Generate immutable setters with type-changing singleton updates

record.ml

@@ -11,3 +11,20 @@ let () =
   assert (Colour.red colour = 3);
   assert (Colour.green colour = 4);
   assert (Legacy.value Legacy.{ value = "grey" } = "grey")
+
+module Box = struct
+  type 'a t = { contents : 'a } [@@fieldglass generate]
+end
+
+module Mutable = struct
+  type t = { mutable count : int; label : string } [@@fieldglass generate]
+end
+
+let () =
+  let original = Colour.{ red = 3; green = 4; blue = 5 } in
+  assert (Colour.set_red 8 original = Colour.{ red = 8; green = 4; blue = 5 });
+  assert (Colour.red original = 3);
+  assert (Box.set_contents "grey" Box.{ contents = 9 } = Box.{ contents = "grey" });
+  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)