[Haskell] Higher types in contexts

Is there any deep reason why I can't write a polymorphic type in a context? I think the record update problem can be (sort of) solved if you could write: class Has r Rev (forall a. [a] -> [a]) => HRClass r where setHRClass :: (forall a.[a] -> [a]) -> r -> r but polymorphic types are not allowed in contexts. Is this one of the problems SPJ considers "Hard" or is it a feasible extension? Barney.