-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-----------------------------------------------------------------------------
---
+-- |
-- 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.
--
module Data.Either (
Either(..),
- either -- :: (a -> c) -> (b -> c) -> Either a b -> c
+ either, -- :: (a -> c) -> (b -> c) -> Either a b -> c
+ lefts, -- :: [Either a b] -> [a]
+ rights, -- :: [Either a b] -> [b]
+ partitionEithers, -- :: [Either a b] -> ([a],[b])
) where
+#include "Typeable.h"
+
#ifdef __GLASGOW_HASKELL__
-import GHC.Maybe
+import GHC.Base
+import GHC.Show
+import GHC.Read
#endif
+
+import Data.Typeable
+
+#ifdef __GLASGOW_HASKELL__
+{-
+-- just for testing
+import Test.QuickCheck
+-}
+
+{-|
+
+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, Read, Show)
+
+-- | 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__ */
+
+INSTANCE_TYPEABLE2(Either,eitherTc,"Either")
+
+-- | Extracts from a list of 'Either' all the 'Left' elements
+-- All the 'Left' elements are extracted in order.
+
+lefts :: [Either a b] -> [a]
+lefts x = [a | Left a <- x]
+
+-- | Extracts from a list of 'Either' all the 'Right' elements
+-- All the 'Right' elements are extracted in order.
+
+rights :: [Either a b] -> [b]
+rights x = [a | Right a <- x]
+
+-- | Partitions a list of 'Either' into two lists
+-- All the 'Left' elements are extracted, in order, to the first
+-- component of the output. Similarly the 'Right' elements are extracted
+-- to the second component of the output.
+
+partitionEithers :: [Either a b] -> ([a],[b])
+partitionEithers = foldr (either left right) ([],[])
+ where
+ left a ~(l, r) = (a:l, r)
+ right a ~(l, r) = (l, a:r)
+
+{-
+{--------------------------------------------------------------------
+ Testing
+--------------------------------------------------------------------}
+prop_partitionEithers :: [Either Int Int] -> Bool
+prop_partitionEithers x =
+ partitionEithers x == (lefts x, rights x)
+-}
+