[Haskell-cafe] Instancing "Typeable" for monad transformers?

John Millikin jmillikin at gmail.com
Wed Feb 2 07:02:36 CET 2011

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.

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