Is there any reasonable way to do this if I want to cast a monadic value? For example:
castState :: (Typeable a, Typeable s, Typeable1 m, Typeable b) => a -> Maybe (StateT s m b) castState = Data.Typeable.cast
None of the common monad transformers declare instances of Typeable, so I don't know if the concept itself even works. The use case here is one of my library users wants to return an Iteratee from code running in "hint", which requires any extracted values be typeable. My first attempt at an extension-free instance is something like this:
import Data.Enumerator import Data.Typeable
instance (Typeable a, Typeable1 m) => Typeable1 (Iteratee a m) where typeOf1 i = rep where typed :: (a -> b) -> b -> a -> a typed _ _ a = a
ia :: a -> Iteratee a m b ia = undefined
im :: m c -> Iteratee a m b im = undefined
rep = mkTyConApp (mkTyCon "Data.Enumerator.Iteratee") [tyA, tyM] tyA = typeOf (typed ia i undefined) tyM = typeOf1 (typed im i undefined)
which, besides being ugly, I have no idea if it's correct.