Proposal: more general unionWith for Data.Map

[Henning Thielemann]

Milan Straka schrieb:

> If others agree, it is indeed simple to generalize type of
> unionWith[Key] from
>   unionWithKey :: Ord k => (k -> a -> a -> a) -> Map k a -> Map k a -> Map k a
> to
>   unionWithKey :: Ord k => (k -> a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a
> These functions are used quite frequently in my opinion, so it will
> probably result in quite a lot of code breaks (unionWith (++) has to be
> changed to unionWith (\a b -> Just (a ++ b))).

Just for consistency with other "With" functions, the result of the
merger function should be 'a' not 'Maybe a'. Maybe 'unionUpdateWithKey'
or so would show the similarity to 'update'.