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Reply-To: david.baelde@ens-lyon.org
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References: <476C4EB3-59D2-4756-927C-C1697E7AE4D8@math.harvard.edu>
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Date: Fri, 4 Feb 2011 12:49:28 +0100
Message-ID: <AANLkTinRM_4vpCVVWGtJ7G5Se5CjF5S8uiSibcE8uYMx@mail.gmail.com>
From: David Baelde <david.baelde@gmail.com>
To: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
Cc: Nicolas Ojeda Bar <nojb@math.harvard.edu>, caml-list@yquem.inria.fr
Content-Type: text/plain; charset=ISO-8859-1
Subject: Re: [Caml-list] reference cells

On Mon, Jan 31, 2011 at 5:46 PM, Guillaume Yziquel
<guillaume.yziquel@citycable.ch> wrote:
>> # let x = ref 10;;
>> val x : int ref = {contents = 10}
>> # Obj.tag (Obj.repr x);;
>> - : int = 0

A funny side note about that test: the fact that you pass your
reference to a function (Obj.repr) prevents the compiler from
optimizing it. So even if you try (let x = ref 10 in Obj.tag (Obj.repr
x)) you'll see the same tag, while (as Alain said) Ocaml can actually
avoid heap allocation in similar expressions where your reference is
clearly local.

The lambda code remains the best option:

$ cat test.ml
let () = let x = ref 10 in Printf.printf "%d\n" (Obj.tag (Obj.repr !x))
let () = let x = ref 10 in Printf.printf "%d\n" (Obj.tag (Obj.repr x))

$ ocamlc -dlambda test.ml
(setglobal Test!
  (let
    (match/61
       (let (x/58 10)
         (apply (field 1 (global Printf!)) "%d\n" (caml_obj_tag (id x/58))))
     match/60
       (let (x/59 (makemutable 0 10))
         (apply (field 1 (global Printf!)) "%d\n" (caml_obj_tag (id x/59)))))
    (makeblock 0)))

Cheers,
-- 
David