On Thursday, 2003-07-10, 15:33, Ross Paterson wrote: > [...] > There are more instances and methods for people to define, even if some of > them imply others. > > As it happens, I would like yet another intermediate class: > > class BiFunctor a where > bimap :: (b' -> b) -> (c -> c') -> a b c -> a b' c' > > (and I have a client for the class: a useful subset of the arrow notation > needs only this, in fact only the contravariant part.) > > Clearly any arrow is also an instance of this class: > > bimap b c f = arr b >>> f >>> arr c > > but you still have to define bimap even if the type is also an arrow. > > 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.
Exactly. Maybe, the problem is not the existence of many classes but the lack of such an "defaults for superclass methods" feature. Wolfgang _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
