[Haskell-cafe] Re: Help understanding type error

Derek Elkins wrote:

On Sat, 2007-09-08 at 12:24 +1000, Stuart Cook wrote:

...

On 9/8/07, Ryan Ingram 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.)

It's documented on the old wiki... and in the papers that introduce local existential types I believe.

E.g. http://citeseer.ist.psu.edu/aufer95type.html, this is where I first read about it. Cheers Ben