[Haskell] Passing typeclasses as parameters?

Hi all. I managed to paint myself in a corner where I did this:

data ShowDynamic = forall a . (Show a) => SD Dynamic a toShowDyn a = SD (toDyn a) a fromShowDyn (SD d a) = fromDynamic d

instance Show ShowDynamic where ...

Then I began to wonder if there was any hack to do:

data AnyDynamic X = forall a . (X a) => AD Dynamic a

which could be kinda neat, but I saw no obvious way to do this. Ideas? - Ville (bound to be slain by curiosity)