Lens.hs

{-# LANGUAGE LiberalTypeSynonyms #-}
-------------------------------------------------------------------------------
-- |
-- Module      :  Data.Either.Lens
-- Copyright   :  (C) 2012 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  portable
--
-- Lenses for working with sums
------------------------------------------------------------------------------
module Data.Either.Lens
  ( traverseLeft
  , traverseRight
  ) where

import Control.Applicative
import Control.Lens

-- | A traversal for tweaking the left-hand value of an 'Either':
--
-- @traverseLeft :: 'Applicative' f => (a -> f b) -> 'Either' a c -> f ('Either' b c)@
traverseLeft :: Traversal (Either a c) (Either b c) a b
traverseLeft f (Left a)  = Left <$> f a
traverseLeft _ (Right c) = pure $ Right c
{-# INLINE traverseLeft #-}

-- | traverse the right-hand value of an 'Either':
--
-- @'traverseRight' = 'Data.Traversable.traverse'@
--
-- Unfortunately the instance for
-- @'Data.Traversable.Traversable' ('Either' c)@ is still missing from base,
-- so this can't just be 'Data.Traversable.traverse'
--
-- @traverseRight :: 'Applicative' f => (a -> f b) -> 'Either' c a -> f ('Either' c a)@
traverseRight :: Traversal (Either c a) (Either c b) a b
traverseRight _ (Left c) = pure $ Left c
traverseRight f (Right a) = Right <$> f a
{-# INLINE traverseRight #-}