Disjoint type classes

I have a large program where I would like to include disjoint type classes into a new type class. My problem can be reduced to the following example (taken from [1]):

class Num a => Dividable a where dividedBy :: a -> a -> a

instance Fractional a => Dividable a where dividedBy = (/)

instance Integral a => Dividable a where dividedBy = div

Which doesn't work because Haskell doesn't detect that "Integral" and "Fractional" are disjoint. Is there a way to implement these type classes with current Haskell implementations? [1] K. Glynn, M. Sulzmann, P.J. Stuckey Type Classes and Constraint Handling Rules http://www.cs.mu.oz.au/tr_submit/test/cover_db/mu_TR_2000_7.html Best regards, Jose Labra