2012/5/31 Iavor Diatchki
Hello,
the notion of a functional dependency is well established, and it was used well before it was introduced to Haskell (for example, take a look at http://en.wikipedia.org/wiki/Functional_dependency). So I'd be weary to redefine it lightly.
Indeed, GHC's functional dependencies are not the same. I see you have already talked about this on the GHC bug tracker. http://hackage.haskell.org/trac/ghc/ticket/1241
1. Check that an instance is consistent with itself. For example, this should be rejected:
instance C a b
because it allows C Int Bool and C Int Char which violate the functional dependency.
This check may not always be as straightforward. When would this be a valid instance? instance K a b ⇒ C a b With a few extra extensions, K could be a type family. C currently has the kind (a -> b -> Constraint), with no mention of functional dependencies. Perhaps the kind of C should include the functional dependencies of C? Etienne Laurin