Also, used normally, the best you're going to get out of Data.Typeable is cast :: a -> Maybe b which means that if your two types don't have runtime equivalence

Also, used normally, the best you're going to get out of Data.Typeable is cast :: a -> Maybe b which means that if your two types don't have runtime equivalence you'll be handled a statically ensured runtime Nothing that you must handle gracefully. The semantics are extremely clean.