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