Joker.hs

-----------------------------------------------------------------------------
-- |
-- Copyright   :  (C) 2008-2015 Edward Kmett
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  portable
--
-- From the Functional Pearl \"Clowns to the Left of me, Jokers to the Right: Dissecting Data Structures\"
-- by Conor McBride.
----------------------------------------------------------------------------
module Data.Bifunctor.Joker
  ( Joker(..)
  ) where

import Control.Applicative
import Data.Biapplicative
import Data.Bifoldable
import Data.Bitraversable
import Data.Foldable
import Data.Traversable

-- | Make a 'Functor' over the second argument of a 'Bifunctor'.
newtype Joker g a b = Joker { runJoker :: g b }
  deriving (Eq,Ord,Show,Read)

instance Functor g => Bifunctor (Joker g) where
  first _ = Joker . runJoker
  {-# INLINE first #-}
  second g = Joker . fmap g . runJoker
  {-# INLINE second #-}
  bimap _ g = Joker . fmap g . runJoker
  {-# INLINE bimap #-}

instance Functor g => Functor (Joker g a) where
  fmap g = Joker . fmap g . runJoker
  {-# INLINE fmap #-}

instance Applicative g => Biapplicative (Joker g) where
  bipure _ b = Joker (pure b)
  {-# INLINE bipure #-}

  Joker mf <<*>> Joker mx = Joker (mf <*> mx)
  {-# INLINE (<<*>>) #-}

instance Foldable g => Bifoldable (Joker g) where
  bifoldMap _ g = foldMap g . runJoker
  {-# INLINE bifoldMap #-}

instance Foldable g => Foldable (Joker g a) where
  foldMap g = foldMap g . runJoker
  {-# INLINE foldMap #-}

instance Traversable g => Bitraversable (Joker g) where
  bitraverse _ g = fmap Joker . traverse g . runJoker
  {-# INLINE bitraverse #-}

instance Traversable g => Traversable (Joker g a) where
  traverse g = fmap Joker . traverse g . runJoker
  {-# INLINE traverse #-}