[Haskell-cafe] checking types with type families

[Simon Peyton-Jones]

| > I think your problem here is that there's no mention of `a' on the
| > left-hand size of from_val's type signature; you either need to use
| > MPTC+fundep to associate what result is compared to a, or else use a
| > phantom type parameter of Val to make it "data Val result a = ..." and
| > then "from_val :: Val result a -> Maybe a".
| 
| Aha!  Why didn't I think of "plain" old MPTC+fundep?  For some reason
| type families feel a lot more fun.  Turns out you can write 'instance
| Typecheck (SomeMonad result) result' and instead of complaining about
| a duplicate symbol it unifies 'result', exactly like I wanted.

I'm interested in situations where you think fundeps work and type families don't.  Reason: no one knows how to make fundeps work cleanly with local type constraints (such as GADTs).  

If you think you have such as case, do send me a test case.  

Thanks

Simon