Recently I've been experimenting with a sort of OOP with GHC, using existential types and (overlapping, undecidable) multi-parameter type classes, but it doesn't seem to work as you might expect because of the way GHC resolves overloaded functions at compile-time. For example, given class A a
data WrappedA = forall a . A a => WrappedA a data A1 = A1 instance A A1 class B b data WrappedB = forall b . B b => WrappedB b data B1 = B1 instance B B1 class AB a b where toBool :: a -> b -> Bool instance (A a,B b) => AB a b where toBool _ _ = False instance AB A1 B1 where toBool _ _ = True instance AB WrappedA WrappedB where toBool (WrappedA a) (WrappedB b) = toBool a b a naive user (like me a month ago) might expect that this to work, so that toBool (WrappedA a) (WrappedB b) will return False unless a is an A1, and b a B1, in which case it returns True. In fact ghc5.02 (rightly) gives an error message with the second instance declaration: Could not unambiguously deduce (AB a b) from the context (A a, B b) The choice of (overlapping) instance declaration depends on the instantiation of `a, b' So is there any other way of doing this sort of dynamic lookup at runtime, in a reasonably neat way? _______________________________________________ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users