Re: Proposal: more general unionWith for Data.Map

On 1/24/12 3:39 PM, Milan Straka wrote:

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

If you're going to go for that level of generality, I'd suggest looking into my currently unpublished Data.Or[1] for combining the a and b arguments with the correct semantics. Naturally, this function will be less efficient for union-like operators since it requires traversing non-intersecting branches of the trees as well. [1] http://code.haskell.org/~wren/data-or/dist/doc/html/data-or/ -- Live well, ~wren