Backwards.hs

-- |
-- Module      :  Control.Applicative.Backwards
-- Copyright   :  (c) Russell O'Connor 2009
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- Making functors with an 'Applicative' instance that performs actions
-- in the reverse order.
--
-- NB: This module is only included in @lens@ for backwards compatibility with
-- @transformers@ versions before 3.0.
module Control.Applicative.Backwards where

import Prelude hiding (foldr, foldr1, foldl, foldl1)
import Control.Applicative
import Data.Foldable
import Data.Traversable

-- | The same functor, but with an 'Applicative' instance that performs
-- actions in the reverse order.
newtype Backwards f a = Backwards { forwards :: f a }

-- | Derived instance.
instance (Functor f) => Functor (Backwards f) where
    fmap f (Backwards a) = Backwards (fmap f a)

-- | Apply @f@-actions in the reverse order.
instance (Applicative f) => Applicative (Backwards f) where
    pure a = Backwards (pure a)
    Backwards f <*> Backwards a = Backwards (a <**> f)

-- | Try alternatives in the same order as @f@.
instance (Alternative f) => Alternative (Backwards f) where
    empty = Backwards empty
    Backwards x <|> Backwards y = Backwards (x <|> y)

-- | Derived instance.
instance (Foldable f) => Foldable (Backwards f) where
    foldMap f (Backwards t) = foldMap f t
    foldr f z (Backwards t) = foldr f z t
    foldl f z (Backwards t) = foldl f z t
    foldr1 f (Backwards t) = foldl1 f t
    foldl1 f (Backwards t) = foldr1 f t

-- | Derived instance.
instance (Traversable f) => Traversable (Backwards f) where
    traverse f (Backwards t) = fmap Backwards (traverse f t)
    sequenceA (Backwards t) = fmap Backwards (sequenceA t)