{-# LANGUAGE MultiParamTypeClasses #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Comonad.Contra -- Copyright : (C) 2011 Edward Kmett -- License : BSD-style (see the file LICENSE) -- -- Maintainer : Edward Kmett -- Stability : provisional -- Portability : MPTCs, fundeps -- -- Use a contravariant dual adjunction from Hask^op to build a 'Monad' to -- 'Comonad' transformer. ---------------------------------------------------------------------------- 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