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Haskell
ParadigmFunctional
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
Websitewww.haskell.org
Major implementations
GHC, Hugs, NHC, JHC, Yhc, UHC
Dialects
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]
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] Frege,[16] Hack,[17] Idris,[18] Isabelle,[8] Java/Generics,[8] LiveScript,[19] Mercury,[8] Ωmega,[citation needed] Perl 6,[20] PureScript,[21] Python,[8][22] Rust,[23] Scala,[8][24] Swift,[25] Timber,[26] Visual Basic 9.0[8][9]

Haskell /ˈhæskəl/[27] is a statically typed, purely functional programming language with type inference and lazy evaluation.[28][29] Type classes, which enable type-safe operator overloading, originated in Haskell.[30] Its main implementation is the Glasgow Haskell Compiler. It is named after logician Haskell Curry.[1]

Haskell is based on the semantics, but not the syntax, of the Miranda programming language, which served to focus the efforts of the initial Haskell working group.[31] The last stable release was made in July 2010 with the next standard planned for 2020.[3]

Haskell is used in academia[32][33] and industry.[34] As of September 2019, Haskell was the 23rd most popular programming language in terms of Google searches[35] for tutorials and made up less than 1% of active users on the GitHub source code repository.[36]

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. 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.[37]

Haskell 1.0 to 1.4

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).

Type classes originated in Haskell

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.[37]

In February 1999, the Haskell 98 language standard was originally published as The Haskell 98 Report.[37] 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.

  • Hierarchical module names. Module names are allowed to consist of dot-separated sequences of capitalised 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 (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 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 were relaxed to allow more programs to type check.
  • Some syntax 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 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.
Haskell supports both sum types and product types

Features

Main article: Haskell features
See also: Glasgow Haskell Compiler § Extensions to Haskell
Haskell features lazy evaluation of expressions, that is, 'call by need'

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. Monads are a general framework that can model different kinds of computation, including error handling, nondeterminism, parsing and software transactional memory. Monads 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

See also: Haskell features § Examples

A "Hello, World!" program 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 function in Haskell, defined in a few different ways: 

-- 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

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 :: 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)

The Haskell logo at the top of this page was generated by Haskell code.[44]

Implementations

All listed implementations are distributed under open source licenses.[45]

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

  • The 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, LLVM (formerly Low Level Virtual Machine) bitcode.[46][47] GHC has become the de facto standard Haskell dialect.[48] There are libraries (e.g., bindings to OpenGL) that work only with GHC. GHC is also distributed with the Haskell platform.
  • The Utrecht Haskell Compiler (UHC) is a Haskell implementation from Utrecht University.[49] It supports almost all Haskell 98 features plus many experimental extensions. It is implemented using attribute grammars and is currently used mostly for research on generated type systems and language extensions.
  • 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.
  • LHC is a whole-program optimizing backend for GHC, based on Urban Boquist’s compiler intermediate language, GRIN.[50] Older versions of LHC were based on Jhc rather than GHC.

Implementations no longer actively maintained include:

  • The Haskell User's Gofer System (Hugs) is a bytecode interpreter. It was once one of the implementations used most widely, alongside the GHC compiler,[51] but has now been mostly replaced by GHCi. It also comes with a graphics library.
  • 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 web browsers.
  • HBC is an early implementation supporting Haskell 1.4. It was implemented by Lennart Augustsson in, and based on, Lazy ML. It has not been actively developed for some time.

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

  • Gofer was an educational dialect of Haskell, with a feature called constructor classes, developed by Mark Jones. It was supplanted by Hugs (see Haskell User's Gofer System, above).
  • Helium is 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.
  • Eta and Frege are dialects of Haskell targeting the Java Virtual Machine.

Applications

  • Darcs is a revision control system written in Haskell, with several innovative features, such as more precise control of patches to apply.
  • Cabal is a tool for building and packaging Haskell libraries and programs.[52]
  • Linspire GNU/Linux chose Haskell for system tools development.[53]
  • Xmonad is a window manager for the X Window System, written fully in Haskell.[54]
  • Git-annex is a tool to manage (big) data files under Git version control. It also provides a distributed file synchronization system (git-annex assistant).
  • GHC is also often a testbed for advanced functional programming features and optimizations in other programming languages.
  • Pandoc is a tool to convert one markup format into another.
  • The Shake build system, aiming to be reliable, robust, and fast.[55]
  • ShellCheck – A shell script static analysis tool.[56]

Industry

  • Facebook implements its anti-spam programs[57] in Haskell, as open-source software.[58]
  • Swift Navigation, a high precision GPS manufacturer, implements significant portions of its product in Haskell, providing some open source software.[59]
  • Bluespec SystemVerilog (BSV) is a language for semiconductor design that is an extension of Haskell. Also, Bluespec, Inc.'s tools are implemented in Haskell.
  • GitHub implemented Semantic, an open-source library for analysis, diffing, and interpretation of untrusted source code, in Haskell.[60]
  • Cryptol, a language and toolchain for developing and verifying cryptography algorithms, is implemented in Haskell.
  • Ampersand[61], a language[62] and toolchain[63] for building information systems with reactive behaviour, is being used by Ordina and TNO (both in the Netherlands) for purposes such as conceptual analysis, analysis of archimate-models, legal analysis and prototyping large information systems. The compiler[64] is written in Haskell. Web applications realised with Ampersand comprise the Semantic Treehouse[65] and RAP3[66].
  • seL4, the first formally verified microkernel,[67] used Haskell as a prototyping language for the OS developer.[67]: 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.[67]: p.3  The Haskell code thus served as an intermediate prototype before final C refinement.[67]: p.3 

Web

There are several web frameworks written for Haskell, including:[68]

Criticism

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.[70][71] In addition to purely practical considerations such as improved performance,[72] 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."[73] 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[74] 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.[75] 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 data types and inductive reasoning,[76] and the "inferiority" of Haskell's (old) class system compared to ML's module system.[77]

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.[78].

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:

Java virtual machine (JVM) based:

Other related languages include:

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

Haskell has served as a testbed for many new ideas in language design. There have been many Haskell variants produced, exploring new language ideas, including:

  • Parallel Haskell:
  • Distributed Haskell (formerly Goffin) and Eden.[citation needed]
  • Eager Haskell, based on speculative evaluation.
  • Several object-oriented versions: Haskell++, and Mondrian.
  • Generic Haskell, a version of Haskell with type system support for generic programming.
  • O'Haskell, an extension of Haskell adding object-orientation and concurrent programming support that "has ... been superseded by Timber."[87]
  • Disciple, a strict-by-default (laziness available by annotation) dialect of Haskell that supports destructive update, computational effects, type directed field projections and allied functional aspects.
  • Hume, a strict functional language for embedded systems 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:

Since 2006, a series of organized hackathons has occurred, the Hac series, aimed at improving the programming language tools and libraries.[88]

References

  1. ^ a b c Hudak et al. 2007.
  2. ^ a b Marlow, Simon (24 November 2009). "Announcing Haskell 2010". Haskell (Mailing list). Retrieved 12 March 2011. {{cite mailing list}}: Unknown parameter |mailinglist= ignored (|mailing-list= suggested) (help)
  3. ^ a b Riedel, Herbert (28 April 2016). "ANN: Haskell Prime 2020 committee has formed". Haskell-prime (Mailing list). Retrieved 6 May 2017. {{cite mailing list}}: Unknown parameter |mailinglist= ignored (|mailing-list= suggested) (help)
  4. ^ a b c d e f g h i j k l m 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}}: Cite journal requires |journal= (help); Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  8. ^ a b c d e f g h i j Hudak et al. 2007, pp. 12-45–46.
  9. ^ a b Meijer, Erik (2006). "Confessions of a Used Programming Language Salesman: Getting the Masses Hooked on Haskell". Oopsla 2007. CiteSeerX 10.1.1.72.868.
  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.{{cite news}}: CS1 maint: location (link)
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  13. ^ Heller, Martin (18 October 2011). "Turn up your nose at Dart and smell the CoffeeScript". JavaWorld. InfoWorld. Retrieved 9 February 2012.
  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.
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  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. ^ "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. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
  21. ^ Freeman, Phil (2016). "PureScript by Example". Leanpub. Retrieved 23 April 2017.
  22. ^ Kuchling, A. M. "Functional Programming HOWTO". Python v2.7.2 documentation. Python Software Foundation. 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.
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  28. ^ Type inference originally using Hindley-Milner type inference
  29. ^ a b c Peyton Jones 2003.
  30. ^ "Type classes, first proposed during the design of the Haskell programming language, ..." —John Garrett Morris (2013), "Type Classes and Instance Chains: A Relational Approach"
  31. ^ Edward Kmett, Edward Kmett – Type Classes vs. the World
  32. ^ "Haskell in education". Retrieved 15 February 2016.
  33. ^ "Haskell in research". Retrieved 15 February 2016.
  34. ^ "Haskell in industry". Retrieved 15 February 2016.
  35. ^ "PYPL PopularitY of Programming Language index". pypl.github.io. 6 September 2019. Retrieved 6 September 2019.{{cite web}}: CS1 maint: url-status (link)
  36. ^ Frederickson, Ben. "Ranking Programming Languages by GitHub Users". www.benfrederickson.com. Retrieved 6 September 2019.{{cite web}}: CS1 maint: url-status (link)
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  42. ^ Computer Language Benchmarks Game
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  44. ^ Haskell.org, Thompson-Wheeler logo in Haskell. (2010) The public domain Haskell script which generates .svg of this logo. The logo is a monad bind (>>=) overlaid by a lambda (λ).
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  49. ^ Utrecht Haskell Compiler
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  54. ^ xmonad.org
  55. ^ Shake Build System
  56. ^ ShellCheck
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  59. ^ Swift-Nav (2018)
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  63. ^ "Ampersand: Build database applications faster than anyone else, and keep your data pollution free as a bonus".
  64. ^ "Ampersand Automates Agile Information System Development".
  65. ^ "SETU".
  66. ^ "Repository for Ampersand Projects".
  67. ^ a b c d 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. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
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  77. ^ Robert Harper. "Modules matter most". Closed access icon
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  83. ^ "Glasgow Parallel Haskell".
  84. ^ "7.15. Parallel Haskell".
  85. ^ "4.12. Using SMP parallelism".
  86. ^ Todd Allen Amicon. "Computation Structures Group- MIT- LCS".
  87. ^ "O'Haskell".
  88. ^ "Hackathon – HaskellWiki".

Further reading

Reports
Textbooks
Tutorials
History


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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