Haskell
Haskell
2007-03-04 16:26:28
2007-03-09 13:04:52
Definition
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{\em Haskell} is a computer programming language designed by a committee in 1990 to consolidate the best features of the many purely functional programming languages that were created in the late 1980s. Haskell is thus neither a procedural programming language nor an object-oriented one, although it offers monads such as {\tt do} to support procedural programming and classes with inheritance to support object-oriented programming (there is also a variant of Haskell called O'Haskell which includes more support for object-oriented programming). In general, Haskell programs are most naturally written declaratively.
The standard version of the language is Haskell 98; Haskell 2007 hasn't been released yet but is expected to be only a minor revision of Haskell 98.
The standard Haskell prelude includes the function {\tt gcd}, which computes the greatest common divisor of two integers. The following Haskell code is a reimplementation of the {\tt gcd} function.
% gcd.hs -- compute the gcd of two integers
% Copyright (C) 2007 by Michael Slone.
%
% This program is free software. You may read, study,
% copy, modify, and distribute modified copies of this
% program according to the terms of the GNU General
% Public License, version 2.
%
% The first line,
% mygcd :: Int -> Int -> Int ,
% is a type declaration. It indicates that mygcd expects to
% receive two integers and expects to return a single integer.
% The function is defined using equations. If no equation
% applies, an error occurs. For example, trying ``mygcd 5 5.2''
% yields the following error message in ghci.
%
% *Main> mygcd 5 5.2
%
% <interactive>:1:8:
% No instance for (Fractional Int)
% arising from the literal `5.2' at <interactive>:1:8-10
% Probable fix: add an instance declaration for (Fractional Int)
% In the second argument of `mygcd', namely `5.2'
% In the definition of `it': it = mygcd 5 5.2
%
% Whitespace can matter in Haskell, so if you wish to use this
% code, please copy directly from the TeX source of this page
% instead of the HTML version.
\begin{verbatim}
-- gcd.hs -- compute the gcd of two integers
-- View this page in TeX mode for documentation and license.
mygcd :: Int -> Int -> Int
mygcd m n
| (n < 0) = mygcd m (abs n)
| (n == 0) = m
| (m < n) = mygcd n m
| otherwise = mygcd n (mymod m n)
mydiv :: Int -> Int -> Int
mydiv m n
| (m < 0) = negate (mydiv (negate m) n)
| (n < 0) = negate (mydiv m (negate n))
| (m < n) = 0
| otherwise = 1 + mydiv (m-n) n
mymod :: Int -> Int -> Int
mymod m n = m - n * (mydiv m n)
\end{verbatim}
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