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
_______________________________________________
Haskell mailing list
Haskell@haskell.org
http://www.haskell.org/mailman/listinfo/haskell
