[Haskell-cafe] Functional dependenices in class declarations

[Frank]

In ghc 6.4 (fine work - many gratulations to all who did it!) it is
permitted
to have type parameters which are reachable (7.4.3.1 context of type
signatures).
I tried the following:

module TypeDependencies where

class A a b | a -> b 

class (A a b) => B b  where
    op :: a -> a
    
x :: A a b => a -> a
x = id

Which compiles fine for the function x, but does complain
"not in scope: type variable 'a' " for the class (A a b) => B b.

Is this an intentional restriction that 'reachable' type variables are only
permitted in type definitions, but not in classes? or do I misunderstand
something?

Any help appreciated!
Andrew Frank