Re: [Haskell-cafe] GADT and problems with rigid type variables

On Sun, Aug 22, 2010 at 7:47 PM, Daniel Peebles wrote:

You could also do some (in my opinion) fairly nasty stuff with Dynamic or Typeable, adding a constraint to the Eq and attempting to cast at runtime (returning False if the cast returns Nothing).

This is what he's talking about:

{-# LANGUAGE GADTs #-}

import Data.Typeable (Typeable, cast)

data Foo where Foo :: (Typeable t, Eq t) => t -> Foo

instance Eq Foo where (Foo a) == (Foo b) = maybe False (== b) (cast a)

Cheers, -- Felipe.