[GHC] #8092: Map.unionWithMaybe

[GHC]

#8092: Map.unionWithMaybe
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       Reporter:  sjcjoosten               |             Owner:
           Type:  feature request          |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  libraries/base           |           Version:  7.6.3
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Easy (less than 1 hour)  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 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.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8092>
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