Fix difference (#68)

* Fix difference

* Update changelog

Author: natefaubion

CHANGELOG.md

@@ -10,6 +10,8 @@ New features:
 
 Bugfixes:
 
+- Fixes Map/Set difference, which was broken due to the internal rewrite (#68 @natefaubion)
+
 Other improvements:
 
 ## [v3.1.0](https://github.com/purescript/purescript-ordered-collections/releases/tag/v3.1.0) - 2023-08-11

src/Data/Map/Internal.purs

@@ -813,7 +813,7 @@ unsafeIntersectionWith = mkFn4 \comp app l r -> case l, r of
 unsafeDifference :: forall k v w. Fn3 (k -> k -> Ordering) (Map k v) (Map k w) (Map k v)
 unsafeDifference = mkFn3 \comp l r -> case l, r of
   Leaf, _ -> Leaf
-  _, Leaf -> Leaf
+  _, Leaf -> l
   _, Node _ _ rk _ rl rr -> do
     let (Split _ ll lr) = runFn3 unsafeSplit comp rk l
     let l' = runFn3 unsafeDifference comp ll rl

test/Test/Data/Map.purs

@@ -251,8 +251,12 @@ mapTests = do
   log "difference"
   quickCheck $ \(TestMap m1) (TestMap m2) ->
     let d = M.difference (m1 :: M.Map SmallKey Int) (m2 :: M.Map SmallKey String)
-    in and (map (\k -> M.member k m1) (A.fromFoldable $ M.keys d)) &&
-       and (map (\k -> not $ M.member k d) (A.fromFoldable $ M.keys m2))
+    in
+      if M.size d == 0 then
+        and (map (\k -> M.member k m2) (A.fromFoldable $ M.keys m1))
+      else
+        and (map (\k -> M.member k m1) (A.fromFoldable $ M.keys d)) &&
+        and (map (\k -> not $ M.member k d) (A.fromFoldable $ M.keys m2))
 
   log "size"
   quickCheck $ \xs ->