[Haskell] functional dependencies not satisfactory?

[Wolfgang Jeltsch]

Hello,

I came across the following problem:

I define a class with a functional dependency like so:

    class C a b | a -> b

Then I want to define a datatype using the fact that the second argument of C 
is dependent on the first:

    data C a b => T a = MkT a b

But unfortunately, this doesn’t work, at least not in GHC.

I can try this:

    data T a = forall b. C a b => MkT a b

But if I do pattern matching on a value of T a, GHC doesn’t recognize that the 
type of MkT’s second argument is determined by the type of the first.  For 
example, the following function definition is not accepted:

    useB :: C a b => T a -> (b -> ()) -> ()
    useB (MkT a b) f = f b

In my opinion, the problem is that GHC doesn’t see that because of the 
functional dependency the type exists b. C a b => b is at least as general as 
the type forall b. C a b => b.  Is there a solution to this problem?

Best wishes,
Wolfgang