[Haskell-cafe] Fundeps and overlapping instances

[Etienne Laurin]

2012/5/31 Iavor Diatchki <iavor.diatchki at gmail.com>:
> 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