Hi,
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...
I agree with the ultimate generality, but the problem is efficiency. For example running intersection (fromList [1..1000]) (fromList [1001..2000]) will end very soon with empty result, but merge (Just . const) (const Nothing) (const Nothing) (fromList [1..1000]) (fromList [1001..2000]) will call the (const Nothing) for 2000 times. Cheers, Milan