Small optimisations in unify

Reviewed By: donsbot

Differential Revision: D67144007

fbshipit-source-id: 568d171bb3532ab960c04e61375fa578fba22ff3

glean/db/Glean/Query/Typecheck/Unify.hs

@@ -17,6 +17,7 @@ import Control.Monad.Except
 import Control.Monad.State
 import qualified Data.IntMap as IntMap
 import qualified Data.Map as Map
+import qualified Data.Map.Merge.Lazy as Map
 import Data.Maybe
 import qualified Data.Text as Text
 import Compat.Prettyprinter hiding ((<>), enclose)
@@ -90,11 +91,23 @@ unify a@(HasTy fa ra x) b@(HasTy fb rb y)
     (Just x, Just y) | x /= y -> unifyError a b
     (Nothing, _) -> return rb
     _otherwise -> return ra
-  mapM_ (uncurry unify) $ Map.intersectionWith (,) fa fb
-  z <- freshTyVarInt
-  let all = HasTy (Map.union fa fb) rec z
-  extend x all
-  extend y all
+  union <- Map.mergeA
+    Map.preserveMissing
+    Map.preserveMissing
+    (Map.zipWithAMatched $ \_ a b -> do unify a b; return a)
+    fa fb
+  -- if either a or b is the same as the unified type, avoid creating
+  -- a new type variable.
+  let size = Map.size union
+  if size == Map.size fa && ra == rec
+    then extend y a
+    else if size == Map.size fb && rb == rec
+      then extend x b
+      else do
+        z <- freshTyVarInt
+        let all = HasTy union rec z
+        extend x all
+        extend y all
 
 unify a@(HasTy _ (Just Sum) _) b@RecordTy{} =
   unifyError a b
@@ -103,9 +116,8 @@ unify a@(HasTy m _ x) b@(RecordTy fs) = do
     case Map.lookup f m of
       Nothing -> return ()
       Just ty' -> unify ty ty'
-  forM_ (Map.keys m) $ \n ->
-    when (n `notElem` map fieldDefName fs) $
-      unifyError a b
+  when (not (Map.null (foldr (Map.delete . fieldDefName) m fs))) $
+    unifyError a b
   extend x (RecordTy fs)
 
 unify a@(HasTy _ (Just Record) _) b@SumTy{} =