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[[Image:Haskell Logo.jpg|frame|Haskell logo]]
'''Haskell''' is a standardized [[purely functional|pure]] [[functional_programming|functional]] [[programming language]] with [[non-strict programming language|non-strict semantics]]. Named after the logician [[Haskell Curry]], it was created by a committee formed in [[1987]] for the express purpose of defining such a language. The direct predecessor of Haskell was [[Miranda programming language|Miranda]] from [[1985]].
The latest semi-official language standard is '''Haskell 98''', intended to specify a minimal, portable version of the language for teaching and as a base for future extensions.
The language continues to evolve rapidly, with [[Hugs]] and [[Glasgow Haskell Compiler|GHC]] (see below) representing the current [[de facto#De facto standards|''de facto'' standard]].
Characterizing syntax features in Haskell include [[pattern matching]], [[currying]], [[list comprehension]]s, [[guard (computing)|guard]]s, and definable [[operator (programming)|operator]]s. The language also supports [[recursion|recursive]] functions and [[algebraic data type]]s, as well as [[lazy evaluation]]. Unique concepts include [[Monads_in_functional_programming|monad]]s, and [[type class]]es.
The combination of such features can make [[function (programming)|functions]] which would be difficult to write in a procedural programming language almost trivial to implement in Haskell.
The language is, as of [[as of 2002|2002]], the [[Lazy_evaluation|lazy]] [[functional programming language|functional language]] on which the most research is being performed. Several variants have been developed: parallelizable versions from [[Massachusetts Institute of Technology|MIT]] and [[Glasgow University|Glasgow]], both called Parallel Haskell; more parallel and distributed versions called Distributed Haskell (formerly Goffin) and [[Eden_programming_language|Eden]]; a [[speculative execution|speculatively evaluating]] version called Eager Haskell and several [[object orientation|object oriented]] versions: Haskell++, [[O'Haskell]] and [[Mondrian programming language|Mondrian]].
Although Haskell has a comparatively small user community, its strengths have been well applied to a few projects. [[Autrijus Tang]]'s [[Pugs]] is an implementation for the forthcoming Perl 6 language with an interpreter and compilers that proved useful already after just a few months of its writing. [[DARCS]] is a revision control system, with several innovative features.
There is also a Haskell-like language that offers a new method of support for [[GUI]] development called [[Clean programming language|Concurrent Clean]]. Its biggest deviation from Haskell is in the use of [[uniqueness type]]s for input as opposed to [[Monads in functional programming|monads]].
== Examples ==
=== Anatomy of a Haskell function ===
The [[Hello world program|"Hello World"]] of functional languages is the [[factorial]] function. Expressed as pure Haskell:
fac :: Integer -> Integer
fac 0 = 1
fac n | n>0 = n * fac (n-1)
This describes the factorial as a recursive function, with a single terminating base case. It is similar to the descriptions of factorials found in mathematics textbooks. Much of Haskell code is similar to mathematics in facility and syntax.
The first line of the factorial function shown is optional, and describes the ''types'' of this function. It can be read as ''the function fac'' (fac) ''has type'' (::) ''from integer to integer'' (Integer -> Integer). That is, it takes an integer as an argument, and returns another integer.
The second line relies on [[pattern matching]], an important part of Haskell programming. Note that parameters of a function are not in parentheses but separated by spaces. When the function's argument is 0 (zero) it will return the integer 1 (one). For all other cases the third line is tried. This is the [[recursion]], and executes the function again until the base case is reached. A [[guard (computing)|guard]] protects the third line from negative arguments that would run down unterminated.
The "Prelude" is a number of small functions analogous to C's standard library. Using the Prelude and writing in the "point free" (insert classic Haskell joke here) style of unspecified arguments, it becomes:
fac = product . enumFromTo 1
The above is close to mathematical definitions such as ''f = g <small>o</small> h'' (see [[function composition]]), and indeed, it's ''not'' an assignment of a value to a variable.
=== More complex examples ===
A simple [[RPN]] calculator:
calc = foldl f [] . words
where
f (x:y:zs) "+" = y+x:zs
f (x:y:zs) "-" = y-x:zs
f (x:y:zs) "*" = y*x:zs
f (x:y:zs) "/" = y/x:zs
f xs y = (read y :: Float):xs
A function which returns a stream of the [[Fibonacci numbers]] in linear time:
fibs = 0 : 1 : (zipWith (+) fibs (tail fibs))
The same function, presented with GHC's [[parallel list comprehension]] syntax:
fibs = 0 : 1 : [ a+b | a <- fibs | b <- tail fibs ]
The earlier factorial function, this time using a sequence of functions:
fac n = (foldl (.) id [\x -> x*k | k <- [1..n]]) 1
A remarkably concise function that returns the list of [[Hamming number]]s in order:
hamming = 1 : map (*2) hamming # map (*3) hamming # map (*5) hamming
where xxs@(x:xs) # yys@(y:ys)
| x==y = x : xs#ys
| x<y = x : xs#yys
| x>y = y : xxs#ys
== Implementations ==
The following all comply fully, or very nearly, with the Haskell 98 standard, and are distributed under [[open source]] licences. There are currently no commercial Haskell implementations.
* '''''[[Hugs]]''''' ([http://www.haskell.org/hugs/]) is a [[bytecode]] interpreter. It offers fast compilation of programs and reasonable execution speed. It also comes with a simple graphics library. Hugs is good for people learning the basics of Haskell, but is by no means a "toy" implementation. It is the most portable and lightweight of the Haskell implementations.
* '''''[[Glasgow Haskell Compiler|Glasgow Haskell Compiler]]''''' ([http://www.haskell.org/ghc/]). The Glasgow Haskell Compiler compiles to native code on a number of different architectures, and can also compile to C. GHC is probably the most popular Haskell compiler, and there are quite a few useful libraries (e.g. bindings to [[OpenGL]]) that will only work with GHC.
* '''''nhc98''''' ([http://www.cs.york.ac.uk/fp/nhc98/]) is another bytecode compiler, but the bytecode runs significantly faster than with Hugs. Nhc98 focuses on minimising memory usage, and is a particularly good choice for older, slower machines.
* '''''Jhc''''' ([http://repetae.net/john/computer/jhc/]) a haskell compiler emphasising speed and efficiency of generated programs as well as exploration of new program transformations.
* '''''Gofer''''' An educational version of Haskell, Gofer was developed by Mark Jones. It was supplanted by HUGS.
* '''''HBC''''' ([http://www.cs.chalmers.se/~augustss/hbc/hbc.html]) is another native-code Haskell compiler. It hasn't been actively developed for some time, but is still usable.
*'''''Helium''''' ([http://www.cs.uu.nl/helium/]) is a newer dialect of Haskell. The focus is on making it easy to learn. It currently lacks typeclasses, making it incompatible with many Haskell programs.
== Extensions ==
*[[O'Haskell]] is an extension of Haskell adding [[object-oriented programming|object-orientation]] and [[concurrent programming]] support.
== External links ==
* [http://www.haskell.org The Haskell Home Page]
* [http://www.haskell.org/hawiki/ The Haskell Wiki]
* [http://www.haskell.org/tutorial/ A Gentle Introduction to Haskell 98] ([http://www.haskell.org/tutorial/haskell-98-tutorial.pdf pdf] format)
* [http://haskell.org/papers/NSWC/jfp.ps Haskell vs. Ada vs. C++ vs. Awk vs. ... An Experiment in Software Prototyping Productivity]
* [http://www.willamette.edu/~fruehr/haskell/evolution.html The Evolution of a Haskell Programmer] - a slightly humorous overview of different programming styles available in Haskell
* [http://haskell.readscheme.org An Online Bibliography of Haskell Research]
* [http://www.haskell.org/humor/press.html Haskell Humor]
* [http://www.research.microsoft.com/~simonpj/papers/haskell-retrospective Wearing the hair shirt: a retrospective on Haskell]. Simon Peyton Jones, invited talk at POPL 2003.
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[[Category:Haskell dialects|*]]
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[[Category:Declarative programming languages]]
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