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.<div><br></div><div>-Edward Kmett<br><br>
<div class="gmail_quote">On Wed, Feb 2, 2011 at 1:02 AM, John Millikin <span dir="ltr"><<a href="mailto:jmillikin@gmail.com">jmillikin@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Is there any reasonable way to do this if I want to cast a monadic<br>
value? For example:<br>
<br>
> castState :: (Typeable a, Typeable s, Typeable1 m, Typeable b) => a -> Maybe (StateT s m b)<br>
> castState = Data.Typeable.cast<br>
<br>
None of the common monad transformers declare instances of Typeable,<br>
so I don't know if the concept itself even works.<br>
<br>
The use case here is one of my library users wants to return an<br>
Iteratee from code running in "hint", which requires any extracted<br>
values be typeable. My first attempt at an extension-free instance is<br>
something like this:<br>
<br>
> import Data.Enumerator<br>
> import Data.Typeable<br>
><br>
> instance (Typeable a, Typeable1 m) => Typeable1 (Iteratee a m) where<br>
> typeOf1 i = rep where<br>
> typed :: (a -> b) -> b -> a -> a<br>
> typed _ _ a = a<br>
><br>
> ia :: a -> Iteratee a m b<br>
> ia = undefined<br>
><br>
> im :: m c -> Iteratee a m b<br>
> im = undefined<br>
><br>
> rep = mkTyConApp (mkTyCon "Data.Enumerator.Iteratee") [tyA, tyM]<br>
> tyA = typeOf (typed ia i undefined)<br>
> tyM = typeOf1 (typed im i undefined)<br>
<br>
which, besides being ugly, I have no idea if it's correct.<br>
<br>
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</blockquote></div><br></div>