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 |
| Test case | |
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