haskell extensions question

[Diego Yanivello]

hi,
   I am using the haskell extensions (in Hugs - Nov 2002) and I have some 
troubles with that. For example:

class A a where
    a :: a -> String
    a = const "a"

class B b where
    b :: b -> String
    b = const "b"

instance (A b) => B b where
    b = a

instance B ()

-----------------
In this case, Hugs says:

ERROR "Test.hs":12 - Overlapping instances for class "B"
*** This instance   : B ()
*** Overlaps with   : B a
*** Common instance : B ()

but I don't have an instance "A ()" from which haskell can infer an 
instance "B ()".
can somebody explain me that it is happening here?

Thanks in advanced...

     Diego