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

[Ganesh Sittampalam]

On Mon, 15 Sep 2008, Tom Hawkins wrote:

> 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  = ...

Apart from the suggestions about Data.Typeable etc, one possibility is to 
enumerate the different possible types that will be used as parameters to 
Const in different constructors, either in Expr or in a new type.

So IntConst :: Int -> Expr Int, etc

Or Const :: Const a -> Expr a and IntConst :: Int -> Const Int etc

Not very pleasant though.

Ganesh