{-# LANGUAGE Trustworthy #-}{-# 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.-------------------------------------------------------------------------------moduleData.Either(Either(..),either,-- :: (a -> c) -> (b -> c) -> Either a b -> clefts,-- :: [Either a b] -> [a]rights,-- :: [Either a b] -> [b]partitionEithers,-- :: [Either a b] -> ([a],[b]))where#include "Typeable.h"#ifdef __GLASGOW_HASKELL__importGHC.BaseimportGHC.ShowimportGHC.Read#endifimportData.TypeableimportGHC.Generics(Generic)#ifdef __GLASGOW_HASKELL__{--- just for testingimport Test.QuickCheck-}{-|The 'Either' type represents values with two possibilities: a value oftype @'Either' a b@ is either @'Left' a@ or @'Right' b@.The 'Either' type is sometimes used to represent a value which iseither correct or an error; by convention, the 'Left' constructor isused to hold an error value and the 'Right' constructor is used tohold a correct value (mnemonic: \"right\" also means \"correct\").-}dataEitherab=Lefta|Rightbderiving(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)->Eitherab->ceitherf_(Leftx)=fxeither_g(Righty)=gy#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::[Eitherab]->[a]leftsx=[a|Lefta<-x]-- | Extracts from a list of 'Either' all the 'Right' elements-- All the 'Right' elements are extracted in order.rights::[Eitherab]->[b]rightsx=[a|Righta<-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::[Eitherab]->([a],[b])partitionEithers=foldr(eitherleftright)([],[])wherelefta~(l,r)=(a:l,r)righta~(l,r)=(l,a:r){-{-------------------------------------------------------------------- Testing--------------------------------------------------------------------}prop_partitionEithers :: [Either Int Int] -> Boolprop_partitionEithers x = partitionEithers x == (lefts x, rights x)-}