TypeFamilies vs. FunctionalDependencies & type-level recursion

[Simon Peyton-Jones]

| By equality superclasses, do you just mean being able to say "a ~ b"
| in a class context? 

Yes.  Or (F a ~ b).

| Unless I'm missing something, that is not sufficient to do a lot of
| things I would like to do, as those things require both
| OverlappingInstances and FunctionalDependencies (as well as
| UndecidableInstances). 

Correct. Hence Oleg's second point "2. combination with overlapping instances".

Oleg claims to have a good story here.  I'd like to see how he uses it to solve your problem.

Simon