Add unionsWith to combine a list of hash maps

Data/HashMap/Base.hs

@@ -36,6 +36,7 @@ module Data.HashMap.Base
     , union
     , unionWith
     , unions
+    , unionsWith
 
       -- * Transformations
     , map
@@ -773,6 +774,13 @@ unions :: (Eq k, Hashable k) => [HashMap k v] -> HashMap k v
 unions = L.foldl' union empty
 {-# INLINE unions #-}
 
+-- | Construct a map containing all elements from a list of maps.
+-- If a key occurs in both maps, the provided function (first argument) will be
+-- used to compute the result.
+unionsWith :: (Eq k, Hashable k) => (v -> v -> v) -> [HashMap k v] -> HashMap k v
+unionsWith f = L.foldl' (unionWith f) empty
+{-# INLINE unionsWith #-}
+
 ------------------------------------------------------------------------
 -- * Transformations
 

Data/HashMap/Lazy.hs

@@ -55,6 +55,7 @@ module Data.HashMap.Lazy
     , union
     , unionWith
     , unions
+    , unionsWith
 
       -- * Transformations
     , HM.map

Data/HashMap/Strict.hs

@@ -55,6 +55,7 @@ module Data.HashMap.Strict
     , union
     , unionWith
     , unions
+    , unionsWith
 
       -- * Transformations
     , map

tests/HashMapProperties.hs

@@ -142,6 +142,10 @@ pUnions :: [[(Key, Int)]] -> Bool
 pUnions xss = M.toAscList (M.unions (map M.fromList xss)) ==
               toAscList (HM.unions (map HM.fromList xss))
 
+pUnionsWith :: [[(Key, Int)]] -> Bool
+pUnionsWith xss = M.toAscList (M.unionsWith (-) (map M.fromList xss)) ==
+                  toAscList (HM.unionsWith (-) (map HM.fromList xss))
+
 ------------------------------------------------------------------------
 -- ** Transformations
 
@@ -265,6 +269,7 @@ tests =
     , testProperty "union" pUnion
     , testProperty "unionWith" pUnionWith
     , testProperty "unions" pUnions
+    , testProperty "unionsWith" pUnionsWith
     -- Transformations
     , testProperty "map" pMap
     -- Folds