You're talking about something else: the dictionaries (Ord a, Ord b) from which the Ord (a,b) dictionary were constructed. We don't have a very good name for these guys, but "superclass" isn't a good one.
Otherwise I agree with all you say. Your idea of using type families is cool.
| data OrdDict a = | { (<) :: a -> a -> Bool | , ... | , super :: Super a | } | | type family Super a :: * | type instance Super Int = () | type instance Super [a] = OrdDict a | type instance Super (a,b) = (OrdDict a, OrdDict b) | | A similar solution is possible with a data family Super (but obviously I'm | in favor of type families :)
Can you say why? A data family would work fine here. But it's not a big deal.
Just a matter of taste, and familiarity.
So the other question is whether this is useful. How often do people write stuff like this? f :: Ord [a] => a -> a -> Bool f x y = x>y
This paper has some examples, I believe: Modular Generic Programming with Extensible Superclasses Martin Sulzmann and Meng Wang In Workshop on Generic Programming (WGP'06) http://www.comp.nus.edu.sg/~sulzmann/publications/wgp06-modulargeneric.ps
Nevertheless, I hadn't realised it was possible before, and now I can see it is.
I'd be nice to know of people who need or would like to have this feature. Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/