First idea: instead of rejecting unpack pragmas on polymorphic fields, have them require a class constraint on the field types. Example: data UnboxPair a b = (Unbox a, Unbox b) => UP {-# UNPACK #-} !a {-# UNPACK #-} !b The Unbox type class would be similar in spirit to the class with the same name in the vector package, but be implemented internally by GHC. To a first approximation instances would only exist for fields that unpack to non-pointer types (e.g. Int.) Johan I don't understand how this would work. What methods does the class Unbox have? What is the final representation of a UP constructor? How many words? Pointers or non-pointers? At the moment each constructor has a fixed layout. I'm really not sure what you have in mind here. More detail needed! Simon