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|
-----------------------------------------------------------------------------
-- |
-- Module : Data.Strict.Either
-- Copyright : (c) 2006-2007 Roman Leshchinskiy
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : Roman Leshchinskiy <rl@cse.unsw.edu.au>
-- Stability : experimental
-- Portability : portable
--
-- Strict @Either@.
--
-- Same as the standard Haskell @Either@, but @Left _|_ = Right _|_ = _|_@
--
-----------------------------------------------------------------------------
module Data.Strict.Either (
Either(..)
, either
, isLeft, isRight
, fromLeft, fromRight
) where
import Prelude hiding( Either(..), either )
-- | The strict choice type.
data Either a b = Left !a | Right !b deriving(Eq, Ord, Read, Show)
instance Functor (Either a) where
fmap _ (Left x) = Left x
fmap f (Right y) = Right (f y)
-- | Case analysis: 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
-- | Yields 'True' iff the argument is of the form @Left _@.
--
isLeft :: Either a b -> Bool
isLeft (Left _) = True
isLeft _ = False
-- | Yields 'True' iff the argument is of the form @Right _@.
--
isRight :: Either a b -> Bool
isRight (Right _) = True
isRight _ = False
-- | Extracts the element out of a 'Left' and throws an error if the argument
-- is a 'Right'.
fromLeft :: Either a b -> a
fromLeft (Left x) = x
fromLeft _ = error "Data.Strict.Either.fromLeft: Right"
-- | Extracts the element out of a 'Right' and throws an error if the argument
-- is a 'Left'.
fromRight :: Either a b -> b
fromRight (Right x) = x
fromRight _ = error "Data.Strict.Either.fromRight: Left"
|
