{-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE Trustworthy #-} {-# LANGUAGE TypeOperators #-} ----------------------------------------------------------------------------- -- | -- 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 4.9.0.0 ----------------------------------------------------------------------------- module Data.Functor.Compose ( Compose(..), ) where import Data.Functor.Classes import Control.Applicative import Data.Coerce (coerce) import Data.Data (Data) import Data.Type.Equality (TestEquality(..), (:~:)(..)) import GHC.Generics (Generic, Generic1) import Text.Read (Read(..), readListDefault, readListPrecDefault) 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 -- ^ @since 4.9.0.0 , Generic -- ^ @since 4.9.0.0 , Generic1 -- ^ @since 4.9.0.0 ) -- Instances of lifted Prelude classes -- | @since 4.9.0.0 instance (Eq1 f, Eq1 g) => Eq1 (Compose f g) where liftEq eq (Compose x) (Compose y) = liftEq (liftEq eq) x y -- | @since 4.9.0.0 instance (Ord1 f, Ord1 g) => Ord1 (Compose f g) where liftCompare comp (Compose x) (Compose y) = liftCompare (liftCompare comp) x y -- | @since 4.9.0.0 instance (Read1 f, Read1 g) => Read1 (Compose f g) where liftReadPrec rp rl = readData $ readUnaryWith (liftReadPrec rp' rl') "Compose" Compose where rp' = liftReadPrec rp rl rl' = liftReadListPrec rp rl liftReadListPrec = liftReadListPrecDefault liftReadList = liftReadListDefault -- | @since 4.9.0.0 instance (Show1 f, Show1 g) => Show1 (Compose f g) where liftShowsPrec sp sl d (Compose x) = showsUnaryWith (liftShowsPrec sp' sl') "Compose" d x where sp' = liftShowsPrec sp sl sl' = liftShowList sp sl -- Instances of Prelude classes -- | @since 4.9.0.0 instance (Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a) where (==) = eq1 -- | @since 4.9.0.0 instance (Ord1 f, Ord1 g, Ord a) => Ord (Compose f g a) where compare = compare1 -- | @since 4.9.0.0 instance (Read1 f, Read1 g, Read a) => Read (Compose f g a) where readPrec = readPrec1 readListPrec = readListPrecDefault readList = readListDefault -- | @since 4.9.0.0 instance (Show1 f, Show1 g, Show a) => Show (Compose f g a) where showsPrec = showsPrec1 -- Functor instances -- | @since 4.9.0.0 instance (Functor f, Functor g) => Functor (Compose f g) where fmap f (Compose x) = Compose (fmap (fmap f) x) a <$ (Compose x) = Compose (fmap (a <$) x) -- | @since 4.9.0.0 instance (Foldable f, Foldable g) => Foldable (Compose f g) where foldMap f (Compose t) = foldMap (foldMap f) t -- | @since 4.9.0.0 instance (Traversable f, Traversable g) => Traversable (Compose f g) where traverse f (Compose t) = Compose <$> traverse (traverse f) t -- | @since 4.9.0.0 instance (Applicative f, Applicative g) => Applicative (Compose f g) where pure x = Compose (pure (pure x)) Compose f <*> Compose x = Compose (liftA2 (<*>) f x) liftA2 f (Compose x) (Compose y) = Compose (liftA2 (liftA2 f) x y) -- | @since 4.9.0.0 instance (Alternative f, Applicative g) => Alternative (Compose f g) where empty = Compose empty (<|>) = coerce ((<|>) :: f (g a) -> f (g a) -> f (g a)) :: forall a . Compose f g a -> Compose f g a -> Compose f g a -- | The deduction (via generativity) that if @g x :~: g y@ then @x :~: y@. -- -- @since 4.14.0.0 instance (TestEquality f) => TestEquality (Compose f g) where testEquality (Compose x) (Compose y) = case testEquality x y of -- :: Maybe (g x :~: g y) Just Refl -> Just Refl -- :: Maybe (x :~: y) Nothing -> Nothing