Local qualification in data types

Hi, I'm playing around with the type system trying to encode various stuff in it. I've gotten an error though which it would be useful to understand. Say I have a type class: class Foo a and then I want a datatype like this: data Bar a = Foo a => C1 | C2 ... | C3 This works fine syntactically but ghc says that I must have at least one existential type variable in the constraint. Hugs doesn't require that but has other problems.... Is there a good reason for not allowing the above? Cheers, /Josef