On Sat, 2007-09-08 at 12:24 +1000, Stuart Cook wrote:
> 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.)
It's documented on the old wiki... and in the papers that introduce
local existential types I believe.
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