Control.Monad.Trans.Cont

ContT.hs

newtype ContT r m a = ContT { runContT :: (a -> m r) -> m r }

instance Functor (ContT r m) where
  -- :: (a -> b) -> ContT r m a -> ContT r m b
  fmap a2b ca = ContT $ \b2r -> runContT ca (b2r . a2b)

instance Applicative (ContT r m) where
  -- :: a -> ContT r m a
  pure a = ContT $ \a2r -> a2r a
  -- :: ContT r m (a -> b) -> ContT r m a -> ContT r m b
  ca2b <*> ca = ContT $ \b2r ->
                  runContT ca2b $ \a2b ->
                    runContT ca $ \a -> b2r (a2b a)

instance Monad (ContT r m) where
  -- :: ContT r m a -> (a -> ContT r m b) -> ContT r m b
  ca >>= a2cb = ContT $ \b2r ->
                  runContT ca $ \a ->
                    runContT (a2cb a) b2r