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.

Note that placing a functional dependency constraint is just one way to allow class methods that don't mention all class variables. If the instances for the class do not satisfy the functional dependency (as in your example), you can refactor your class hierarchy, instead. For example:

class D a where k :: a

class D a => C a b where f :: a -> b

instance D Int where k = 2

instance C Int b where f _ = Nothing

I hope this helps,

-Iavor

On Wed, May 30, 2012 at 1:31 PM, Etienne Laurin <etienne@atnnn.com> wrote:

Hello,

I disagree with your example.

> 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.

Functional dependencies are not used to pick types, they are used to
pick instances.

class C a b | a → b where
k ∷ a
f ∷ a → Maybe b

The functional dependency allows you to have a method such as k that
doesn't use all the arguments of the class.

I expect to be able to make a instance that works for any b.

instance C Int b where
k = 2
f _ = Nothing

Etienne Laurin