[Haskell-cafe] Phantoms

[Henning Thielemann]

On Wed, 6 Aug 2008, Andrew Coppin wrote:

> I just (re)discovered that I can do things like
>
>   data Foo x = Foo Int Int
>
> Now "Foo Int" and "Foo Double" are, as far as the type checker cares,
> two completely different types, even though in fact they are the same.
> This is actually Quite Useful, in the particular case I'm working on.
>
> Here's the thing though... How do I get it so that "Foo Int" and "Foo
> Double" produce slightly different strings when printed?

The non-hacky Haskell 98 solution is:

class ShowPhantom a where
   showPhantom :: Foo a -> String

You can call this from the 'show' implemention for 'Foo a'.