On 3/27/08, Dan Doel
class Typeable t where data TypeRep t :: * typeRep :: TypeRep t
This is a really interesting idea. Sadly, I can't think of a way to make it work. The biggest problem is the TEquality class; by putting teq in a typeclass, the decision of which teq to call (and thus, whether or not to return Just Refl or Nothing) is made at compile time. The GADT solution puts that decision off until run-time; the case statement in teq allows a decision to be made at that point. What we'd really like to do is something like the following: class Typeable t where data TypeRep t :: * typeRep :: TypeRep t typeEq :: forall t2. TypeRep t2 -> Maybe (TEq t t2) instance Typeable Int where data TypeRep Int = TInt typeRep = TInt typeEq rep = case rep of TInt -> Just Refl _ -> Nothing But you can't case analyze t2 in this way; each TypeRep is an independent data type and there's no way to determine which one you have (aside from some sort of typeable/dynamic constraint, and then we have a circular problem!) Perhaps there is a solution, but I'm not clever enough to find it. -- ryan