foo :: MyTypeClass resultType => resultType foo = fromString (toString (undefined :: resultType))

but I can't seem to do this -- GHC's scoped type variables don't seem to help either, or at least I couldn't get it working.

In GHC, type variables in the signature bear no relationship to (local) type variables in the body of the function. Simon Peyton Jones and Mark Shields explain that decision in their paper: http://research.microsoft.com/~simonpj/papers/scoped-tyvars They argue that a type signature does not have to immediately precede the body of the function. A type signature may be far away. In that case, relying in the body of the function something that was bound far earlier in the code did not seem reasonable. Incidentally, Chameleon took an opposite decision. Thus, if you want to use resultType in the body of the function, you have to bind it somewhere in the body of the function: foo :: MyTypeClass resultType => resultType foo::rt = fromString (toString (undefined :: rt)) Omitting the type signature, as in foo = fromString $ toString undefined works too. This is almost the same example that has been explained in the thread "Solved: Typing problems with polymorphic recursion and typeclasses" on this thread two weeks ago.