[Haskell-cafe] Functional dependencies with Type Classes

Hi, suppose there are two (identical) classes:

class Res a b | a -> b where getRes :: a -> b

and

class Res2 t where type Member t getRes2 :: t -> Member t

It is easy to automatically make every instance of Res2 an instance of res:

instance Res2 a => Res a (Member a) where getRes x = getRes2 x

However, declaring every instance of Res an instance of Res2 seems impossible, as the following doesn't compile

instance Res a b => Res2 a where type Member a = b getRes2 x = getRes x

Question is: How to do this? The reason I need it is because I use a library which uses functional dependencies, but my classes shall be type families. Regards, Henning