Record field update using 'with' syntax

["Denis Bueno" <dbueno@gmail.com>]

Suppose a module M1 with the record

    type foo = {x : int; y : int; z : int};;

and a member of its type. In another module, a qualified field
matching like the following works:

    match instance with
      {M1.x = 5; y = 5; z = 100} -> (* something *)

Why doesn't the following work in a function?

    {M1.instance with x = 20}

It would seem easy to determine that x must name a field of a record inside M1.

Thanks for your help. (Example in OCaml 3.09.2 below.)

-Denis

Example:

,----
|         Objective Caml version 3.09.2
|
| # module M1 = struct
|       type foo = {x : int; y : int; z : int};;
|       let foo = {x = 10; y = 10; z = 10};;
|   end;;
| module M1 : sig type foo = { x : int; y : int; z : int; } val foo : foo end
| # match M1.foo with | {M1.x = 5; y = 5} -> ();;
| Characters 0-43:
| Warning P: this pattern-matching is not exhaustive.
| Here is an example of a value that is not matched:
| {x=0}
|   match M1.foo with | {M1.x = 5; y = 5} -> ();;
|   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Exception: Match_failure ("", 1, 0).
| # {M1.foo with x = 5};;
| Characters 0-19:
|   {M1.foo with x = 5};;
|   ^^^^^^^^^^^^^^^^^^^
| Unbound record field label x
| #
`----