Fix the Semigroup instance for Foreign.Object.Object (#19)

The Semigroup instance for Foreign.Object.Object currently appends values from the right to values from the left: `singleton k a <> singleton k b` is equivalent to `singleton k (b <> a)` and not to `singleton k (a <> b)` as expected.

Author: kl0tl

src/Foreign/Object.purs

@@ -273,7 +273,7 @@ mapWithKey :: forall a b. (String -> a -> b) -> Object a -> Object b
 mapWithKey f m = runFn2 _mapWithKey m f
 
 instance semigroupObject :: (Semigroup a) => Semigroup (Object a) where
-  append m1 m2 = mutate (\s1 -> foldM (\s2 k v2 -> OST.poke k (runFn4 _lookup v2 (\v1 -> v1 <> v2) k m2) s2) s1 m1) m2
+  append m1 m2 = mutate (\s1 -> foldM (\s2 k v1 -> OST.poke k (runFn4 _lookup v1 (\v2 -> v1 <> v2) k m2) s2) s1 m1) m2
 
 instance monoidObject :: (Semigroup a) => Monoid (Object a) where
   mempty = empty

test/Test/Foreign/Object.purs

@@ -11,12 +11,13 @@ import Data.List as L
 import Data.List.NonEmpty as NEL
 import Data.Maybe (Maybe(..))
 import Data.NonEmpty ((:|))
+import Data.Semigroup.First (First(..))
+import Data.Semigroup.Last (Last(..))
 import Data.Traversable (sequence, traverse)
 import Data.TraversableWithIndex (traverseWithIndex)
 import Data.Tuple (Tuple(..), fst, snd, uncurry)
 import Effect (Effect)
 import Effect.Console (log)
-import Foreign.Object (Object)
 import Foreign.Object as O
 import Foreign.Object.Gen (genForeignObject)
 import Partial.Unsafe (unsafePartial)
@@ -257,3 +258,13 @@ objectTests = do
       { expected: entries
       , actual: O.size (O.fromFoldable (map (\x -> Tuple (show x) x) (A.range 1 entries)))
       }
+
+  log "Semigroup instance"
+  assertEqual
+    { expected: O.singleton "a" (First 1)
+    , actual: O.singleton "a" (First 1) <> O.singleton "a" (First 2)
+    }
+  assertEqual
+    { expected: O.singleton "a" (Last 2)
+    , actual: O.singleton "a" (Last 1) <> O.singleton "a" (Last 2)
+    }