Re: Proposal: more general unionWith for Data.Map

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