[Haskell-cafe] Functional dependencies and Peano numbers

Hi, there is a lot of buzz around functional dependencies on the mailing list and Planet Haskell. I've read some of the tutorials and I think I understand how they work but I still haven't figured out where they can be useful. * Can someone give me a real world (preferably hackagedb) example where functional dependencies are used? * Can someone give me a real world (preferably hackagedb) example where Peano numbers a la "data Zero; data Succ n" are used? Finally... * All the examples involving functional dependencies are on the form

a b c | a b -> c

but can they also be on a form similar to

a b c d e f g h| b c -> d e f | b d g -> h

(i.e. d,e,f are decided by the b,c-combination while h is decided by the b,d,g-combination)? -- Oscar