ExternsDiff: compute deps of TypeSynonyms properly (#7)

Fixes a bug where type synonym changes are incorrectly computed when diffing externs.

See https://github.com/zyla/purs-recompilation-repro for a reproducer of the bug.

Bug explanation: Externs files can have multiple entries with the same Ref, but the code in ExternsDiff assumed uniqueness. This led to buggy dependency graph construction.

The fix is to merge duplicate entries.

Author: zyla

src/Language/PureScript/Make/ExternsDiff.hs

@@ -354,19 +354,21 @@ isEmpty (ExternsDiff _ refs)
 type Tuple4 m a = (m a, m a, m a, m a)
 
 -- | Returns refs as a tuple of four (added, removed, changed, unchanged).
-splitRefs :: Ord r => Eq a => [a] -> [a] -> (a -> Maybe r) -> Tuple4 [] r
+splitRefs :: forall ref a deps. Monoid deps => Ord ref => Eq a => [a] -> [a] -> (a -> Maybe (ref, deps)) -> Tuple4 [] (ref, deps)
 splitRefs new old toRef =
   M.foldrWithKey go (added, [], [], []) oldMap
   where
-    toMap = M.fromList . mapMaybe (((<$>) . flip (,)) <*> toRef)
+    toMap :: [a] -> Map ref (deps, [a])
+    toMap = M.fromListWith (<>) . mapMaybe (\decl -> do (ref, deps) <- toRef decl; pure (ref, (deps, [decl])))
     newMap = toMap new
     oldMap = toMap old
-    added = M.keys $ M.difference newMap oldMap
-    go ref decl (a, r, c, u) = case M.lookup ref newMap of
-      Nothing -> (a, r <> [ref], c, u)
-      Just newDecl
-        | decl /= newDecl -> (a, r, ref : c, u)
-        | otherwise -> (a, r, c, ref : u)
+    added = fmap (\(ref, (deps, _)) -> (ref, deps)) $ M.toList $ M.difference newMap oldMap
+    go :: ref -> (deps, [a]) -> Tuple4 [] (ref, deps) -> Tuple4 [] (ref, deps)
+    go ref (deps, decls) (a, r, c, u) = case M.lookup ref newMap of
+      Nothing -> (a, r <> [(ref, deps)], c, u)
+      Just (_, newDecls)
+        | decls /= newDecls -> (a, r, (ref, deps) : c, u)
+        | otherwise -> (a, r, c, (ref, deps) : u)
 
 -- | Traverses the type and finds all the refs within.
 typeDeps :: P.Type a -> S.Set (ModuleName, Ref)