module Control.Comonad.Contra
( Contra
, runContra
, contra
, ContraT(..)
) where
import Prelude hiding (sequence)
import Control.Comonad
import Control.Monad (liftM)
import Data.Functor.Identity
import Data.Functor.Contravariant
import Data.Functor.Contravariant.DualAdjunction
type Contra f g = ContraT f g Identity
newtype ContraT f g m a = ContraT { runContraT :: f (m (g a)) }
contra :: Contravariant f => f (g a) -> Contra f g a
contra = ContraT . contramap runIdentity
runContra :: Contravariant f => Contra f g a -> f (g a)
runContra = contramap Identity . runContraT
instance (Contravariant f, Contravariant g, Monad m) => Functor (ContraT f g m) where
fmap f (ContraT g) = ContraT $ contramap (liftM (contramap f)) g
instance (DualAdjunction f g, Monad m) => Comonad (ContraT f g m) where
extract = rightAdjunctOp return . runContraT
extend f = ContraT . contramap (>>= leftAdjunctOp (f . ContraT)) . runContraT