Re: [Haskell-cafe] Re: Comparing GADTs for Eq and Ord

Take a look at http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap Tom Hawkins wrote:

On Mon, Sep 15, 2008 at 3:11 PM, apfelmus wrote:

...

So, in other words, in order to test whether terms constructed with Equal are equal, you have to compare two terms of different type for equality. Well, nothing easier than that:

(===) :: Expr a -> Expr b -> Bool Const === Const = True (Equal a b) === (Equal a' b') = a === a' && b === b' _ === _ = False

instance Eq (Expr a) where (==) = (===)

OK. But let's modify Expr so that Const carries values of different types:

data Expr :: * -> * where Const :: a -> Term a Equal :: Term a -> Term a -> Term Bool

How would you then define:

Const a === Const b = ...

-Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe