were made in response to these criticisms. Cabal itself now has a much more sophisticated build system, heavily inspired by
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people with ...
, which became the default with version 3.0.


Related languages

Clean is a close, slightly older relative of Haskell. Its biggest deviation from Haskell is in the use of
uniqueness type In computing, a unique type guarantees that an object is used in a single-threaded way, with at most a single reference to it. If a value has a unique type, a function applied to it can be optimized to update the value in-place in the object code ...
s instead of monads for I/O and side-effects. A series of languages inspired by Haskell, but with different type systems, have been developed, including: *
Agda Agda may refer to: * Agda (programming language), the programming language and theorem prover * Agda (Golgafrinchan), the character in ''The Hitchhiker's Guide to the Galaxy'' by Douglas Adams * Liten Agda, the heroine of a Swedish legend * Agda M ...
, a functional language with
dependent type In computer science and logic, a dependent type is a type whose definition depends on a value. It is an overlapping feature of type theory and type systems. In intuitionistic type theory, dependent types are used to encode logic's quantifiers lik ...
s. *
Cayenne Cayenne (; ; gcr, Kayenn) is the capital city of French Guiana, an overseas region and Overseas department, department of France located in South America. The city stands on a former island at the mouth of the Cayenne River on the Atlantic Oc ...
, with dependent types. *
Elm Elms are deciduous and semi-deciduous trees comprising the flowering plant genus ''Ulmus'' in the plant family Ulmaceae. They are distributed over most of the Northern Hemisphere, inhabiting the temperate and tropical-montane regions of North ...
, a functional language to create web front-end apps, no support for user-defined or higher- kinded
type classes In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class T and ...
or instances. *
Epigram An epigram is a brief, interesting, memorable, and sometimes surprising or satirical statement. The word is derived from the Greek "inscription" from "to write on, to inscribe", and the literary device has been employed for over two mille ...
, a functional language with dependent types suitable for proving properties of programs. * Idris, a general purpose functional language with dependent types, developed at the
University of St Andrews (Aien aristeuein) , motto_lang = grc , mottoeng = Ever to ExcelorEver to be the Best , established = , type = Public research university Ancient university , endowment ...
. *
PureScript PureScript is a strongly-typed, purely-functional programming language that compiles to JavaScript. It can be used to develop web applications, server side apps, and also desktop applications with use of Electron. Its syntax is mostly comparabl ...
compiles to JavaScript. *
Ωmega The Omega interpreter is a strict pure functional programming interpreter similar to the Hugs Haskell interpreter. The syntax closely resembles that of Haskell but with important differences: * Omega is strict (Hugs is lazy); * Ability to int ...
, strict and more. Other related languages include: *
Curry A curry is a dish with a sauce seasoned with spices, mainly associated with South Asian cuisine. In southern India, leaves from the curry tree may be included. There are many varieties of curry. The choice of spices for each dish in tradit ...
, a functional/logic programming language based on Haskell. Notable Haskell variants include: * Generic Haskell, a version of Haskell with type system support for
generic programming Generic programming is a style of computer programming in which algorithms are written in terms of types ''to-be-specified-later'' that are then ''instantiated'' when needed for specific types provided as parameters. This approach, pioneered b ...
. * Hume, a strict functional language for
embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...
s based on processes as stateless automata over a sort of tuples of one element mailbox channels where the state is kept by feedback into the mailboxes, and a mapping description from outputs to channels as box wiring, with a Haskell-like expression language and syntax.


Conferences and workshops

The Haskell community meets regularly for research and development activities. The main events are: *
International Conference on Functional Programming The ACM SIGPLAN International Conference on Functional Programming (ICFP) is an annual academic conference in the field of computer science sponsored by the ACM SIGPLAN, in association with IFIP Working Group 2.8 (Functional Programming). The con ...
(ICFP) * Haskell Symposium (formerly the Haskell Workshop) * Haskell Implementors Workshop * Commercial Users of Functional Programming (CUFP) Starting in 2006, a series of organized ''hackathons'' has occurred, the Hac series, aimed at improving the programming language tools and libraries.


References


Bibliography

; Reports * * ; Textbooks * * * * * * *
full text
* ; Tutorials * *Learn You a Haskell for Great Good! - A community version (
learnyouahaskell.github.io
''. An up-to-date community maintained version of the renowned "Learn You a Haskell" (LYAH) guide.
Yet Another Haskell Tutorial
by Hal Daumé III; assumes far less prior knowledge than official tutorial * * ; History * *


External links

*
Official wikiHaskell Weekly
{{Authority control Academic programming languages Articles with example Haskell code Educational programming languages Functional languages Literate programming Pattern matching programming languages Programming languages created in 1990 Statically typed programming languages