Haskell /ˈhæskəl/^{[27]} is a standardized, generalpurpose compiled purely functional programming language, with nonstrict semantics and strong static typing.^{[28]} It is named after logician Haskell Curry.^{[1]} The latest standard of Haskell is Haskell 2010. As of May 2016, a group is working on the next version, Haskell 2020.^{[29]}
Haskell features a type system with type inference^{[30]} and lazy evaluation.^{[31]} Type classes first appeared in the Haskell programming language.^{[32]} Its main implementation is the Glasgow Haskell Compiler.
Haskell is based on the semantics, but not the syntax, of the language Miranda, which served to focus the efforts of the initial Haskell working group.^{[33]} Haskell is used widely in academia^{[34]}^{[35]} and industry.^{[36]}
Haskell  

Paradigm  functional, lazy/nonstrict, modular 
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 appeared  1990^{[1]} 
Stable release 
Haskell 2010^{[2]} / July 2010

Preview release 
Haskell 2020 announced^{[3]}

Typing discipline  static, strong, inferred 
OS  Crossplatform 
Filename extensions  .hs , .lhs 
Website  www 
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, Cayenne,^{[8]} Clean,^{[8]} Clojure,^{[12]} CoffeeScript,^{[13]} Curry,^{[8]} Elm, Epigram, Escher,^{[14]} F#,^{[15]} Frege,^{[16]} Hack,^{[17]} Idris,^{[18]} Isabelle,^{[8]} Java/Generics,^{[8]} LiveScript,^{[19]} Mercury,^{[8]} Ωmega, Perl 6,^{[20]} PureScript,^{[21]} Python,^{[8]}^{[22]} Rust,^{[23]} Scala,^{[8]}^{[24]} Swift,^{[25]} Timber,^{[26]} Visual Basic 9.0^{[8]}^{[9]} 
Following the release of Miranda by Research Software Ltd, in 1985, interest in lazy functional languages grew. By 1987, more than a dozen nonstrict, 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 functionallanguage design.^{[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).
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.^{[28]} The language continues to evolve rapidly, with the Glasgow Haskell Compiler (GHC) implementation representing the current de facto standard.^{[38]}
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 wellused and uncontroversial features previously enabled via compilerspecific flags.
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.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 (>=)
.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 encouraged to support several others that correspond to extensions added in Haskell 2010.Haskell features lazy evaluation, 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,^{[28]} and multiple implementations exist. Its main implementation, the Glasgow Haskell Compiler (GHC), is both an interpreter and nativecode 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 highperformance implementation of concurrency and parallelism.^{[42]}
An active, growing community exists around the language, and more than 5,400 thirdparty opensource libraries and tools are available in the online package repository Hackage.^{[43]}
A "Hello world" program in Haskell:^{[a]}
module Main where main :: IO () main = putStrLn "Hello, World!"
The factorial function in Haskell, defined in a few different ways (the type annotation is optional):
 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]  Pointfree style factorial = foldr (*) 1 . enumFromTo 1
An efficient implementation of the Fibonacci numbers as an infinite list:
 Type annotation (optional, same for each implementation) fib :: Int > Integer  With selfreferencing data fib n = fibs !! n where fibs = 0 : scanl (+) 1 fibs  0,1,1,2,3,5,...  Same, coded directly fib n = fibs !! n where fibs = 0 : 1 : next fibs next (a : t@(b:_)) = (a+b) : next t  Similar idea, using zipWith fib n = fibs !! n where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)  Using a generator function fib n = fibs (0,1) !! n where fibs (a,b) = a : fibs (b,a+b)
The Int type refers to a machinesized integer (used as a list subscript with the !! operator), while Integer is an arbitraryprecision integer. For example, using Integer, the factorial code above easily computes factorial 100000
as a number of 456,574 digits, 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)
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:
Haskell web frameworks exist,^{[60]} including:
JanWillem Maessen, in 2002, and Simon Peyton Jones, in 2003, discussed problems associated with lazy evaluation while also acknowledging the theoretical motives for it,^{[62]}^{[63]} in addition to purely practical considerations such as improved performance.^{[64]} 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."^{[65]} To address these, researchers from Utrecht University developed an advanced interpreter called Helium, which improved the userfriendliness of error messages by limiting the generality of some Haskell features, and in particular removing support for type classes.
Ben Lippmeier designed Disciple^{[66]} as a strictbydefault (lazy by explicit annotation) dialect of Haskell with a typeandeffect system, to address Haskell's difficulties in reasoning about lazy evaluation and in using traditional data structures such as mutable arrays.^{[67]} He argues (p. 20) that "destructive update furnishes the programmer with two important and powerful tools... a set of efficient arraylike 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 nonstrict evaluation, that lazy evaluation complicates the definition of data types and inductive reasoning,^{[68]} and the "inferiority" of Haskell's (old) class system compared to ML's module system.^{[69]}
It was consistently criticised by developers due to the lack of good management of different versions of a particular library by default build tool, Cabal. This has been addressed by the release of the Stack, but cabal is still shipped as the default build tool.
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 sideeffects.
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:
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:
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.^{[79]}
As Haskell separates domains (types) and behaviour (type classes) you may find correspondences with the algebra world that underlies the standard typeclasses of the Haskell basic library.
The Num class has the operation signatures required for a Ring, except for the (+) and (*) neutral elements, which are predefined as literals.^{[80]}
Operation laws like (+) and (*) associativity and addition commutativity are not related with the typeclass, but rather proofs to be checked on the instances.
$ ghci Prelude> :type 0 0 :: Num a => a  0 belongs to any type that has a Num instance, a Ring Prelude> :type 1 1 :: Num a => a  neutral for the product of a type that implements Num
The Word data types (Word, WordN) implement the Num type class with modular arithmetic whereas the data types Int and IntN use two's complement arithmetic that does not match the decimal arithmetic (see Integer overflow):
Prelude> (signum maxBound ::Int) == signum (maxBound +1) False  ??? maxBound successor doesn't obey arithmetic rule (it doesn't throw exceptions) Prelude> (signum maxBound ::Int) == signum (maxBound *2) False  ??? Int product overflow (no exceptions either) Prelude> (minBound ::Int) /= minBound  where Int minBound == sign bit set followed by zeros False  ??? Negate overflow
Possible workarounds to make it match with regular arithmetic, throwing an exception on overflow, are
The Fractional type class adds to Num the multiplicative inverse in the recip function (for "reciprocal") and, as a consequence, the division. It corresponds to a Field.^{[83]}
The Real type class requires Num and Ord, corresponds to an Ordered ring,^{[84]} which serves to Integer numbers, Rational^{[85]} and Floating point numbers.^{[86]}
The Integral type class adds operations for Euclidean division to the required Real and Enum classes, corresponding to a Euclidean ring, which is an integral ring.^{[87]}
The Floating type class adds to Fractional the calculus functions (sqrt, trigonometric functions, logarithms) common to floating point (Float, Double) and complex numbers.^{[88]}^{[86]}^{[89]}
Exponentiation comes in three flavours:
(^) :: (Num a, Integral ex) => a > ex > a  (^) admits nonnegative exponents of an euclidean domain, throws an error if the exponent is negative
(^^) :: (Fractional a, Integral ex) => a > ex > a  (^^) admits all exponents of an euclidean domain
(**) :: (Floating a, Floating ex) => a > ex > a
Conversions between Euclidean types preserve the representation, not the value. Numeric type downcasting does not throw an overflow exception:
$ ghci Prelude> import Data.Int Prelude Data.Int> fromIntegral (32767 :: Int16) :: Int8 1 Prelude Data.Int> fromInteger (2^64 :: Integer) :: Int32 0
main
in this simple example makes the module Main
a nicety, which instead would have been a necessity in a multimodule example. Rather, the first two lines are provided for consistency with larger examples.F# also draws from Haskell particularly with regard to two advanced language features called sequence expressions and workflows.
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 battletest ideas. Of course, it also greatly benefited from the experiences hardwon by many other languages in the field, drawing ideas from ObjectiveC, Rust, Haskell, Ruby, Python, C#, CLU, and far too many others to list.
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