Jay Cox complained that the following is not possible: | data S m a = Nil | Cons a (m (S m a)) | | instance (Show a, Show (m (S m a))) => Show (S m a) where | show Nil = "Nil" | show (Cons x y) = "Cons " ++ show x ++ " " ++ show y Ken Shan answered: | Here's how I've been handling such situations: | | data S m a = Nil | Cons a (m (S m a)) | | -- "ShowF f" means that the functor f "preserves Show" | class ShowF f where | showsPrecF :: (Int -> a -> ShowS) -> (Int -> f a -> ShowS) Actually, this class definition can be simplified to: class ShowF f where showsPrecF :: Show a => Int -> f a -> ShowS And the rest of Ken's code accordingly. /Koen. -- Koen Claessen http://www.cs.chalmers.se/~koen phone:+46-31-772 5424 mailto:koen@cs.chalmers.se ----------------------------------------------------- Chalmers University of Technology, Gothenburg, Sweden