[project @ 2003-09-05 15:06:48 by ross]

[ghc-base.git] / Data / Either.hs

diff --git a/Data/Either.hs b/Data/Either.hs
index f3cd106..65b3661 100644 (file)
--- a/Data/Either.hs
+++ b/Data/Either.hs
@@ -1,15 +1,13 @@
 {-# OPTIONS -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 :  non-portable
---
--- $Id: Either.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
+-- Portability :  portable
 --
 -- The Either type, and associated operations.
 --
@@ -21,5 +19,26 @@ module Data.Either (
  ) where
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.Maybe
+import GHC.Base
 #endif
+
+#ifndef __HUGS__
+{-|
+
+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  /* __HUGS__ */