[Haskell] Swapping parameters and type classes

Hi all If I have this type: data Foo a b = ... and this class class Bar (x :: * -> *) where ... I can imagine two ways to make Foo an instance of Bar. Either I must "apply" the 'a' or the 'b' in (Foo a b). Otherwise it will not have the right kind. To "apply" the 'a' I can do: instance Bar (Foo a) where ... But what if I want to "apply" the 'b' ? How do I do that ? Greetings, Mads Lindstrøm