Experiment

src/Nixfmt/Pretty.hs

@@ -394,6 +394,22 @@ prettyApp indentFunction pre hasPost f a =
         where
           -- If the brackets are on different lines, keep them like that
           sur = if sourceLine paropen /= sourceLine parclose then hardline else line
+      -- Same for sets. Limit is two items
+      absorbInner (Term (Set krec paropen@Ann{trailComment = post'} binders parclose)) =
+        -- \| length (unItems binders) <= 2 && all isSimpleBinding binders =
+        pretty (fmap (,hardspace) krec)
+          <> pretty (paropen{trailComment = Nothing})
+          <> surroundWith sep (nest $ pretty post' <> prettyItems binders)
+          <> pretty parclose
+        where
+          isSimpleBinding (Assignment sels (LoneAnn _) rhs (LoneAnn _)) =
+            all isSimpleSelector sels && isSimple rhs
+          isSimpleBinding _ = False
+          -- If the braces are on different lines, keep them like that
+          sep =
+            if sourceLine paropen /= sourceLine parclose
+              then line
+              else line
       absorbInner expr = pretty expr
 
       -- Render the last argument of a function call

test/diff/apply/out.nix

@@ -122,16 +122,10 @@
       # multiline
     } argument;
 
-    name3 =
-      function arg
-        {
-          asdf = 1;
-          # multiline
-        }
-        {
-          qwer = 12345;
-        }
-        argument;
+    name3 = function arg {
+      asdf = 1;
+      # multiline
+    } { qwer = 12345; } argument;
   }
   {
     name4 = function arg { asdf = 1; } {