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{-# LANGUAGE CPP, NoImplicitPrelude #-}
#ifdef __GLASGOW_HASKELL__
{-# LANGUAGE DeriveDataTypeable, DeriveGeneric, StandaloneDeriving #-}
#endif
-----------------------------------------------------------------------------
-- |
-- 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
import GHC.Generics (Generic)
#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, Generic)
-- | 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)
-}
|
