I have only one thing to add to this discussion:
On Fri, Sep 3, 2010 at 5:16 AM, Ivan Lazar Miljenovic
2b) Is it OK to promote functions that use a class to being class methods? When I was discussing this in #haskell several people mentioned that defining something like liftA2 for the Set instance of (restricted) Applicative would make more sense than the default <*> (since (a -> b) isnt' an instance of Ord).
One problem with defining both <*> and liftA2 in the class in terms of each other is that omitting them from instances is no longer a compile-time error, nor is it even an obvious runtime error - it's frequently an infinite loop, which is unpleasant. Though I understand that it's nice to be able to choose the methods you want to define, I think static error detection is nice too, so I tend to avoid providing extra class methods with defaults unless the alternative is much worse. On that note, why *do* we have missing instance methods filled in with bottoms, so that even without defaults, it's a warning rather than an error? It seems like quite an un-Haskelly thing to do. I know there may be some cases where you do not need to define a particular method, but imo you should be required to explicitly opt out of it if that's the case.