# Disjoint type classes

**Jose Emilio Labra Gayo
**
labra@lsi.uniovi.es

*Thu, 18 Feb 1988 08:17:58 +0100*

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