[Haskell-cafe] Monad for Set?

[Matthew Brecknell]

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.