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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
