module Control.Monad.Contra
( Contra
, runContra
, contra
, ContraT(..)
) where
import Prelude hiding (sequence)
import Control.Applicative
import Control.Comonad
import Control.Monad (ap)
import Data.Functor.Identity
import Data.Functor.Contravariant
import Data.Functor.Contravariant.Adjunction
type Contra f g = ContraT f g Identity
newtype ContraT f g w a = ContraT { runContraT :: g (w (f a)) }
contra :: Contravariant g => g (f a) -> Contra f g a
contra = ContraT . contramap runIdentity
runContra :: Contravariant g => Contra f g a -> g (f a)
runContra = contramap Identity . runContraT
instance (Adjunction f g, Functor w) => Functor (ContraT f g w) where
fmap f (ContraT g) = ContraT $ contramap (fmap (contramap f)) g
instance (Adjunction f g, Comonad w) => Applicative (ContraT f g w) where
pure = ContraT . leftAdjunct extract
(<*>) = ap
instance (Adjunction f g, Comonad w) => Monad (ContraT f g w) where
return = ContraT . leftAdjunct extract
ContraT m >>= f = ContraT $ contramap (extend (rightAdjunct (runContraT . f))) m