From 2391b3b42e75b3a1fc90629c46646355644150cd Mon Sep 17 00:00:00 2001
From: Cole Haus <colehaus@cryptolab.net>
Date: Fri, 13 Apr 2018 20:53:14 -0700
Subject: [PATCH 1/2] Add `unionsWith`

Straightforward combo of `unionWith` and `unions`. Haskell's `container` has it.
---
 src/Data/Map.purs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/Data/Map.purs b/src/Data/Map.purs
index e764370b..cba4d51b 100644
--- a/src/Data/Map.purs
+++ b/src/Data/Map.purs
@@ -608,6 +608,9 @@ union = unionWith const
 unions :: forall k v f. Ord k => Foldable f => f (Map k v) -> Map k v
 unions = foldl union empty
 
+unionsWith :: forall k v f. Ord k => Foldable f => (v -> v -> v) -> f (Map k v) -> Map k v
+unionsWith f = foldl (unionWith f) empty
+
 -- | Test whether one map contains all of the keys and values contained in another map
 isSubmap :: forall k v. Ord k => Eq v => Map k v -> Map k v -> Boolean
 isSubmap m1 m2 = LL.all f $ (toUnfoldable m1 :: LL.List (Tuple k v))

From 22e3ee612d6c3868b59a6f58000e7cfdb4b6a8c3 Mon Sep 17 00:00:00 2001
From: Cole Haus <colehaus@cryptolab.net>
Date: Thu, 10 May 2018 00:30:29 -0700
Subject: [PATCH 2/2] Test `unionsWith`

---
 src/Data/Map.purs       |  1 +
 test/Test/Data/Map.purs | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/src/Data/Map.purs b/src/Data/Map.purs
index cba4d51b..798ef32a 100644
--- a/src/Data/Map.purs
+++ b/src/Data/Map.purs
@@ -32,6 +32,7 @@ module Data.Map
   , union
   , unionWith
   , unions
+  , unionsWith
   , isSubmap
   , size
   , mapWithKey
diff --git a/test/Test/Data/Map.purs b/test/Test/Data/Map.purs
index bc38e615..e53f81aa 100644
--- a/test/Test/Data/Map.purs
+++ b/test/Test/Data/Map.purs
@@ -23,6 +23,9 @@ import Test.QuickCheck.Gen (elements, oneOf)
 
 newtype TestMap k v = TestMap (M.Map k v)
 
+unwrap :: forall k v. TestMap k v -> M.Map k v
+unwrap (TestMap m) = m
+
 instance arbTestMap :: (Eq k, Ord k, Arbitrary k, Arbitrary v) => Arbitrary (TestMap k v) where
   arbitrary = TestMap <$> genMap arbitrary arbitrary
 
@@ -214,6 +217,26 @@ mapTests = do
           Just v              -> Just v == v2
           Nothing             -> not (in1 || in2)
 
+  log "unionsWith"
+  for_ [Tuple (+) 0, Tuple (*) 1] $ \(Tuple op ident) ->
+    quickCheck $ \(testMaps :: Array (TestMap SmallKey Int)) k ->
+      let testMaps' = unwrap <$> testMaps
+          u = M.unionsWith op testMaps'
+      in case M.lookup k u of
+           Nothing -> A.all (not <<< M.member k) testMaps'
+           Just v -> (v == _) <<< A.foldl op ident <<< map (fromMaybe ident <<< M.lookup k) $ testMaps'
+
+  log "unionsWith argument order"
+  quickCheck $ \(testMaps :: Array (TestMap SmallKey Int)) k ->
+    let testMaps' = unwrap <$> testMaps
+        u = M.unionsWith (-) testMaps'
+    in case M.lookup k u of
+         Just v ->
+           case A.uncons <<< A.mapMaybe (M.lookup k) $ testMaps' of
+             Nothing -> false
+             Just { head, tail } -> v == foldl (-) head tail
+         Nothing -> A.all (not <<< M.member k) testMaps'
+
   log "size"
   quickCheck $ \xs ->
     let xs' = nubBy ((==) `on` fst) xs