Haskell (programming language)

Last updated

Logo of the Haskell programming language.svg
Paradigm Purely functional
Designed by Lennart 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;31 years ago (1990) [1]
Stable release
Haskell 2010 [2] / July 2010;10 years ago (2010-07)
Preview release
Haskell 2020 announced [3]
Typing discipline Inferred, static, strong
OS Cross-platform
Filename extensions .hs, .lhs
Website www.haskell.org
Major implementations
GHC, Hugs, NHC, JHC, Yhc, UHC
Helium, Gofer
Influenced by
Clean, [4] FP, [4] Gofer, [4] Hope and Hope+, [4] Id, [4] ISWIM, [4] KRC, [4] Lisp, [4] Miranda, [4] ML and Standard ML, [4] Orwell, SASL, [4] Scheme, [4] SISAL [4]
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] Frege, [16] Hack, [17] Idris, [18] Isabelle, [8] Java/Generics, [8] LiveScript, [19] Mercury, [8] Ωmega, PureScript, [20] Python, [8] [21] Raku, [22] Rust, [23] Scala, [8] [24] Swift, [25] Timber, [26] Visual Basic 9.0 [8] [9]

Haskell ( /ˈhæskəl/ [27] ) is a general-purpose, statically typed, purely functional programming language with type inference and lazy evaluation. [28] [29] Designed for teaching, research and industrial application, Haskell has pioneered a number of advanced programming language features such as type classes, which enable type-safe operator overloading. Haskell's main implementation is the Glasgow Haskell Compiler (GHC). It is named after logician Haskell Curry. [1]


Haskell's semantics are historically based on those of the Miranda programming language, which served to focus the efforts of the initial Haskell working group. [30] 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 was planned for 2020. [3] [ needs update ]

Haskell is used in academia and industry. [31] [32] As of May 2021, Haskell was the 28th most popular programming language in terms of Google searches [33] for tutorials and made up less than 1% of active users on the GitHub source code repository. [34]


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. At the conference on Functional Programming Languages and Computer Architecture (FPCA '87) in Portland, Oregon, there was a strong consensus that a committee be formed to define an 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. [35]

Haskell 1.0 to 1.4

Type classes, which enable type-safe operator overloading, were first proposed by Philip Wadler and Stephen Blott for Standard ML but were first implemented in Haskell between 1987 and version 1.0. [36] [37]

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 of type classes 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. Base-classes.svg
Hierarchy of type classes 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. [35]

In February 1999, the Haskell 98 language standard was originally published as The Haskell 98 Report. [35] In January 2003, a revised version was published as Haskell 98 Language and Libraries: The Revised Report. [29] The language continues to evolve rapidly, with the Glasgow Haskell Compiler (GHC) implementation representing the current de facto standard. [38]

Haskell 2010

In early 2006, the process of defining a successor to the Haskell 98 standard, informally named Haskell Prime, began. [39] 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 [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.


Haskell features lazy evaluation, lambda expressions, pattern matching, list comprehension, type classes and type polymorphism. It is a purely functional language, which means that functions generally have no 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 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 overloading to the language, [40] but since finding many more uses. [41]

The construct that represents side-effects is an example of a monad: a general framework which can model various computations such as error handling, nondeterminism, parsing and software transactional memory. They are defined as ordinary datatypes, but Haskell provides some syntactic sugar for their use.

Haskell has an open, published specification, [29] and multiple implementations exist. Its main implementation, the Glasgow Haskell Compiler (GHC), is both an interpreter and native-code compiler that runs on most platforms. GHC is noted for its rich type system incorporating recent innovations such as generalized algebraic data types and type families. The Computer Language Benchmarks Game also highlights its high-performance implementation of concurrency and parallelism. [42]

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. [43]

Code examples

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

moduleMain(main)where-- not needed in interpreter, is the default in a module filemain::IO()-- the compiler can infer this type definitionmain=putStrLn"Hello, World!"

The factorial function in Haskell, defined in a few different ways:

-- [[Type signature|Type annotation]] (optional, same for each implementation)factorial::(Integrala)=>a->a-- Using recursion (with the "ifthenelse" expression)factorialn=ifn<2then1elsen*factorial(n-1)-- Using recursion (with pattern matching)factorial0=1factorialn=n*factorial(n-1)-- Using recursion (with guards)factorialn|n<2=1|otherwise=n*factorial(n-1)-- Using a list and the "product" functionfactorialn=product[1..n]-- Using fold (implements "product")factorialn=foldl(*)1[1..n]-- Point-free stylefactorial=foldr(*)1.enumFromTo1

As the Integer type has arbitrary-precision, 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 over lists, where the first element is taken as the pivot:

-- Type annotation (optional, same for each implementation)quickSort::Orda=>[a]->[a]-- Using list comprehensionsquickSort[]=[]-- The empty list is already sortedquickSort(x:xs)=quickSort[a|a<-xs,a<x]-- Sort the left part of the list++[x]++-- Insert pivot between two sorted partsquickSort[a|a<-xs,a>=x]-- Sort the right part of the list-- Using filterquickSort[]=[]quickSort(x:xs)=quickSort(filter(<x)xs)++[x]++quickSort(filter(>=x)xs)


All listed implementations are distributed under open source licenses. [44]

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

Implementations no longer actively maintained include:

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

Notable applications



Notable web frameworks written for Haskell include: [58]


Jan-Willem Maessen, in 2002, and Simon Peyton Jones, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it. [59] [60] In addition to purely practical considerations such as improved performance, [61] 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." [62] 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 classes.

Ben Lippmeier designed Disciple [63] 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. [64] 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, [65] and the "inferiority" of Haskell's (old) class system compared to ML's module system. [66]

Haskell's build tool, Cabal, has historically been criticised for poorly handling multiple versions of the same library, a problem known as "Cabal hell". The Stackage server and Stack build tool were made in response to these criticisms. [67] Cabal itself now has a much more sophisticated build system, heavily inspired by Nix, [68] which became the default with version 3.0.

Clean is a close, slightly older relative of Haskell. Its biggest deviation from Haskell is in the use of uniqueness types 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:

Other related languages include:

Notable Haskell variants include:

Conferences and workshops

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

Since 2006, a series of organized hackathons has occurred, the Hac series, aimed at improving the programming language tools and libraries. [69]

Related Research Articles

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 map values to other values, rather than a sequence of imperative statements which update the running state of the program.

ML is a general-purpose functional programming language. It is known for its use of the polymorphic Hindley–Milner type system, which automatically assigns the types of most expressions without requiring explicit type annotations, and ensures type safety – there is a formal proof that a well-typed ML program does not cause runtime type errors. ML provides pattern matching for function arguments, garbage collection, imperative programming, call-by-value and currying. It is used heavily in programming language research and is one of the few languages to be completely specified and verified using formal semantics. Its types and pattern matching make it well-suited and commonly used to operate on other formal languages, such as in compiler writing, automated theorem proving, and formal verification.

OCaml is a general-purpose, multi-paradigm programming language which extends the Caml dialect of ML with object-oriented features. OCaml was created in 1996 by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy, Ascánder Suárez, and others.

Standard ML (SML) is a general-purpose modular functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of theorem provers.

Clean (programming language)

Clean is a general-purpose purely functional computer programming language. For much of the language's active development history it was called Concurrent Clean, but this was dropped at some point. Clean is being developed by a group of researchers from the Radboud University in Nijmegen since 1987.

Programming languages can be grouped by the number and types of paradigms supported.

F Sharp (programming language) Microsoft programming language

F# is a functional-first, general purpose, strongly typed, multi-paradigm programming language that encompasses functional, imperative, and object-oriented programming methods. F# is most often used as a cross-platform Common Language Infrastructure (CLI) language on .NET, but it can also generate JavaScript and graphics processing unit (GPU) code.

D (programming language) Multi-paradigm system programming language

D, also known as Dlang, is a multi-paradigm system programming language created by Walter Bright at Digital Mars and released in 2001. Andrei Alexandrescu joined the design and development effort in 2007. Though it originated as a re-engineering of C++, D is a distinct language. It has redesigned some core C++ features, while also sharing characteristics of other languages, notably Java, Python, Ruby, C#, and Eiffel.

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, libraries, and optimisations that streamline the process of generating and executing code. GHC is the most commonly used Haskell compiler. The lead developers are Simon Peyton Jones and Simon Marlow.

LLVM Compiler backend for multiple programming languages

The LLVM compiler infrastructure project is a set of compiler and toolchain technologies, which 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 representation (IR) that serves as a portable, high-level assembly language that can be optimized with a variety of transformations over multiple passes.

Hope is a small functional programming language developed in the 1970s at the University of Edinburgh. It predates Miranda and Haskell and is contemporaneous with ML, also developed at the University. Hope was derived from NPL, a simple functional language developed by Rod Burstall and John Darlington in their work on program transformation. NPL and Hope are notable for being the first languages with call-by-pattern evaluation and algebraic data types.

SIGPLAN is the Association for Computing Machinery's Special Interest Group on programming languages.

C-- is a 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-level languages rather than written by human programmers. Unlike many other intermediate languages, its representation is plain ASCII text, not bytecode or another binary format.

Simon Peyton Jones British computer scientist

Simon Peyton Jones is a British computer scientist who researches the implementation and applications of functional programming languages, particularly lazy functional programming.

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 a type variable a, and means that a can only be instantiated to a type whose members support the overloaded operations associated with T.

Bluespec, Inc. is a semiconductor tool design company co-founded by Professor Arvind of MIT in June 2003. Arvind had previously founded Sandburst in 2000, which specialized in producing chips for 10G-bit Ethernet routers; for this task, Arvind had developed the Bluespec language, a high-level functional hardware description programming language which was essentially Haskell extended to handle chip design and electronic design automation in general. The main designer and implementor of Bluespec was Lennart Augustsson. Bluespec is partially evaluated and compiled to the term rewriting system (TRS). It comes with a SystemVerilog frontend.

In functional programming, a generalized algebraic data type is a generalization of parametric algebraic data types.

This article describes the features in Haskell.

In computer science, a type family associates data types with other data types, using a type-level function defined by an open-ended collection of valid instances of input types and the corresponding output types.


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 comparable to that of Haskell. In addition, it introduces row polymorphism and extensible records. Also, contrary to Haskell, PureScript adheres to a strict evaluation strategy.


  1. 1 2 3 Hudak et al. 2007.
  2. 1 2 Marlow, Simon (24 November 2009). "Announcing Haskell 2010". Haskell (Mailing list). Retrieved 12 March 2011.
  3. 1 2 Riedel, Herbert (28 April 2016). "ANN: Haskell Prime 2020 committee has formed". Haskell-prime (Mailing list). Retrieved 6 May 2017.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 Peyton Jones 2003, p. xi
  5. Norell, Ulf (2008). "Dependently Typed Programming in Agda" (PDF). Gothenburg: Chalmers University. Retrieved 9 February 2012.
  6. Hudak et al. 2007, pp. 12–38, 43.
  7. Stroustrup, Bjarne; Sutton, Andrew (2011). "Design of Concept Libraries for C++" (PDF). Archived from the original (PDF) on 10 February 2012.Cite journal requires |journal= (help)
  8. 1 2 3 4 5 6 7 8 9 10 Hudak et al. 2007, pp. 12-45–46.
  9. 1 2 Meijer, Erik (2006). "Confessions of a Used Programming Language Salesman: Getting the Masses Hooked on Haskell". Oopsla 2007. CiteSeerX .
  10. Meijer, Erik (1 October 2009). "C9 Lectures: Dr. Erik Meijer – Functional Programming Fundamentals, Chapter 1 of 13". Channel 9 . Microsoft. Retrieved 9 February 2012.
  11. Drobi, Sadek (4 March 2009). "Erik Meijer on LINQ". InfoQ. QCon SF 2008: C4Media Inc. Retrieved 9 February 2012.CS1 maint: location (link)
  12. Hickey, Rich. "Clojure Bookshelf". Listmania!. Archived from the original on 3 October 2017. Retrieved 3 October 2017.
  13. Heller, Martin (18 October 2011). "Turn up your nose at Dart and smell the CoffeeScript". InfoWorld . Retrieved 2020-07-15.
  14. "Declarative programming in Escher" (PDF). Retrieved 7 October 2015.
  15. Syme, Don; Granicz, Adam; Cisternino, Antonio (2007). Expert F#. Apress. p. 2. F# also draws from Haskell particularly with regard to two advanced language features called sequence expressions and workflows.
  16. Wechsung, Ingo. "The Frege Programming Language" (PDF). Retrieved 26 February 2014.
  17. "Facebook Introduces 'Hack,' the Programming Language of the Future". WIRED. 20 March 2014.
  18. "Idris, a dependently typed language" . Retrieved 26 October 2014.
  19. "LiveScript Inspiration" . Retrieved 4 February 2014.
  20. Freeman, Phil (2016). "PureScript by Example". Leanpub. Retrieved 23 April 2017.
  21. Kuchling, A. M. "Functional Programming HOWTO". Python v2.7.2 documentation. Python Software Foundation. Retrieved 9 February 2012.
  22. "Glossary of Terms and Jargon". Perl Foundation Perl 6 Wiki. The Perl Foundation. Archived from the original on 21 January 2012. Retrieved 9 February 2012.
  23. "The Rust Reference: Appendix: Influences" . Retrieved 3 February 2016.
  24. Fogus, Michael (6 August 2010). "MartinOdersky take(5) toList". Send More Paramedics. Retrieved 9 February 2012.
  25. Lattner, Chris (3 June 2014). "Chris Lattner's Homepage". Chris Lattner. Retrieved 3 June 2014. The Swift language is the product of tireless effort from a team of language experts, documentation gurus, compiler optimization ninjas, and an incredibly important internal dogfooding group who provided feedback to help refine and battle-test ideas. Of course, it also greatly benefited from the experiences hard-won by many other languages in the field, drawing ideas from Objective-C, Rust, Haskell, Ruby, Python, C#, CLU, and far too many others to list.
  26. "Timber/History" . Retrieved 7 October 2015.
  27. Chevalier, Tim (28 January 2008). "anybody can tell me the pronunciation of "haskell"?". Haskell-cafe (Mailing list). Retrieved 12 March 2011.
  28. Type inference originally using Hindley-Milner type inference
  29. 1 2 3 Peyton Jones 2003.
  30. Edward Kmett, Edward Kmett – Type Classes vs. the World
  31. Mossberg, Erik (8 June 2020), erkmos/haskell-companies , retrieved 22 June 2020
  32. O'Sullivan, Bryan; Goerzen, John; Stewart, Donald Bruce (15 November 2008). Real World Haskell: Code You Can Believe In. "O'Reilly Media, Inc.". pp. xxviii–xxxi. ISBN   978-0-596-55430-9.
  33. "PYPL PopularitY of Programming Language index". pypl.github.io. May 2021. Archived from the original on 7 May 2021. Retrieved 16 May 2021.
  34. Frederickson, Ben. "Ranking Programming Languages by GitHub Users". www.benfrederickson.com. Retrieved 6 September 2019.
  35. 1 2 3 Peyton Jones 2003, Preface.
  36. "Type classes, first proposed during the design of the Haskell programming language, ..." John Garrett Morris (2013), "Type Classes and Instance Chains: A Relational Approach"
  37. Wadler, Philip (October 1988). "How to make ad-hoc polymorphism less ad hoc".
  38. "Haskell Wiki: Implementations" . Retrieved 18 December 2012.
  39. "Welcome to Haskell'". The Haskell' Wiki.
  40. Wadler, P.; Blott, S. (1989). "How to make ad-hoc polymorphism less ad hoc". Proceedings of the 16th ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages. ACM: 60–76. doi:10.1145/75277.75283. ISBN   978-0-89791-294-5. S2CID   15327197.
  41. Hallgren, T. (January 2001). "Fun with Functional Dependencies, or Types as Values in Static Computations in Haskell". Proceedings of the Joint CS/CE Winter Meeting. Varberg, Sweden.
  42. Computer Language Benchmarks Game
  43. "HackageDB statistics". Hackage.haskell.org. Archived from the original on 3 May 2013. Retrieved 26 June 2013.
  44. "Implementations" at the Haskell Wiki
  45. "The LLVM Backend". GHC Trac.
  46. Terei, David A.; Chakravarty, Manuel M. T. (2010). "An LLVM Backend for GHC". Proceedings of ACM SIGPLAN Haskell Symposium 2010. ACM Press.
  47. C. Ryder and S. Thompson (2005). "Porting HaRe to the GHC API"
  48. Utrecht Haskell Compiler
  49. Hudak et al. 2007, pp. 12–22.
  50. "The Haskell Cabal" . Retrieved 8 April 2015.
  51. "Linspire/Freespire Core OS Team and Haskell". Debian Haskell mailing list. May 2006.
  52. xmonad.org
  53. "Fighting spam with Haskell". Facebook Code. 26 June 2015. Retrieved 11 August 2019.
  54. "Open-sourcing Haxl, a library for Haskell". Facebook Code. 10 June 2014. Retrieved 11 August 2019.
  55. https://github.com/input-output-hk/cardano-node
  56. Parsing, analyzing, and comparing source code across many languages: github/semantic, GitHub, 7 June 2019, retrieved 7 June 2019
  57. 1 2 3 4 A formal proof of functional correctness was completed in 2009. Klein, Gerwin; Elphinstone, Kevin; Heiser, Gernot; Andronick, June; Cock, David; Derrin, Philip; Elkaduwe, Dhammika; Engelhardt, Kai; Kolanski, Rafal; Norrish, Michael; Sewell, Thomas; Tuch, Harvey; Winwood, Simon (October 2009). "seL4: Formal verification of an OS kernel" (PDF). 22nd ACM Symposium on Operating System Principles. Big Sky, MT, USA.
  58. "Web/Frameworks – HaskellWiki". wiki.haskell.org. Retrieved 11 August 2019.
  59. Jan-Willem Maessen. Eager Haskell: Resource-bounded execution yields efficient iteration. Proceedings of the 2002 Association for Computing Machinery (ACM) SIGPLAN workshop on Haskell.
  60. Simon Peyton Jones. Wearing the hair shirt: a retrospective on Haskell. Invited talk at POPL 2003.
  61. "Lazy evaluation can lead to excellent performance, such as in The Computer Language Benchmarks Game".
  62. Heeren, Bastiaan; Leijen, Daan; van IJzendoorn, Arjan (2003). "Helium, for learning Haskell" (PDF). Proceedings of the 2003 ACM SIGPLAN Workshop on Haskell: 62–71. doi:10.1145/871895.871902. ISBN   1581137583. S2CID   11986908.
  63. "DDC – HaskellWiki". Haskell.org. 3 December 2010. Retrieved 26 June 2013.
  64. Ben Lippmeier, Type Inference and Optimisation for an Impure World, Australian National University (2010) PhD thesis, chapter 1
  65. Robert Harper. "The point of laziness". Closed Access logo transparent.svg
  66. Robert Harper. "Modules matter most". Closed Access logo transparent.svg
  67. "Solving Cabal Hell". www.yesodweb.com. Retrieved 11 August 2019.
  68. "Announcing cabal new-build: Nix-style local builds" . Retrieved 1 October 2019.
  69. "Hackathon – HaskellWiki".

Further reading