Generate labelled updaters and propagate callback exceptions

record.ml

@@ -28,3 +28,13 @@ let () =
   let original = Mutable.{ count = 1; label = "grey" } in
   let changed = Mutable.set_count 2 original in
   assert (original.Mutable.count = 1 && changed.Mutable.count = 2)
+
+let () =
+  let calls = ref 0 in
+  let original = Colour.{ red = 3; green = 4; blue = 5 } in
+  let changed = Colour.update_green ~f:(fun n -> incr calls; n + 1) original in
+  assert (Colour.green changed = 5 && !calls = 1);
+  assert (Box.update_contents ~f:string_of_int Box.{ contents = 9 } = Box.{ contents = "9" });
+  (try ignore (Colour.update_red ~f:(fun _ -> failwith "callback") original); assert false
+   with Failure message -> assert (message = "callback"));
+  assert (Colour.red original = 3)