Casting dynamic values

It would occasionally be nice to have a function cast :: (Typeable a,Typeable b) => a -> Maybe b which returns Just a if a and b have the same type, and Nothing otherwise. This may seem rather a curious need, but it arises with existential types; if you have data A = forall a . (context) => A a (context including Typeable a) then this allows you to getB :: (Typeable b) => A -> Maybe b getB (A a) = cast a and so extract a value from A, if you can guess its type. Clearly we can implement cast = fromDynamic . toDyn My question is: is this the most efficient way of doing it, or is there a better way?