Generate labelled updaters and propagate callback exceptions

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 ->