On 9/8/07, Ryan Ingram <[EMAIL PROTECTED]> wrote: > 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe