On 9/8/07, Ryan Ingram
This does what you want, I think:
{-# LANGUAGE ExistentialQuantification #-} module Exist where
data Showable = forall a. (Show a) => Showable a instance Show Showable where showsPrec p (Showable a) = showsPrec p a show (Showable a) = show a -- You have to use the default implementation of showList -- because a list could be heterogeneous
data T a = forall b. (Show b) => T b a
extShow :: T a -> Showable extShow (T b _) = Showable b
Wow, I'm impressed! Making the existential wrapper an instance of its own typeclass solves quite a few problems. While the idiom is obvious in hindsight, I don't think I've seen it documented before. (Looking around just now I found some oblique references to the technique, but nothing that really called attention to itself.) Stuart