content: {-# 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.
  --
  -----------------------------------------------------------------------------
  
  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)
  -}
  
  