[Haskell] class variable dynamic inference

Hi, I have defined some functors (with respective Functor instances) and a class that derives the equivalent non-functor representation class (Functor f) => Rep f a b | f a -> b My problem is that I need to define another class class (Functor f,Rep f d c) => FunctorOf f d | d -> f where in'' :: c -> d out'' :: d -> c where the variable c would be implicit and dynamically derived from the (f d) context. Of course this is not allowed as long as the variable c is not in the class context. Is there any way I can hack this around? defining the class functions like in'' :: Rep f d c => c -> d doesn't solve my problem as well. Thanks in advance, hugo