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Haskell

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For other uses, seeHaskell (disambiguation).

Haskell
Logo of Haskell
ParadigmPurely functional
Designed byLennart Augustsson,Dave Barton, Brian Boutel, Warren Burton, Joseph Fasel, Kevin Hammond, Ralf Hinze,Paul Hudak,John Hughes,Thomas Johnsson, Mark Jones,Simon Peyton Jones,John Launchbury,Erik Meijer,John Peterson, Alastair Reid, Colin Runciman,Philip Wadler
First appeared1990;34 years ago(1990)[1]
Stable release
Haskell 2010[2] / July 2010;14 years ago(2010-07)
Preview release
Haskell 2020 announced[3]
Typing disciplineInferred,static,strong
OSCross-platform
Filename extensions.hs,.lhs
Websitehaskell.org
Majorimplementations
GHC,Hugs,NHC, JHC,Yhc,UHC
Dialects
Gofer
Influenced by
Clean,[4]FP,[4]Gofer,[4]Hopeand Hope+,[4]Id,[4]ISWIM,[4]KRC,[4]Lisp,[4]
Miranda,[4]MLandStandard ML,[4]Orwell,SASL,[4]Scheme,[4]SISAL[4]
Influenced
Agda,[5]Bluespec,[6]C++11/Concepts,[7]
C#/LINQ,[8][9][10][11]CAL,[citation needed]Cayenne,[8]Clean,[8]Clojure,[12]
CoffeeScript,[13]Curry,[8]Elm,
Epigram,[citation needed]Escher,[14]F#,[15]Hack,[16]Idris,[17]
Isabelle,[8]Java/Generics,[8]LiveScript,[18]
Mercury,[8]Ωmega,PureScript,[19]Python,[8][20]Raku,[21]
Rust,[22]Scala,[8][23]Swift,[24]
Visual Basic 9.0[8][9]

Haskell(/ˈhæskəl/[25]) is ageneral-purpose,statically-typed,purely functionalprogramming languagewithtype inferenceandlazy evaluation.[26][27]Designed for teaching, research, and industrial applications, Haskell has pioneered a number of programming languagefeaturessuch astype classes,which enabletype-safeoperator overloading,andmonadicinput/output(IO). It is named afterlogicianHaskell Curry.[1]Haskell's main implementation is theGlasgow Haskell Compiler(GHC).

Haskell'ssemanticsare historically based on those of theMirandaprogramming language, which served to focus the efforts of the initial Haskell working group.[28]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.[29][30][31]As of May 2021,Haskell was the 28th most popular programming language byGoogle searchesfor tutorials,[32]and made up less than 1% of active users on theGitHubsource code repository.[33]

History[edit]

After the release ofMirandaby 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 wasproprietary software.At the conference onFunctional Programming Languages and Computer Architecture(FPCA '87) inPortland, Oregon,there was a strong consensus that a committee be formed to define anopen standardfor such languages. The committee's purpose was to consolidate existingfunctional languagesinto a common one to serve as a basis for future research in functional-language design.[34]

Haskell 1.0 to 1.4[edit]

Haskell was developed by a committee, attempting to bring together off the shelf solutions where possible.

Type classes,which enabletype-safeoperator overloading,were first proposed byPhilip Wadlerand Stephen Blott to address the ad-hoc handling of equality types and arithmetic overloading in languages at the time.[35]

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.[36]In version 1.3,monadicIO 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.[1]The committee's efforts resulted in a series of language definitions (1.0, 1.1, 1.2, 1.3, 1.4).

Hierarchy oftype classesin 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[edit]

In late 1997, the series culminated inHaskell 98,intended to specify a stable, minimal, portable version of the language and an accompanying standardlibraryfor 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.[34]

In February 1999, the Haskell 98 language standard was originally published asThe Haskell 98 Report.[34]In January 2003, a revised version was published asHaskell 98 Language and Libraries: The Revised Report.[27]The language continues to evolve rapidly, with theGlasgow Haskell Compiler(GHC) implementation representing the currentde factostandard.[37]

Haskell 2010[edit]

In early 2006, the process of defining a successor to the Haskell 98 standard, informally namedHaskell Prime,began.[38]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, namedHaskell 2010,was announced in November 2009[2]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.Listinstead ofList), 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.
  • Theforeign function interface(FFI) allows bindings to other programming languages. Only bindings toCare 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-calledn+kpatterns (definitions of the formfact (n+1) = (n+1) * fact n) were no longer allowed. Thissyntactic sugarhad misleading semantics, in which the code looked like it used the(+)operator, but in fact desugared to code using(-)and(>=).
  • The rules oftype inferencewere relaxed to allow more programs to type check.
  • Somesyntaxissues (changes in the formal grammar) were fixed:pattern guardswere added, allowing pattern matching within guards; resolution ofoperator fixitywas specified in a simpler way that reflected actual practice; an edge case in the interaction of the language'slexical syntaxof operators and comments was addressed, and the interaction of do-notation and if-then-else was tweaked to eliminate unexpected syntax errors.
  • TheLANGUAGEpragmawas specified. By 2010, dozens of extensions to the language were in wide use, and GHC (among other compilers) provided theLANGUAGEpragma to specify individual extensions with a list of identifiers. Haskell 2010 compilers are required to support theHaskell2010extension and are encouraged to support several others, which correspond to extensions added in Haskell 2010.

Future standards[edit]

The next formal specification had been planned for 2020.[3]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.[39][40]

Features[edit]

Main article:Haskell features
See also:Glasgow Haskell Compiler § Extensions to Haskell

Haskell featureslazy evaluation,lambda expressions,pattern matching,list comprehension,type classesandtype polymorphism.It is apurely functional language,which means that functions generally have noside effects.A distinct construct exists to represent side effects,orthogonalto the type of functions. A pure function can return a side effect that is subsequently executed, modeling theimpure functionsof other languages.

Haskell has astrong,statictype system based onHindley–Milner type inference.Its principal innovation in this area is type classes, originally conceived as a principled way to addoverloadingto the language,[41]but since finding many more uses.[42]

The construct that represents side-effects is an example of amonad:a general framework which can model various computations such as error handling,nondeterminism,parsingandsoftware transactional memory.They are defined as ordinary datatypes, but Haskell provides somesyntactic sugarfor their use.

Haskell has an open, published specification,[27]andmultiple implementations exist.Its main implementation, theGlasgow Haskell Compiler(GHC), is both aninterpreterandnative-codecompilerthat runs on most platforms. GHC is noted for its rich type system incorporating recent innovations such asgeneralized algebraic data typesand type families.The Computer Language Benchmarks Gamealso highlights its high-performance implementation ofconcurrencyandparallelism.[43]

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 repositoryHackage.[44]

Code examples[edit]

See also:Haskell features § Examples

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

moduleMain(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!"

Thefactorialfunction in Haskell, defined in a few different ways (the first line is thetype annotation,which is optional and is the same for each implementation): 

factorial::(Integrala)=>a->a

-- Using recursion (with the "ifthenelse" expression)
factorialn=ifn<2
then1
elsen*factorial(n-1)

-- Using recursion (with pattern matching)
factorial0=1
factorialn=n*factorial(n-1)

-- Using recursion (with guards)
factorialn
|n<2=1
|otherwise=n*factorial(n-1)

-- Using a list and the "product" function
factorialn=product[1..n]

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

-- Point-free style
factorial=foldr(*)1.enumFromTo1

Using Haskell'sFixed-point combinatorallows this function to be written without any explicit recursion. 

importData.Function(fix)

factorial=fixfac
wherefacfx
|x<2=1
|otherwise=x*f(x-1)


As theIntegertype hasarbitrary-precision,this code will compute values such asfactorial 100000(a 456,574-digit number), with no loss of precision.

An implementation of an algorithm similar toquick sortover lists, where the first element is taken as the pivot: 

-- Type annotation (optional, same for each implementation)
quickSort::Orda=>[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)

Implementations[edit]

All listed implementations are distributed underopen source licenses.[45]

Implementations that fully or nearly comply with the Haskell 98 standard include:

  • TheGlasgow Haskell Compiler(GHC) compiles to native code on many different processor architectures, and toANSI C,via one of twointermediate languages:C--,or in more recent versions,LLVM(formerly Low Level Virtual Machine) bitcode.[46][47]GHC has become thede factostandard Haskell dialect.[48]There are libraries (e.g., bindings toOpenGL) that work only with GHC. GHC was also distributed with theHaskell 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 fromUtrecht University.[49]It supports almost all Haskell 98 features plus many experimental extensions. It is implemented usingattribute grammarsand is primarily used for research on generated type systems and language extensions.

Implementations no longer actively maintained include:

  • The Haskell User's Gofer System (Hugs) is abytecodeinterpreter. It was once one of the implementations used most widely, alongside the GHC compiler,[50]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 byLennart Augustssonin, 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 aJavaScriptbackend, allowing users to run Haskell programs inweb browsers.

Implementations not fully Haskell 98 compliant, and using a variant Haskell language, include:

  • Eta and Frege are dialects of Haskell targeting theJava virtual machine.
  • Goferis an educational dialect of Haskell, with a feature calledconstructor classes,developed by Mark Jones. It is supplanted by Haskell User's Gofer System (Hugs).
  • Helium, a newer dialect of Haskell. The focus is on making learning easier via clearer error messages by disabling type classes as a default.

Notable applications[edit]

  • The proof assistantAgdais written in Haskell.[51]
  • Cabalis a tool forbuildingand packaging Haskell libraries and programs.[52]
  • Darcsis arevision control systemwritten in Haskell, with several innovative features, such as more precise control of patches to apply.
  • GHCis also often a testbed for advanced functional programming features and optimizations in other programming languages.
  • Git-annexis a tool to manage (big) data files underGitversion control. It also provides a distributed file synchronization system (git-annex assistant).
  • LinspireLinux chose Haskell for system tools development.[53]
  • Pandocis a tool to convert one markup format into another.
  • Pugsis acompilerandinterpreterfor theRakuprogramming language(formerly Perl 6).
  • TidalCyclesis a domain special language for live coding musical patterns, embedded in Haskell.[54]
  • Xmonadis awindow managerfor theX Window System,written fully in Haskell.[55]
  • GarganText[56]is a collaborative tool to map through semantic analysis texts on anyweb browser,written fully in Haskell andPureScript,which is used for instance in the research community to draw up state-of-the-art reports and roadmaps.[57]

Industry[edit]

Web[edit]

Notableweb frameworkswritten for Haskell include:[66]

Criticism[edit]

Jan-Willem Maessen, in 2002, andSimon Peyton Jones,in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it.[67][68]In addition to purely practical considerations such as improved performance,[69]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."[70]To address the error messages researchers from Utrecht University developed an advanced interpreter calledHelium,which improved the user-friendliness of error messages by limiting the generality of some Haskell features. In particular it disables type classes by default.[71]

Ben Lippmeier designed Disciple[72]as astrict-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.[73]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,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,[74]and the "inferiority" of Haskell's (old) class system compared to ML's module system.[75]

Haskell's build tool,Cabal,has historically been criticized for poorly handling multiple versions of the same library, a problem known as "Cabal hell". The Stackage server andStackbuild tool were made in response to these criticisms.[76]Cabal itself now has a much more sophisticated build system, heavily inspired byNix,[77]which became the default with version 3.0.

Related languages[edit]

Cleanis a close, slightly older relative of Haskell. Its biggest deviation from Haskell is in the use ofuniqueness typesinstead 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 withdependent types.
  • Cayenne,with dependent types.
  • Elm,a functional language to create web front-end apps, no support for user-defined or higher-kindedtype classesor instances.
  • Epigram,a functional language with dependent types suitable for proving properties of programs.
  • Idris,a general purpose functional language with dependent types, developed at theUniversity of St Andrews.
  • PureScripttranspiles to JavaScript.
  • Ωmega,a strict language that allows introduction of newkinds,and programming at the type level.

Other related languages include:

  • Curry,a functional/logic programming language based on Haskell.

Notable Haskell variants include:

  • Generic Haskell,a version of Haskell with type system support forgeneric programming.
  • Hume,a strict functional language forembedded systemsbased 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[edit]

The Haskell community meets regularly for research and development activities. The main events are:

Starting in 2006, a series of organizedhackathonshas occurred, the Hac series, aimed at improving the programming language tools and libraries.[79]

References[edit]

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Bibliography[edit]

Reports
Textbooks
Tutorials
History

External links[edit]

Wikibooks has a book on the topic of:Haskell
Wikibooks has a book on the topic of:Write Yourself a Scheme in 48 Hours
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