[Haskell-cafe] Re: GATD and pattern matching

[Felipe Lessa]

On Thu, Jun 10, 2010 at 11:14:42PM +0200, Daniel Fischer wrote:
> Show can work (should with the constraint on Equ), Eq is hairy.
>
> instance Show t => Show (Obs t) where
>     show (Equ a b) = show a ++ " `Equal` " ++ show b
>     show (Plus a b) = ...
>     show (Konst x) = "Konst " ++ show x
>     ...
>
> For an Eq instance, you have the problem that
>
> Equ (Konst True) (Konst False)
> and
> Equ Player Turn
>
> both have the type Obs Bool, but have been constructed from different
> types, so you can't compare (Konst True) and Player.
> I don't see a nice way to work around that.

I didn't test, but something like this could work:

  Equ :: (Show a, Eq a, Typeable a) => Obs a -> Obs a -> Obs Bool

  (Equ a b) == (Equ c d) = eqTypeable (a,b) (c,d)

  eqTypeable :: (Typeable a, Eq a, Typeable b, Eq b) => a -> b -> Bool
  eqTypeable x y = case cast y of
                     Just y' -> x == y'
                     Nothing -> False

Maybe not ;).

Cheers,

--
Felipe.