[Haskell-cafe] Re: GATD and pattern matching

[Felipe Lessa]

On Sat, Jun 12, 2010 at 12:13:14AM +0200, Dupont Corentin wrote:
> Thanks all, it works fine (see below).
>
> I lamentably try to make the same for show:
> > showTypeable :: (Typeable a) => a -> String
> > showTypeable x = case cast x of
> >                      Just x' -> show x'
> >                      Nothing -> ""
>
> Because it really upsets me to add this show constraints to the Equ
> constructor ;)
> what if i want to make an Obs instance with non showable elements, with no
> intention to show it of course?

Ad hoc solution:

  class MaybeShow a where
    maybeShow :: a -> Maybe String

  instance Show a => MaybeShow a where
    maybeShow = Just . show

  instance MaybeShow a where
    maybeShow = Nothing

  data MyData where
    Something :: MaybeShow a => a -> MyData

  instance MaybeShow MyData where
    maybeShow (Something x) =
      fmap (\s -> "Something (" ++ s ++ ")") (maybeShow x)

Hahahaha :).  Try to guess without using GHC/GHCi:

  1) Which extensions are required to make the code compile.

  2) After compiled, if it works as intended or not.

Cheers,

--
Felipe.