[GHC] #665: re-work of insert API

#665: re-work of insert API
--------------------------------+-------------------------------------------
    Reporter:  jpbernardy       |        Owner:          
        Type:  feature request  |       Status:  new     
    Priority:  normal           |    Milestone:          
   Component:  libraries/base   |      Version:  6.4.1   
    Severity:  normal           |     Keywords:  Data Map
          Os:  Unknown          |   Difficulty:  Unknown 
Architecture:  Unknown          |  
--------------------------------+-------------------------------------------
I have been using 'Data.Map' from the library distributed with GHC/Hugs,
 and I quite like it.  Often though, I find myself needing a function
 that behaves like this:

 > import Data.Map (Map); import qualified Data.Map as Map hiding (Map)
 >
 > insert1 :: Ord k => k -> (Maybe v -> v) -> Map k v -> (v, Map k v)
 > insert1 k f m   = let v = f (Map.lookup k m)
 >                   in (v, Map.insert k v m)

 In words, we use a function to determine what to place in the map,
 depending on what is already in the map.  In the result, we return
 the new map, and the value that was inserted in the map.  Here is an
 example of how we may use this function:

 > example m           = insert1 0 v m
 >   where v Nothing   = 0
 >         v (Just n)  = n + 1

 Clearly, I can implement this function myself as above.  However, I was
 wondering if I can achieve the same, without searching in the map twice.
 I came up with the following:

 > insert2 :: Ord k => k -> (Maybe v -> v) -> Map k v -> (v, Map k v)
 > insert2 k f m   = (v,m')
 >   where
 >   v             = f r
 >   (r, m')       = Map.insertLookupWithKey g k v m
 >   g _ x _       = x

 Is there a more direct way to implement this function?  The use of
 lazyness
 in this case seems like an overkill.  If not, I would like to suggest that
 we
 add it to the API.  Also, it seems that some of the functions in
 the library are quite easy to implement in terms of this function,
 so perhaps they don't need to be in the API?

 > insertWith' :: Ord k => (v -> v -> v) -> k -> v -> Map k v -> Map k v
 > insertWith' f k v m = snd (insert2 k (maybe v (f v)) m)
 >
 > insertWithKey' :: Ord k => (k -> v -> v -> v) -> k -> v -> Map k v ->
 Map k v
 > insertWithKey' f k v m = snd (insert2 k (maybe v (f k v)) m)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/665>
GHC <http://www.haskell.org/ghc/>
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