You need a language extension. Check out Section 7 of "Derivable type classes" http://research.microsoft.com/~simonpj/Papers/derive.htm Alas, I have not implemented the idea yet. (Partly because no one ever reports it as a problem; you are the first!) Simon | One day I was playing around with types and I came across this type: | | >data S m a = Nil | Cons a (m (S m a)) | | The idea being one could use generic functions with whatever | monad m (of course m doesn't need to be a monad but my | original idea was to be able to make mutable lists with some | sort of monad m.) | | Anyway in attempting to define a generic show instance for | the above datatype I finally came upon: | | >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 | | which boggles my mind. But hugs +98 and ghci | -fallow-undecidable-instances both allow it to compile but when i try