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|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
-----------------------------------------------------------------------------
-- |
-- 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,
lefts,
rights,
isLeft,
isRight,
partitionEithers,
) where
import GHC.Base
import GHC.Show
import GHC.Read
import Data.Typeable
{-
-- 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, Typeable)
instance Functor (Either a) where
fmap _ (Left x) = Left x
fmap f (Right y) = Right (f y)
instance Monad (Either e) where
return = Right
Left l >>= _ = Left l
Right r >>= k = k r
-- | 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
-- | 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)
-- | Return `True` if the given value is a `Left`-value, `False` otherwise.
isLeft :: Either a b -> Bool
isLeft (Left _) = True
isLeft (Right _) = False
-- | Return `True` if the given value is a `Right`-value, `False` otherwise.
isRight :: Either a b -> Bool
isRight (Left _) = False
isRight (Right _) = True
{-
{--------------------------------------------------------------------
Testing
--------------------------------------------------------------------}
prop_partitionEithers :: [Either Int Int] -> Bool
prop_partitionEithers x =
partitionEithers x == (lefts x, rights x)
-}
|
