content: {-# LANGUAGE CPP #-}
  #if __GLASGOW_HASKELL__ >= 702
  {-# LANGUAGE Safe #-}
  #endif
  #if __GLASGOW_HASKELL__ >= 706
  {-# LANGUAGE PolyKinds #-}
  #endif
  #if __GLASGOW_HASKELL__ >= 710
  {-# LANGUAGE AutoDeriveTypeable #-}
  #endif
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Data.Functor.Reverse
  -- Copyright   :  (c) Russell O'Connor 2009
  -- License     :  BSD-style (see the file LICENSE)
  --
  -- Maintainer  :  R.Paterson@city.ac.uk
  -- Stability   :  experimental
  -- Portability :  portable
  --
  -- Making functors whose elements are notionally in the reverse order
  -- from the original functor.
  -----------------------------------------------------------------------------
  
  module Data.Functor.Reverse (
      Reverse(..),
    ) where
  
  import Control.Applicative.Backwards
  import Data.Functor.Classes
  #if MIN_VERSION_base(4,12,0)
  import Data.Functor.Contravariant
  #endif
  
  import Prelude hiding (foldr, foldr1, foldl, foldl1, null, length)
  import Control.Applicative
  import Control.Monad
  #if MIN_VERSION_base(4,9,0)
  import qualified Control.Monad.Fail as Fail
  #endif
  import Data.Foldable
  import Data.Traversable
  import Data.Monoid
  
  -- | The same functor, but with 'Foldable' and 'Traversable' instances
  -- that process the elements in the reverse order.
  newtype Reverse f a = Reverse { getReverse :: f a }
  
  instance (Eq1 f) => Eq1 (Reverse f) where
      liftEq eq (Reverse x) (Reverse y) = liftEq eq x y
      {-# INLINE liftEq #-}
  
  instance (Ord1 f) => Ord1 (Reverse f) where
      liftCompare comp (Reverse x) (Reverse y) = liftCompare comp x y
      {-# INLINE liftCompare #-}
  
  instance (Read1 f) => Read1 (Reverse f) where
      liftReadsPrec rp rl = readsData $
          readsUnaryWith (liftReadsPrec rp rl) "Reverse" Reverse
  
  instance (Show1 f) => Show1 (Reverse f) where
      liftShowsPrec sp sl d (Reverse x) =
          showsUnaryWith (liftShowsPrec sp sl) "Reverse" d x
  
  instance (Eq1 f, Eq a) => Eq (Reverse f a) where (==) = eq1
  instance (Ord1 f, Ord a) => Ord (Reverse f a) where compare = compare1
  instance (Read1 f, Read a) => Read (Reverse f a) where readsPrec = readsPrec1
  instance (Show1 f, Show a) => Show (Reverse f a) where showsPrec = showsPrec1
  
  -- | Derived instance.
  instance (Functor f) => Functor (Reverse f) where
      fmap f (Reverse a) = Reverse (fmap f a)
      {-# INLINE fmap #-}
  
  -- | Derived instance.
  instance (Applicative f) => Applicative (Reverse f) where
      pure a = Reverse (pure a)
      {-# INLINE pure #-}
      Reverse f <*> Reverse a = Reverse (f <*> a)
      {-# INLINE (<*>) #-}
  
  -- | Derived instance.
  instance (Alternative f) => Alternative (Reverse f) where
      empty = Reverse empty
      {-# INLINE empty #-}
      Reverse x <|> Reverse y = Reverse (x <|> y)
      {-# INLINE (<|>) #-}
  
  -- | Derived instance.
  instance (Monad m) => Monad (Reverse m) where
  #if !(MIN_VERSION_base(4,8,0))
      return a = Reverse (return a)
      {-# INLINE return #-}
  #endif
      m >>= f = Reverse (getReverse m >>= getReverse . f)
      {-# INLINE (>>=) #-}
  #if !(MIN_VERSION_base(4,13,0))
      fail msg = Reverse (fail msg)
      {-# INLINE fail #-}
  #endif
  
  #if MIN_VERSION_base(4,9,0)
  instance (Fail.MonadFail m) => Fail.MonadFail (Reverse m) where
      fail msg = Reverse (Fail.fail msg)
      {-# INLINE fail #-}
  #endif
  
  -- | Derived instance.
  instance (MonadPlus m) => MonadPlus (Reverse m) where
      mzero = Reverse mzero
      {-# INLINE mzero #-}
      Reverse x `mplus` Reverse y = Reverse (x `mplus` y)
      {-# INLINE mplus #-}
  
  -- | Fold from right to left.
  instance (Foldable f) => Foldable (Reverse f) where
      foldMap f (Reverse t) = getDual (foldMap (Dual . f) t)
      {-# INLINE foldMap #-}
      foldr f z (Reverse t) = foldl (flip f) z t
      {-# INLINE foldr #-}
      foldl f z (Reverse t) = foldr (flip f) z t
      {-# INLINE foldl #-}
      foldr1 f (Reverse t) = foldl1 (flip f) t
      {-# INLINE foldr1 #-}
      foldl1 f (Reverse t) = foldr1 (flip f) t
      {-# INLINE foldl1 #-}
  #if MIN_VERSION_base(4,8,0)
      null (Reverse t) = null t
      length (Reverse t) = length t
  #endif
  
  -- | Traverse from right to left.
  instance (Traversable f) => Traversable (Reverse f) where
      traverse f (Reverse t) =
          fmap Reverse . forwards $ traverse (Backwards . f) t
      {-# INLINE traverse #-}
  
  #if MIN_VERSION_base(4,12,0)
  -- | Derived instance.
  instance Contravariant f => Contravariant (Reverse f) where
      contramap f = Reverse . contramap f . getReverse
      {-# INLINE contramap #-}
  #endif
  