I just saw this http://stackoverflow.com/questions/25827227/why-cant-there-be-an-instance-of... Personally, I think the suggested type for callCC newtype ContT m a = ContT { runContT :: forall r. (a -> m r) -> m r } class MonadCont m where callCC :: (forall b. (a -> m b) -> m b) -> m a is much more intuitive and resembles the actual type of ContT much more closely then the current callCC and it can be Fixed. So I was wondering if there was an other reason than convention for not using that type. Silvio