Proposal: more general unionWith for Data.Map

[John Meacham]

I have needed this before

merge :: (a -> b -> Maybe c) -> (a -> Maybe c) -> (b -> Maybe c) ->
Map k a -> Map k b -> Map k c
mergeWithKey :: (k -> a -> b -> Maybe c) -> (k -> a -> Maybe c) ->
(k-> b -> Maybe c) -> Map k a -> Map k b -> Map k c

where the function arguments are
what to use when the key appears in both maps
what to use when the key appears in just the left map
what to use when the key appears in just the right map.

It seems like it would be the ultimate fully general fast map merging routine.

intersection = merge (Just . const) (const Nothing) (const Nothing)
union = merge (Just . const) Just Just
xor = merge ( const Nothing . const) Just Just

and so forth...

    John


    John