-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Either
-- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/core/LICENSE)
+-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
--- $Id: Either.hs,v 1.4 2002/04/24 16:31:39 simonmar Exp $
---
-- The Either type, and associated operations.
--
-----------------------------------------------------------------------------
#ifdef __GLASGOW_HASKELL__
import GHC.Base
-#endif
+{-|
+
+The 'Either' type represents values with two possibilities: a value of
+type @'Either' a b@ is either @'Left' a@ or @'Right' b@.
+
+The 'Either' type is sometimes used to represent a value which is
+either correct or an error; by convention, the 'Left' constructor is
+used to hold an error value and the 'Right' constructor is used to
+hold a correct value (mnemonic: \"right\" also means \"correct\").
+-}
data Either a b = Left a | Right b deriving (Eq, Ord )
+-- | Case analysis for the 'Either' type.
+-- If the value is @'Left' a@, apply the first function to @a@;
+-- if it is @'Right' b@, apply the second function to @b@.
either :: (a -> c) -> (b -> c) -> Either a b -> c
either f _ (Left x) = f x
either _ g (Right y) = g y
+#endif /* __GLASGOW_HASKELL__ */