[Haskell-cafe] Phantoms

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?