Data/Either.hs

{-# OPTIONS_GHC -XNoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Data.Either
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- The Either type, and associated operations.
--
-----------------------------------------------------------------------------

module Data.Either (
 Either(..),
 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.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)
-}