{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE Safe #-}
-- |
-- Module      :  Data.Functor.Compose
-- Copyright   :  (c) Ross Paterson 2010
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
-- Composition of functors.
-- @since

module Data.Functor.Compose (
  ) where

import Data.Functor.Classes

import Control.Applicative
import Data.Data (Data)
import Data.Foldable (Foldable(foldMap))
import Data.Traversable (Traversable(traverse))
import GHC.Generics (Generic, Generic1)

infixr 9 `Compose`

-- | Right-to-left composition of functors.
-- The composition of applicative functors is always applicative,
-- but the composition of monads is not always a monad.
newtype Compose f g a = Compose { getCompose :: f (g a) }
  deriving (Data, Generic, Generic1)

-- Instances of lifted Prelude classes

instance (Eq1 f, Eq1 g) => Eq1 (Compose f g) where
    liftEq eq (Compose x) (Compose y) = liftEq (liftEq eq) x y

instance (Ord1 f, Ord1 g) => Ord1 (Compose f g) where
    liftCompare comp (Compose x) (Compose y) =
        liftCompare (liftCompare comp) x y

instance (Read1 f, Read1 g) => Read1 (Compose f g) where
    liftReadsPrec rp rl = readsData $
        readsUnaryWith (liftReadsPrec rp' rl') "Compose" Compose
        rp' = liftReadsPrec rp rl
        rl' = liftReadList rp rl

instance (Show1 f, Show1 g) => Show1 (Compose f g) where
    liftShowsPrec sp sl d (Compose x) =
        showsUnaryWith (liftShowsPrec sp' sl') "Compose" d x
        sp' = liftShowsPrec sp sl
        sl' = liftShowList sp sl

-- Instances of Prelude classes

instance (Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a) where
    (==) = eq1

instance (Ord1 f, Ord1 g, Ord a) => Ord (Compose f g a) where
    compare = compare1

instance (Read1 f, Read1 g, Read a) => Read (Compose f g a) where
    readsPrec = readsPrec1

instance (Show1 f, Show1 g, Show a) => Show (Compose f g a) where
    showsPrec = showsPrec1

-- Functor instances

instance (Functor f, Functor g) => Functor (Compose f g) where
    fmap f (Compose x) = Compose (fmap (fmap f) x)

instance (Foldable f, Foldable g) => Foldable (Compose f g) where
    foldMap f (Compose t) = foldMap (foldMap f) t

instance (Traversable f, Traversable g) => Traversable (Compose f g) where
    traverse f (Compose t) = Compose <$> traverse (traverse f) t

instance (Applicative f, Applicative g) => Applicative (Compose f g) where
    pure x = Compose (pure (pure x))
    Compose f <*> Compose x = Compose ((<*>) <$> f <*> x)

instance (Alternative f, Applicative g) => Alternative (Compose f g) where
    empty = Compose empty
    Compose x <|> Compose y = Compose (x <|> y)