Hi, all I thought the right type for ContT should be newtype ContT m a = ContT {runContT :: forall r. (a-> m r) -> m r} and other control operators shift :: Monad m => (forall r . (a-> ContT m r) -> ContT m r) -> ContT m a reset :: Monad m => ContT m a -> ContT m a callCC :: ((a-> (forall r . ContT m r)) -> ContT m a) -> ContT m a unfortunately, I can not make callCC type check, and don't know how to do it. I managed to make shift, reset type check reset :: Monad m => ContT m a -> ContT m a reset e = ContT $ \ k -> runContT e return >>= k shift :: Monad m => (forall r . (a-> ContT m r) -> ContT m r) -> ContT m a shift e = ContT $ \ (k :: a -> m r) -> runContT ((e $ \ v -> ContT $ \c -> k v >>= c) :: ContT m r) return but still, I cann't use shift, reset in recursive jumpings like this? newtype H r m = H (H r m -> ContT m r) unH (H x) = x test = flip runContT return $ reset $ do jump <- shift (\f -> f (H f)) lift . print $ "hello" unH jump jump Have anyone tried this before? Best, bob