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

[Edward Kmett]

I would happily supply a patch to add the Typeable (and the few Data
instances that can be made) to transformers. I had to make similar ones in
my comonad-transformers package anyways.

-Edward Kmett

On Wed, Feb 2, 2011 at 1:02 AM, John Millikin <jmillikin at gmail.com> wrote:

> 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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