From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mail1-relais-roc.national.inria.fr (mail1-relais-roc.national.inria.fr [192.134.164.82])
	by walapai.inria.fr (8.13.6/8.13.6) with ESMTP id p9F9D8tJ006894
	for <caml-list@sympa-roc.inria.fr>; Sat, 15 Oct 2011 11:13:08 +0200
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: Aq4BANFNmU7RVdK2kWdsb2JhbABDqF0IIgEBAQEJCQ0HEieBbgEBAQMBEgIsARsdAQMBCwYFCw0uIgERAQUBHAYTCBqHXZlaCotRgmCEdT2IbgIFCod6BJN6jSo9g3A
X-IronPort-AV: E=Sophos;i="4.69,350,1315173600"; 
   d="scan'208";a="124230897"
Received: from mail-iy0-f182.google.com ([209.85.210.182])
  by mail1-smtp-roc.national.inria.fr with ESMTP/TLS/RC4-SHA; 15 Oct 2011 11:13:03 +0200
Received: by iaky10 with SMTP id y10so5961156iak.27
        for <caml-list@inria.fr>; Sat, 15 Oct 2011 02:13:02 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=gamma;
        h=mime-version:in-reply-to:references:date:message-id:subject:from:to
         :cc:content-type:content-transfer-encoding;
        bh=+etmumlPGz1VcfuFurYgq9oC1kOBl5ITLLdGIBXFGEE=;
        b=M/dCP2ppo1HCrTFP33TpFXbl9zmTk81Awo6NPOkCHyV2njkB0nnrPBvAOD/t2KRhp6
         WseC9OaBtyDzI7HrPX0ZDqx9B+vNh7sulRWL6BXBxx7XFXtkKkHJi4oEYEYpeQaE/HYn
         /LkDNUQ4xC2ikInhRKdn96rdnpSY9V8wFnN+0=
MIME-Version: 1.0
Received: by 10.231.5.102 with SMTP id 38mr1172402ibu.98.1318669981917; Sat,
 15 Oct 2011 02:13:01 -0700 (PDT)
Received: by 10.231.199.5 with HTTP; Sat, 15 Oct 2011 02:13:01 -0700 (PDT)
In-Reply-To: <4E993FE5.7080409@ens-lyon.org>
References: <4E993FE5.7080409@ens-lyon.org>
Date: Sat, 15 Oct 2011 10:13:01 +0100
Message-ID: <CAAxsn=Go+=FPCnWOv3NHJ1runia0Sc82gaiXsXLc6iOoPk83vw@mail.gmail.com>
From: Jeremy Yallop <yallop@gmail.com>
To: Martin Jambon <martin.jambon@ens-lyon.org>
Cc: OCaml Mailing List <caml-list@inria.fr>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by walapai.inria.fr id p9F9D8tJ006894
Subject: Re: [Caml-list] How to mutate immutable record fields?

On 15 October 2011 09:10, Martin Jambon <martin.jambon@ens-lyon.org> wrote:
> Purpose: deserializing a record efficiently, i.e. creating a record
> whose field values are available one after another in an unpredictable
> order.
>
> Problem: Obj.set_field does the job in most cases but not in the example
> below. How to make it work?

> (we don't want to use pure OCaml option refs to store field values
> before putting them into the record because that's too slow)
>
> Requirements:
> - performance is critical
> - code will be machine-generated
> - immutable record fields must be supported

The way I've approached this in the past is to to construct the record
at a mutable version of the type, then convert the value to the
immutable type at the last moment.  That is, for your type

> type t = {
>  foo : int option;
>  bar : int;
> }

I'd have the serialisation code generate a second type, not exposed to
user code:

  module Mutable = struct
    type mutable_t = {
      mutable foo : int option;
      mutable bar : int;
    }
  end

The generated deserialisation code would deal only with values of type
Mutable.t, concluding with a final coercion (Obj.magic) to the exposed
type t.