On Thu, Jul 10, 2003 at 02:33:25PM +0100, Ross Paterson wrote:
Subclasses in Haskell cover a range of relationships, including this sense where things in the subclass automatically belong to the superclass. Other examples include Eq => Ord and Functor vs Monad. In such cases it would be handy if the subclass could define defaults for the superclass methods (e.g. Ord defining (==)), so that the superclass instance could be optional.
I agree, but this needs to be carefully thought out. For instance, remember to consider the case that there is more than one default instance for a given method of a superclass. I am reminded of multiple inheritance considerations. (These difficulties came up before when I was thinking about the numeric heirarchy, and was the reason I proposed a heirarchy which was much less fine-grained than, e.g., in Mechvelliani's proposal.) Peace, Dylan