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In the 1980s, a committee was formed to create a standardized functional programming language with lazy evaluation. Haskell, named after the logician Haskell Curry, was the result of those deliberations. The latest version of the language is Haskell 98.
Some Haskell features are extremely simple recursion, pattern matching, list comprehensions, making difficult functions in other procedural languages almost trivial in Haskell. Haskell is, as of 2002, the functional language on which the most research is being performed. Several variants have been developed: parallelizable versions from MIT and Glasgow, both called Parallel Haskell, more parallel and distributed versions called Goffin and Eden, an eager version called Eager Haskell and several object oriented versions: Haskell++, O'Haskell and Mondrian.
There is also a Haskelllike language that offers support for GUI development called Concurrent Clean. Its biggest deviation from Haskell is in the use of Uniqueness types for input as opposed to Monads.
An educational version of Haskell called Gofer was developed by Mark Jones. It was supplanted by HUGS, the Haskell User's Gofer System (see the Implementations section of this article).
For more comprehensive information, see the haskell.org website, linked at the end of this article.
Examples
The classic definition of the factorial function:
fac 0 = 1
fac n = n * fac (n  1)
The cute definition of the factorial function (using a builtin Haskell list notation and the standard product
function):
fac n = product [1..n]
A naive implementation of a function which returns the nth number in the Fibonacci sequence:
fib 0 = 0
fib 1 = 1
fib n = fib (n  2) + fib (n  1)
A function which returns a list of the Fibonacci numbers in linear time:
fibs = 0 : 1 : (zipWith (+) fibs (tail fibs))
The previous function creates an infinite list, which is possible because of lazy evaluation. One could implement fib
as:
fib n = fibs !! n
(!!
is an operator which gets the nth element of a list).
The Quicksort alogrithm can be elegantly expressed in Haskell with the help of list comprehensions:
qsort [] = []
qsort (x:xs) =
qsort elts_lt_x ++ [x] ++ qsort elts_greq_x
where
elts_lt_x = [y  y < xs, y < x]
elts_greq_x = [y  y < xs, y >= x]
Implementations The following all comply (or very nearly comply) with the Haskell 98 standard, and are distributed under open source licences. There are currently no commerical Haskell implementations.
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