[project @ 2002-07-04 16:22:02 by simonmar]
[packages/old-time.git] / Data / Either.hs
1 {-# OPTIONS -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
3 -- |
4 -- Module : Data.Either
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- The Either type, and associated operations.
13 --
14 -----------------------------------------------------------------------------
15
16 module Data.Either (
17 Either(..),
18 either -- :: (a -> c) -> (b -> c) -> Either a b -> c
19 ) where
20
21 #ifdef __GLASGOW_HASKELL__
22 import GHC.Base
23 #endif
24
25 {-|
26
27 The 'Either' type represents values with two possibilities: a value of
28 type @'Either' a b@ is either @'Left' a@ or @'Right' b@.
29
30 The 'Either' type is sometimes used to represent a value which is
31 either correct or an error; by convention, the 'Left' constructor is
32 used to hold an error value and the 'Right' constructor is used to
33 hold a correct value (mnemonic: \"right\" also means \"correct\").
34 -}
35 data Either a b = Left a | Right b deriving (Eq, Ord )
36
37 either :: (a -> c) -> (b -> c) -> Either a b -> c
38 either f _ (Left x) = f x
39 either _ g (Right y) = g y