default class methods in sub-classes

Apparently I can't do this (in ghc or hugs -- haven't tried others): class Space a where distance :: a -> a -> Int subtract :: a -> a -> a class (Space a) => NormSpace a where norm :: a -> Int distance a b = norm (subtract a b) That is, I can't make (or redefine) a default for the superclass method 'distance' in a subclass. I agree that the Haskell'98 definition doesn't claim that this should be allowed, but it seems like something that would be useful. Are there reasons not to allow this (or that I shouldn't want to do this at all)? Apologies if this is well-covered. I couldn't find any mention of it. :-Dylan Simon