diff --git a/record.ml b/record.ml
index c3d5c66..1aa8a02 100644
--- a/record.ml
+++ b/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)
diff --git a/rewrite.ml b/rewrite.ml
index 4e607e1..0c45f82 100644
--- a/rewrite.ml
+++ b/rewrite.ml
@@ -31,13 +31,17 @@ let generate declaration =
     let name = field.pld_name.txt in
     let getter = lambda ~loc Nolabel source_pattern
         (Exp.field ~loc source (ident ~loc name)) in
-    let replacement = Exp.record ~loc [ident ~loc name, value ~loc "__fg_value"]
+    let replace expression = Exp.record ~loc [ident ~loc name, expression]
         (if List.length fields = 1 then None else Some source) in
     let setter = lambda ~loc Nolabel (variable ~loc "__fg_value")
-        (lambda ~loc Nolabel source_pattern replacement) in
+        (lambda ~loc Nolabel source_pattern (replace (value ~loc "__fg_value"))) in
+    let updater = lambda ~loc (Labelled "f") (variable ~loc "__fg_function")
+        (lambda ~loc Nolabel source_pattern
+           (replace (Exp.apply ~loc (value ~loc "__fg_function")
+              [Nolabel, Exp.field ~loc source (ident ~loc name)]))) in
     List.map (fun (name, body) ->
       Str.value ~loc Nonrecursive [Vb.mk ~loc (variable ~loc name) body])
-      [name, getter; "set_" ^ name, setter]) fields)
+      [name, getter; "set_" ^ name, setter; "update_" ^ name, updater]) fields)
 
 let structure mapper items =
   List.concat (List.map (fun item ->