module Control.Comonad.Context where
import Control.Arrow ((&&&), first)
import Control.Comonad
import Control.Comonad.Context.Class
data Context s a = Context (s -> a) s
runContext f s = (a b, b) where
Context a b = f (Context id s)
instance ComonadContext s (Context s) where
getC (Context f s) = s
modifyC m (Context f c) = f (m c)
instance Functor (Context s) where
fmap f (Context f' s) = Context (f . f') s
instance Comonad (Context s) where
extract (Context f a) = f a
duplicate (Context f a) = Context (Context f) a
newtype ContextT s w a = ContextT { runContextT :: (w s -> a, w s) }
instance Comonad w => ComonadContext s (ContextT s w) where
getC = extract . snd . runContextT
modifyC m (ContextT (f,c)) = f (fmap m c)
instance Functor (ContextT b f) where
fmap f = ContextT . first (f .) . runContextT
instance Comonad w => Comonad (ContextT b w) where
extract = uncurry id . runContextT
duplicate (ContextT (f,ws)) = ContextT (ContextT . (,) f, ws)