Hello, 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)
-Iavor PS: How do people import 'Data.Map'? The above seems clunky...