Merge pull request #5677 from unisonweb/cp/fix-multimap

Fix Multimap.fromList

Author: pchiusano

unison-runtime/src/Unison/Runtime/Foreign/Function.hs

@@ -901,7 +901,9 @@ foreignCallHelper = \case
   List_sort -> mkForeign $ \(l :: Seq Val) -> pure $ Sq.unstableSort l
   Multimap_fromList -> mkForeign $ \(l :: [(Val, Val)]) -> do
     let listVals = l <&> \(k, v) -> (k, Sq.singleton v)
-    evaluate $ Map.fromListWith (<>) listVals
+    -- Haskell Map.fromList calls the semigroup in reverse order, so we correct for it by flipping.
+    let result :: Map Val Val = fmap encodeVal $ Map.fromListWith (flip (<>)) listVals
+    evaluate result
   Set_fromList -> mkForeign $ \(l :: [Val]) -> do
     m <- evaluate $ Map.fromList $ zip l (repeat unitValue)
     pure . Data1 Ty.setRef TT.setWrapTag $ encodeVal m