Re: Proposal: more general unionWith for Data.Map

Hi,

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On Tue, Jan 24, 2012 at 11:42 AM, Christian Sattler wrote:

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I don't care much about the naming, but note that the analogous property already fails for the generalized intersectionWithKey in the development version.

I suspected as much. I think the current intersectionWithKey is broken. We should have a single function, mergeWithKey, that allows people to do the things they currently do using Maybe return values in e.g. intersectionWithKey.

Funnily, I just also used mergeWithKey for the "ultimate combining function" in this thread :)

FYI, the new intersectionWithKey is not released yet -- maybe we could leave intersectionWithKey as it is, and provide mergeWith[Key] instead. Opinions?

Oh, Christian just noted that the performance of mergeWithKey :: Ord k => (k -> Maybe a -> Maybe b -> Maybe c) -> Map k a -> Map k b -> Map k c is too big -- it is at least O(size_of_first_op + size_of_second_op). But intersectionWithKey and unionWithKey is at most O(size_of_smaller * log(size_of_bigger)). So the proposal stands: should we add mergeWithKey :: Ord k => (k -> a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a ? Cheers, Milan