Map.unionWithMaybe

for Data.Map, I needed a "unionWithMaybe" function for my sparse system of linear equations (unionWithMaybe :: Ord k => (a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a). My usage: (.+.) = Map.unionWithMaybe (\a b->case a+b of {0->Nothing;s->Just s}) (I do not think Map.unionWithMaybe can be expressed in terms of other functions without loosing performance, so I recon it would be a nice addition to the library.)

I built this function myself by modifying the Data.Map implementation, but it would be nice to see it in the official version of Data.Map. Here is the file including the modifications I made to it. Feel free to use it, I will agree to whatever license you need to make it public.

(PS: this is the first time I return modified source code to the maintainer, feel free to instruct me on how to do this in the future.)

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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changed weight to 5

added Tfeature request Trac import core libraries labels

Attached file DataMap.hs (download).

added the unionWithMaybe function

closed

Thanks for the suggestion. However, please see http://www.haskell.org/haskellwiki/Library_submissions for the process for proposing changes to the libraries.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedWon'tFix

added Pnormal label