View source for Haskell (programming language)

{{short description|Functional programming language}}
{{Infobox programming language
| name = Haskell
| logo = Logo_of_the_Haskell_programming_language.svg
| logo size = 250px
| logo alt = Logo of Haskell
| paradigm = [[Purely functional programming|Purely functional]]
| released = {{start date and age|df=yes|1990}}{{sfn|Hudak|Hughes|Peyton Jones|Wadler|2007}}
| designer = [[Biography:Lennart Augustsson|Lennart Augustsson]], Dave Barton, Brian Boutel, Warren Burton, Joseph Fasel, Kevin Hammond, Ralf Hinze, [[Biography:Paul Hudak|Paul Hudak]], [[Biography:John Hughes (computer scientist)|John Hughes]], Thomas Johnsson, Mark Jones, [[Biography:Simon Peyton Jones|Simon Peyton Jones]], [[Biography:John Launchbury|John Launchbury]], [[Biography:Erik Meijer (computer scientist)|Erik Meijer]], John Peterson, Alastair Reid, Colin Runciman, [[Biography:Philip Wadler|Philip Wadler]]
| developer = 
| latest release version = <!-- Of language specification, not [[Software:Haskell Platform|Haskell Platform]]! -->Haskell 2010<ref name="2010ann">{{cite mailing list |url=http://www.haskell.org/pipermail/haskell/2009-November/021750.html |title=Announcing Haskell 2010 |date=24 November 2009 |access-date=12 March 2011 |mailing-list=Haskell |last=Marlow |first=Simon}}</ref>
| latest release date = {{start date and age|df=yes|2010|07}}
| latest test version = Haskell 2020 announced<ref name="2020committee">{{cite mailing list|url=https://mail.haskell.org/pipermail/haskell-prime/2016-April/004050.html |title=ANN: Haskell Prime 2020 committee has formed |date=28 April 2016 |access-date=6 May 2017 |mailing-list=Haskell-prime |last=Riedel |first=Herbert}}</ref>
| latest test date = 
| typing = [[Type inference|Inferred]], static, strong
| implementations = [[Software:Glasgow Haskell Compiler|GHC]], [[Software:Hugs|Hugs]], NHC, JHC, [[Yhc]], UHC
| dialects = Helium, Gofer
| influenced by = [[Clean (programming language)|Clean]],<ref name="haskell-report-influences">{{harvnb|Peyton Jones|2003|p=xi}}</ref> [[FP (programming language)|FP]],<ref name="haskell-report-influences"/> [[Gofer (programming language)|Gofer]],<ref name="haskell-report-influences"/> [[Hope (programming language)|Hope]] and Hope<sup>+</sup>,<ref name="haskell-report-influences"/> [[Id (programming language)|Id]],<ref name="haskell-report-influences"/> [[ISWIM]],<ref name="haskell-report-influences"/> [[Kent Recursive Calculator|KRC]],<ref name="haskell-report-influences"/> [[Lisp (programming language)|Lisp]],<ref name="haskell-report-influences"/> [[Miranda (programming language)|Miranda]],<ref name="haskell-report-influences"/> [[ML (programming language)|ML]] and [[Standard ML]],<ref name="haskell-report-influences"/> [[Orwell (programming language)|Orwell]], [[SASL (programming language)|SASL]],<ref name="haskell-report-influences"/> [[Scheme (programming language)|Scheme]],<ref name="haskell-report-influences"/> [[SISAL]]<ref name="haskell-report-influences"/>
| operating system = Cross-platform
| license = 
| website = {{url|https://www.haskell.org}}
| file ext = .hs, .lhs
}}

'''Haskell''' ({{IPAc-en|ˈ|h|æ|s|k|əl}}<ref>{{cite mailing list |url=http://www.haskell.org/pipermail/haskell-cafe/2008-January/038756.html |title=anybody can tell me the pronunciation of "haskell"? |date=28 January 2008 |access-date=12 March 2011 |mailing-list=Haskell-cafe |last=Chevalier |first=Tim}}</ref>) is a [[General-purpose programming language|general-purpose]], statically typed, [[Purely functional programming|purely functional]] [[Programming language|programming language]] with [[Type inference|type inference]] and [[Lazy evaluation|lazy evaluation]].<ref>Type inference originally using [[Type inference#Hindley.E2.80.93Milner type inference algorithm|Hindley-Milner type inference]]</ref>{{sfn|Peyton Jones|2003}} Designed for teaching, research and industrial application, Haskell has pioneered a number of advanced programming language features such as type classes, which enable [[Type safety|type-safe]] [[Operator overloading|operator overloading]]. Haskell's main implementation is the [[Software:Glasgow Haskell Compiler|Glasgow Haskell Compiler]] (GHC). It is named after logician [[Biography:Haskell Curry|Haskell Curry]].{{sfn|Hudak|Hughes|Peyton Jones|Wadler|2007}}

Haskell's semantics are historically based on those of the [[Miranda (programming language)|Miranda]] programming language, which served to focus the efforts of the initial Haskell working group.<ref>Edward Kmett, [https://www.youtube.com/watch?v=hIZxTQP1ifo Edward Kmett – Type Classes vs. the World]</ref> The last formal specification of the language was made in July 2010, while the development of GHC has expanded Haskell via language extensions. The next formal specification {{As of|2020||alt=was planned for 2020|pre=}}.<ref name="2020committee" />{{update-inline|date=March 2021}}

Haskell is used in academia and industry.<ref>{{Citation|last=Mossberg|first=Erik|title=erkmos/haskell-companies|date=2020-06-08|url=https://github.com/erkmos/haskell-companies|access-date=2020-06-22}}</ref><ref>{{cite book |last1=O'Sullivan |first1=Bryan |last2=Goerzen |first2=John |last3=Stewart |first3=Donald Bruce |title=Real World Haskell: Code You Can Believe In |date=15 November 2008 |publisher="O'Reilly Media, Inc." |isbn=978-0-596-55430-9 |pages=xxviii-xxxi |url=https://books.google.com/books?id=nh0okI1a1sQC |language=en}}</ref><ref>{{Cite web|title=Haskell in Production: Riskbook|url=https://serokell.io/blog/haskell-in-industry-riskbook|access-date=2021-09-07|website=Serokell Software Development Company|language=en}}</ref> {{As of|2021|May}}, Haskell was the 28th most popular programming language in terms of Google searches<ref>{{Cite web|last=|first=|date=May 2021|title=PYPL PopularitY of Programming Language index|url=http://pypl.github.io/PYPL.html|url-status=dead|archive-url=https://web.archive.org/web/20210507222106/https://pypl.github.io/PYPL.html|archive-date=7 May 2021|access-date=16 May 2021|website=pypl.github.io|language=en}}</ref> for tutorials and made up less than 1% of active users on the GitHub source code repository.<ref>{{Cite web|url=https://www.benfrederickson.com/ranking-programming-languages-by-github-users/|title=Ranking Programming Languages by GitHub Users|last=Frederickson|first=Ben|website=www.benfrederickson.com|access-date=6 September 2019}}</ref>

== History ==

Following the release of [[Miranda (programming language)|Miranda]] by Research Software Ltd. in 1985, interest in lazy functional languages grew. By 1987, more than a dozen non-[[Strict function|strict]], purely functional programming languages existed. Miranda was the most widely used, but it was [[Proprietary software|proprietary software]]. At the conference on Functional Programming Languages and Computer Architecture (FPCA '87) in {{wipe|Portland, Oregon}}, there was a strong consensus that a committee be formed to define an [[Engineering:Open standard|open standard]] for such languages. The committee's purpose was to consolidate existing functional languages into a common one to serve as a basis for future research in functional-language design.{{sfn|Peyton Jones|2003|loc=Preface}}

=== Haskell 1.0 to 1.4 ===
Type classes, which enable [[Type safety|type-safe]] [[Operator overloading|operator overloading]], were first proposed by [[Biography:Philip Wadler|Philip Wadler]] and Stephen Blott for [[Standard ML]] but were first implemented in Haskell between 1987 and version 1.0.<ref>"Type classes, first proposed during the design of the Haskell programming language, ..."&nbsp;—[https://jgbm.github.io/pubs/morris-dissertation.pdf John Garrett Morris (2013), "Type Classes and Instance Chains: A Relational Approach"]</ref><ref>{{Cite web|url=https://www.researchgate.net/publication/2710954|title=How to make ad-hoc polymorphism less ad hoc|last=Wadler|first=Philip|date=October 1988}}</ref>

<!-- To do: Describe the change-over in I/O, from lazy streams to monads. -->
The first version of Haskell ("Haskell 1.0") was defined in 1990.{{sfn|Hudak|Hughes|Peyton Jones|Wadler|2007}} The committee's efforts resulted in a series of language definitions (1.0, 1.1, 1.2, 1.3, 1.4).

[[File:Base-classes.svg|thumb|right|Hierarchy of [[Type class|type class]]es in the Haskell prelude as of GHC 7.10. The inclusion of Foldable and Traversable (with corresponding changes to the type signatures of some functions), and of Applicative as intermediate between Functor and Monad, are deviations from the Haskell 2010 standard.]]

=== 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 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.{{sfn|Peyton Jones|2003|loc=Preface}}

In February 1999, the Haskell 98 language standard was originally published as ''The Haskell 98 Report''.{{sfn|Peyton Jones|2003|loc=Preface}} In January 2003, a revised version was published as ''Haskell 98 Language and Libraries: The Revised Report''.{{sfn|Peyton Jones|2003}} The language continues to evolve rapidly, with the [[Software:Glasgow Haskell Compiler|Glasgow Haskell Compiler]] (GHC) implementation representing the current ''de facto'' standard.<ref name=HaskellWikiImpl>{{cite web|title=Haskell Wiki: Implementations|url=http://www.haskell.org/haskellwiki/Implementations|access-date=18 December 2012}}</ref>

=== Haskell 2010 ===

In early 2006, the process of defining a successor to the Haskell 98 standard, informally named ''Haskell Prime'', began.<ref>{{cite web|url=https://prime.haskell.org/|title=Welcome to Haskell'|work=The Haskell' Wiki}}</ref> 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<ref name="2010ann"/> 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., <code>Data.List</code> instead of <code>List</code>), 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|foreign function interface]] (FFI) allows bindings to other programming languages. Only bindings to [[C (programming language)|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 <code>fact (n+1) = (n+1) * fact n</code>) were no longer allowed. This [[Syntactic sugar|syntactic sugar]] had misleading semantics, in which the code looked like it used the <code>(+)</code> operator, but in fact desugared to code using <code>(-)</code> and <code>(>=)</code>.
* The rules of [[Type inference|type inference]] were relaxed to allow more programs to type check.
* Some [[Syntax (programming languages)|syntax]] issues (changes in the formal grammar) were fixed: pattern guards were added, allowing pattern matching within guards; resolution of [[Order of operations|operator fixity]] was specified in a simpler way that reflected actual practice; an edge case in the interaction of the language's lexical syntax of operators and comments was addressed, and the interaction of do-notation and if-then-else was tweaked to eliminate unexpected syntax errors.
* The <code>LANGUAGE</code> [[Directive (programming)|pragma]] was specified. By 2010, dozens of extensions to the language were in wide use, and GHC (among other compilers) provided the <code>LANGUAGE</code> pragma to specify individual extensions with a list of identifiers. Haskell 2010 compilers are required to support the <code>Haskell2010</code> extension and are encouraged to support several others, which correspond to extensions added in Haskell 2010.

== Features ==
{{Main|Haskell features}}

Haskell features [[Lazy evaluation|lazy evaluation]], [[Anonymous function|lambda expressions]], [[Pattern matching|pattern matching]], [[List comprehension|list comprehension]], [[Type class|type class]]es and type polymorphism. It is a purely functional language, which means that functions generally have no [[Side effect (computer science)|side effects]]. A distinct construct exists to represent side effects, orthogonal to the type of functions. A pure function can return a side effect that is subsequently executed, modeling the [[Pure function#Impure functions|impure functions]] of other languages.

Haskell has a strong, static 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 [[Polymorphism (computer science)|overloading]] to the language,<ref name="wadler89">{{cite journal|last1=Wadler|first1=P.|first2=S. |last2=Blott|year=1989|title=How to make ad-hoc polymorphism less ad hoc|journal=Proceedings of the 16th ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages|publisher=[[Organization:Association for Computing Machinery|ACM]]|pages=60–76|doi=10.1145/75277.75283|isbn=978-0-89791-294-5|s2cid=15327197}}</ref> but since finding many more uses.<ref name="hallgren01">{{cite journal|last=Hallgren|first=T.|date=January 2001|title=Fun with Functional Dependencies, or Types as Values in Static Computations in Haskell|journal=Proceedings of the Joint CS/CE Winter Meeting|location=Varberg, Sweden|url=http://www.cs.chalmers.se/~hallgren/Papers/wm01.html}}</ref>

The construct that represents side-effects is an example of a [[Monad (functional programming)|monad]]: a general framework which can model various computations such as error handling, [[Nondeterministic algorithm|nondeterminism]], [[Parsing|parsing]] and [[Software transactional memory|software transactional memory]]. They are defined as ordinary datatypes, but Haskell provides some [[Syntactic sugar|syntactic sugar]] for their use.

Haskell has an open, published specification,{{sfn|Peyton Jones|2003}} and multiple implementations exist. Its main implementation, the [[Software:Glasgow Haskell Compiler|Glasgow Haskell Compiler]] (GHC), is both an [[Interpreter (computing)|interpreter]] and [[Machine code|native-code]] [[Compiler|compiler]] that runs on most platforms. GHC is noted for its rich type system incorporating recent innovations such as [[Generalized algebraic data type|generalized algebraic data type]]s and type families. [[The Computer Language Benchmarks Game]] also highlights its high-performance implementation of [[Concurrency (computer science)|concurrency]] and [[Parallel computing|parallelism]].<ref name="shootout">[https://benchmarksgame-team.pages.debian.net/benchmarksgame/fastest/haskell.html 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''.<ref name="hackage-stats">{{cite web|url=http://hackage.haskell.org/cgi-bin/hackage-scripts/stats|archive-url=https://web.archive.org/web/20130503114836/http://hackage.haskell.org/cgi-bin/hackage-scripts/stats|archive-date=3 May 2013 |title=HackageDB statistics |publisher=Hackage.haskell.org |access-date=26 June 2013}}</ref>

== Code examples ==

A "Hello, World!" program in Haskell (only the last line is strictly necessary):

<source lang="haskell">
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!"
</source>

The [[Factorial|factorial]] function in Haskell, defined in a few different ways:
<source lang="haskell">
-- [[Type signature|Type annotation]] (optional, 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 [1..n]

-- Using fold (implements "product")
factorial n = foldl (*) 1 [1..n]

-- Point-free style
factorial = foldr (*) 1 . enumFromTo 1
</source>

As the ''Integer'' type has arbitrary-precision, this code will compute values such as <code>factorial 100000</code> (a 456,574-digit number), with no loss of precision.

An implementation of an algorithm similar to quick sort over lists, where the first element is taken as the pivot:
<source lang="haskell">
-- Type annotation (optional, same for each implementation)
quickSort :: Ord a => [a] -> [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 | a <- xs, a >= x]  -- Sort the right part of the list

-- Using filter
quickSort []     = []
quickSort (x:xs) = quickSort (filter (<x) xs)
                   ++ [x] ++
                   quickSort (filter (>=x) xs)
</source>

== Implementations ==
<!-- To anyone wishing to expand or improve this section: the article "A History of Haskell: Being Lazy With Class", linked to in the Further reading section contains detailed descriptions of all implementations. -->

All listed implementations are distributed under open source licenses.<ref name="implementations">[http://www.haskell.org/haskellwiki/Implementations "Implementations"] at the Haskell Wiki</ref>

Implementations that fully or nearly comply with the Haskell 98 standard, include:
* The [[Software:Glasgow Haskell Compiler|Glasgow Haskell Compiler]] (GHC) compiles to native code on many different processor architectures, and to [[ANSI C]], via one of two intermediate languages: [[C--]], or in more recent versions, [[Software:LLVM|LLVM]] (formerly Low Level Virtual Machine) bitcode.<ref>{{cite web|
url=https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/Backends/LLVM|
title=The LLVM Backend|
website=GHC Trac}}</ref><ref>{{cite conference 
| url = http://www.cse.unsw.edu.au/~chak/papers/TC10.html
| title = An LLVM Backend for GHC
| last1 = Terei
| first1 = David A.
| last2 = Chakravarty
| first2 = Manuel M. T.
| date = 2010
| publisher = ACM Press
| book-title = Proceedings of ACM SIGPLAN Haskell Symposium 2010}}</ref> GHC has become the ''de facto'' standard Haskell dialect.<ref>C. Ryder and S. Thompson (2005). [http://kar.kent.ac.uk/14237/1/Tech_Chris.pdf "Porting HaRe to the GHC API"]</ref> There are libraries (e.g., bindings to [[Software:OpenGL|OpenGL]]) that work only with GHC. GHC is also distributed with the Haskell platform.
* 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 [[Organization:Utrecht University|Utrecht University]].<ref>[http://www.cs.uu.nl/wiki/UHC Utrecht Haskell Compiler]</ref> It supports almost all Haskell 98 features plus many experimental extensions. It is implemented using [[Attribute grammar|attribute grammar]]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 ([[Software:Hugs|Hugs]]) is a [[Software:Bytecode|bytecode]] interpreter. It was once one of the implementations used most widely, alongside the GHC compiler,{{sfn|Hudak|Hughes|Peyton Jones|Wadler|2007|pp=12–22}} 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 [[Biography:Lennart Augustsson|Lennart Augustsson]] in, and based on, [[Lazy ML]]. It has not been actively developed for some time.
* nhc98 is a bytecode compiler focusing on minimizing memory use.
** The York Haskell Compiler ([[Yhc]]) 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]] backend, allowing users to run Haskell programs in [[Software:Web browser|web browser]]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|Java Virtual Machine]].
* Gofer 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 (programming language)|Agda]] is written in Haskell.<ref>{{Citation|title=Agda 2|date=2021-10-15|url=https://github.com/agda/agda|publisher=Agda Github Community|access-date=2021-10-16}}</ref>
*[[Software:Cabal|Cabal]] is a tool for [[Software:Build automation|building]] and packaging Haskell libraries and programs.<ref>{{cite web|url=https://www.haskell.org/cabal/|title=The Haskell Cabal|access-date=8 April 2015}}</ref>
* [[Software:Darcs|Darcs]] is a revision control system written in Haskell, with several innovative features, such as more precise control of patches to apply.
* [[Software:Glasgow Haskell Compiler|GHC]] is also often a testbed for advanced functional programming features and optimizations in other programming languages.
* [[Software:Git-annex|Git-annex]] is a tool to manage (big) data files under [[Software:Git|Git]] version control. It also provides a distributed file synchronization system (git-annex assistant).
* [[Software:Linspire|Linspire]] Linux chose Haskell for system tools development.<ref>{{cite web|url=http://urchin.earth.li/pipermail/debian-haskell/2006-May/000169.html|title=Linspire/Freespire Core OS Team and Haskell|work=Debian Haskell mailing list|date=May 2006}}</ref>
* [[Software:Pandoc|Pandoc]] is a tool to convert one markup format into another.
* [[Pugs (programming)|Pugs]] is a [[Compiler|compiler]] and [[Interpreter (computing)|interpreter]] for the [[Raku (programming language)|Raku]] [[Programming language|programming language]] (formerly Perl 6).
*[[Software:Xmonad|Xmonad]] is a [[Window manager|window manager]] for the [[Software:X Window System|X Window System]], written fully in Haskell.<ref>xmonad.org</ref>

=== Industry ===

*Bluespec SystemVerilog (BSV) is a language for semiconductor design that is an extension of Haskell. Also, Bluespec, Inc.'s tools are implemented in Haskell.
* [[Cryptol]], a language and toolchain for developing and verifying [[Cryptography|cryptography]] algorithms, is implemented in Haskell.
* [[Company:Facebook|Facebook]] implements its anti-spam programs<ref>{{Cite web|url=https://code.fb.com/security/fighting-spam-with-haskell/|title=Fighting spam with Haskell|date=26 June 2015|website=Facebook Code|language=en-US|access-date=11 August 2019}}</ref> in Haskell, maintaining the underlying data access library as [[Open-source software|open-source software]].<ref>{{Cite web|url=https://code.fb.com/web/open-sourcing-haxl-a-library-for-haskell/|title=Open-sourcing Haxl, a library for Haskell|date=10 June 2014|website=Facebook Code|language=en-US|access-date=11 August 2019}}</ref>
*The [[Cardano (blockchain platform)|Cardano blockchain platform]] is implemented in Haskell.<ref>https://github.com/input-output-hk/cardano-node</ref>
*[[GitHub]] implemented Semantic, an open-source library for analysis, diffing, and interpretation of untrusted source code, in Haskell.<ref>{{Citation|title=Parsing, analyzing, and comparing source code across many languages: github/semantic|date=7 June 2019|url=https://github.com/github/semantic|publisher=GitHub|access-date=7 June 2019}}</ref>
* seL4, the first [[Formal methods|formally verified]] [[Microkernel|microkernel]],<ref name="klein-sosp09">
A formal proof of functional correctness was completed in 2009.
{{ cite conference
 | first1 = Gerwin
 | last1 = Klein
 | first2 = Kevin
 | last2 = Elphinstone
 | first3 = Gernot
 | last3 = Heiser
 | first4 = June
 | last4 = Andronick
 | first5 = David
 | last5 = Cock
 | first6 = Philip
 | last6 = Derrin
 | first7 = Dhammika
 | last7 = Elkaduwe
 | first8 = Kai
 | last8 = Engelhardt
 | first9 = Rafal
 | last9 = Kolanski
 | first10 = Michael
 | last10 = Norrish
 | first11 = Thomas
 | last11 = Sewell
 | first12 = Harvey
 | last12 = Tuch
 | first13 = Simon
 | last13 = Winwood
 | title = seL4: Formal verification of an OS kernel
 | book-title = 22nd ACM Symposium on Operating System Principles
 |date=October 2009
 | location = Big Sky, MT, USA
 | url = http://www.sigops.org/sosp/sosp09/papers/klein-sosp09.pdf
}}
</ref> used Haskell as a prototyping language for the OS developer.<ref name="klein-sosp09"/>{{rp|p.2}} At the same time, the Haskell code defined an executable specification with which to reason, for automatic translation by the theorem-proving tool.<ref name="klein-sosp09"/>{{rp|p.3}} The Haskell code thus served as an intermediate prototype before final [[C (programming language)|C]] refinement.<ref name="klein-sosp09"/>{{rp|p.3}}

=== Web ===

Notable [[Software:Web framework|web framework]]s written for Haskell include:<ref>{{Cite web|url=https://wiki.haskell.org/Web/Frameworks|title=Web/Frameworks – HaskellWiki|website=wiki.haskell.org|access-date=11 August 2019}}</ref>

* [[Software:Snap (web framework)|Snap]]
* [[Software:Yesod (web framework)|Yesod]]

==Criticism==

Jan-Willem Maessen, in 2002, and [[Biography:Simon Peyton Jones|Simon Peyton Jones]], in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it.<ref>Jan-Willem Maessen. ''Eager Haskell: Resource-bounded execution yields efficient iteration''. Proceedings of the 2002 [[Organization:Association for Computing Machinery|Association for Computing Machinery]] (ACM) SIGPLAN workshop on Haskell.</ref><ref>Simon Peyton Jones. [http://research.microsoft.com/~simonpj/papers/haskell-retrospective ''Wearing the hair shirt: a retrospective on Haskell'']. Invited talk at POPL 2003.</ref> In addition to purely practical considerations such as improved performance,<ref>{{cite web|url=http://www.haskell.org/pipermail/haskell/2006-June/018127.html|title=Lazy evaluation can lead to excellent performance, such as in The Computer Language Benchmarks Game}}</ref> they note that, in addition to adding some performance overhead, 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."<ref>{{cite journal|first1=Bastiaan |last1=Heeren |first2=Daan |last2=Leijen |first3=Arjan |last3=van IJzendoorn|year=2003|title=Helium, for learning Haskell|journal=Proceedings of the 2003 ACM SIGPLAN Workshop on Haskell|pages=62–71 |doi=10.1145/871895.871902 |isbn=1581137583 |s2cid=11986908 |url=http://www.cs.uu.nl/~bastiaan/heeren-helium.pdf}}</ref> To address these, researchers from Utrecht University developed an advanced interpreter called Helium, which improved the user-friendliness of error messages by limiting the generality of some Haskell features, and in particular removing support for [[Type class|type class]]es.

Ben Lippmeier designed Disciple<ref>{{cite web|url=http://www.haskell.org/haskellwiki/DDC |title=DDC – HaskellWiki |publisher=Haskell.org |date=3 December 2010 |access-date=26 June 2013}}</ref> as a [[Evaluation strategy|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.<ref>Ben Lippmeier, [http://www.cse.unsw.edu.au/~benl/papers/thesis/lippmeier-impure-world.pdf Type Inference and Optimisation for an Impure World], Australian National University (2010) PhD thesis, chapter 1</ref> He argues (p.&nbsp;20) that "destructive update furnishes the programmer with two important and powerful tools&nbsp;... 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."

[[Biography:Robert Harper (computer scientist)|Robert Harper]], 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,<ref>{{cite web|url=http://existentialtype.wordpress.com/2011/04/24/the-real-point-of-laziness/|title=The point of laziness|author=Robert Harper}} {{Closed access}}
</ref> and the "inferiority" of Haskell's (old) class system compared to ML's module system.<ref>{{cite web|url=http://existentialtype.wordpress.com/2011/04/16/modules-matter-most/|author=Robert Harper|title=Modules matter most.}} {{Closed access}}
</ref>

Haskell's build tool, [[Software:Cabal|Cabal]], has historically been criticized for poorly handling multiple versions of the same library, a problem known as "Cabal hell". The Stackage server and [[Software:Stack (Haskell)|Stack]] build tool were made in response to these criticisms.<ref>{{Cite web|url=https://www.yesodweb.com/blog/2012/11/solving-cabal-hell|title=Solving Cabal Hell|website=www.yesodweb.com|access-date=11 August 2019}}</ref> Cabal itself now has a much more sophisticated build system, heavily inspired by [[Software:Nix package manager|Nix]],<ref>{{Cite web|url=http://blog.ezyang.com/2016/05/announcing-cabal-new-build-nix-style-local-builds/|title=Announcing cabal new-build: Nix-style local builds|access-date=1 October 2019}}</ref> which became the default with version 3.0.

== Related languages ==

[[Clean (programming language)|Clean]] is a close, slightly older relative of Haskell. Its biggest deviation from Haskell is in the use of [[Uniqueness type|uniqueness type]]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, a functional language with [[Dependent type|dependent type]]s.
* [[Cayenne (programming language)|Cayenne]], with [[Dependent type|dependent type]]s.
* [[Elm (programming language)|Elm]], a functional language to create web front-end apps, no support for user-defined or higher-[[Kind (type theory)|kinded]] type classes or instances.
* [[Epigram (programming language)|Epigram]], a functional language with dependent types suitable for proving properties of programs.
* [[Idris (programming language)|Idris]], a general purpose functional language with [[Dependent type|dependent type]]s, developed at the [[Organization:University of St Andrews|University of St Andrews]].
* [[PureScript]] compiles to JavaScript.
* Ωmega, strict and more.{{Clarify|reason=What does "and more" mean?|date=June 2021}}

Other related languages include:
* [[Curry (programming language)|Curry]], a functional/logic programming language based on Haskell.

Notable Haskell variants include:
*[[Generic programming#Generic Haskell|Generic Haskell]], a version of Haskell with type system support for [[Generic programming|generic programming]].
*[[Hume (programming language)|Hume]], a strict functional language for [[Embedded system|embedded system]]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]] (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.<ref>{{cite web |title=Hackathon – HaskellWiki |url=http://haskell.org/haskellwiki/Hackathon}}</ref>

== References ==
{{Reflist|30em}}

==Further reading==
; Reports
* {{cite book| editor-first = Simon| editor-last = Peyton Jones|  title = Haskell 98 Language and Libraries: The Revised Report| url = http://haskell.org/onlinereport/| year = 2003| publisher = Cambridge University Press| isbn = 978-0521826143}}
* {{cite book| editor-first = Simon| editor-last = Marlow|  title = Haskell 2010 Language Report| url= https://www.haskell.org/definition/haskell2010.pdf| year = 2010| publisher = Haskell.org }}
; Textbooks
*{{cite book|first=Antony|last=Davie|title=An Introduction to Functional Programming Systems Using Haskell|publisher=Cambridge University Press|year=1992|isbn=978-0-521-25830-2}}
*{{cite book|first=Richard|last=Bird|title=Introduction to Functional Programming using Haskell|edition=2nd|publisher=Prentice Hall Press|year=1998|isbn=978-0-13-484346-9|url=http://www.cs.ox.ac.uk/publications/books/functional/}}
*{{cite book|first=Paul|last=Hudak|title=The Haskell School of Expression: Learning Functional Programming through Multimedia|publisher=Cambridge University Press|location=New York|year=2000|isbn=978-0521643382|url=http://www.cs.yale.edu/homes/hudak/SOE/}}
*{{cite book|first=Graham|last=Hutton|title=Programming in Haskell|year=2007|publisher=Cambridge University Press|isbn=978-0521692694|url=http://www.cs.nott.ac.uk/~gmh/book.html}}
*{{Cite book|first1=Bryan|last1=O'Sullivan|first2=Don|last2=Stewart|author-link2=Don Stewart (computer programmer)|last3=Goerzen|first3=John|author-link3=John Goerzen|title=Real World Haskell|year=2008|publisher=O'Reilly|location=Sebastopol|isbn=978-0-596-51498-3|postscript=&nbsp;([http://book.realworldhaskell.org/read/ full text])|title-link=Real World Haskell}}
*{{cite book|first=Simon|last=Thompson|title=Haskell: The Craft of Functional Programming|edition=3rd|publisher=Addison-Wesley|year=2011|isbn=978-0201882957|url=http://www.haskellcraft.com/}}
*{{cite book|last=Lipovača|first=Miran|title=Learn You a Haskell for Great Good!|url=https://archive.org/details/learnyouhaskellf00lipo_0|date=April 2011|publisher=No Starch Press|location=San Francisco|isbn=978-1-59327-283-8|url-access=registration}} ([http://learnyouahaskell.com/chapters full text])
*{{cite book|first=Richard|last=Bird|title=Thinking Functionally with Haskell |publisher=Cambridge University Press|year=2014|isbn= 978-1-107-45264-0}}
; Tutorials
*{{cite web|url=http://haskell.org/tutorial/|title=A Gentle Introduction To Haskell, Version 98|date=June 2000|first1=Paul |last1=Hudak |first2=John |last2=Peterson |first3=Joseph |last3=Fasel|work=Haskell.org}}
*[http://hal3.name/docs/daume02yaht.pdf Yet Another Haskell Tutorial], by Hal Daumé III; assumes far less prior knowledge than official tutorial
*{{Cite journal|last=Yorgey|first=Brent|date=12 March 2009|title=The Typeclassopedia|journal=The Monad.Reader|issue=13|pages=17–68|url=http://www.haskell.org/wikiupload/8/85/TMR-Issue13.pdf}}
*{{cite book|first=Sandy|last=Maguire|title=Thinking with Types: Type-Level Programming in Haskell|year=2018|url=https://thinkingwithtypes.com/}}
; History
* {{cite journal| first1 = Paul| last1 = Hudak|  first2 = John| last2 = Hughes| author-link2 = John Hughes (computer scientist)| first3 = Simon| last3 = Peyton Jones| author-link3 = Simon Peyton Jones| first4 = Philip| last4 = Wadler| author-link4 = Philip Wadler| title = A History of Haskell: Being Lazy with Class| url = http://research.microsoft.com/~simonpj/papers/history-of-haskell/history.pdf | doi = 10.1145/1238844.1238856| year = 2007| journal = Proceedings of the Third ACM SIGPLAN Conference on History of Programming Languages (HOPL III)| pages = 12–1–55| isbn = 978-1-59593-766-7| s2cid = 52847907}}
* {{cite journal|last=Hamilton|first=Naomi|date=19 September 2008|title=The A-Z of Programming Languages: Haskell|journal=Computerworld|url=http://www.computerworld.com.au/article/261007/a-z_programming_languages_haskell/}}

==External links==
*[https://wiki.haskell.org/ Official wiki]


{{Programming languages}}
[[Category:Academic programming languages]]
[[Category:Educational programming languages]]
[[Category:Functional languages]]
[[Category:Haskell programming language family| ]]
[[Category:Literate programming]]
[[Category:Pattern matching programming languages]]
[[Category:Statically typed programming languages]]

{{Sourceattribution|Haskell (programming language)|1}}