Ronald Guida:
I'm pondering, is it possible to define a Set monad analogous to the List monad?
[snip]
This leads me think of a different solution: What if I could define a Set monad that's smart enough to "know", for any type a, whether or not (Eq a) holds, and degenerate to a blind list if the elements can't be equated. Ultimately, what I would need is a way to overload "join" (or "bind") with two different implementations, one for types that satisfy (Eq a), and another implementation for all other types.
You might find this interesting, in case you haven't yet seen it: http://article.gmane.org/gmane.comp.lang.haskell.cafe/18118 If you also read the rest of that thread, you'll see that with a recent GHC HEAD, you should be able to avoid the need for the Teq witness.