From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <steph@glondu.net>
X-Original-To: caml-list@yquem.inria.fr
Delivered-To: caml-list@yquem.inria.fr
Received: from mail1-relais-roc.national.inria.fr (mail1-relais-roc.national.inria.fr [192.134.164.82])
	by yquem.inria.fr (Postfix) with ESMTP id A057CBC37
	for <caml-list@yquem.inria.fr>; Sun, 25 Oct 2009 16:04:09 +0100 (CET)
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: ApsEADYG5EpeFxRU/2dsb2JhbACEcb9aj2uBMoI6UwQ
X-IronPort-AV: E=Sophos;i="4.44,621,1249250400"; 
   d="scan'208";a="38923668"
Received: from glyptal.glondu.net ([94.23.20.84])
  by mail1-smtp-roc.national.inria.fr with ESMTP; 25 Oct 2009 16:04:02 +0100
Received: from [10.198.42.2] (unknown [10.198.42.2])
	by glyptal.glondu.net (Postfix) with ESMTP id A871B59DEA;
	Sun, 25 Oct 2009 16:03:59 +0100 (CET)
Message-ID: <4AE468DF.806@glondu.net>
Date: Sun, 25 Oct 2009 16:03:59 +0100
From: =?UTF-8?B?U3TDqXBoYW5lIEdsb25kdQ==?= <steph@glondu.net>
User-Agent: Mozilla-Thunderbird 2.0.0.22 (X11/20090701)
MIME-Version: 1.0
To: Mathias Kende <mathias@kende.fr>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] forbidden construct as right hand side of "let rec"
References: <1256250121.4178.37.camel@MATHIAS-ENS>	<4AE0DDF5.8040601@glondu.net> <f23ca73b320d0c61a4a72cd310658e59@quare.fr>
In-Reply-To: <f23ca73b320d0c61a4a72cd310658e59@quare.fr>
X-Enigmail-Version: 0.95.0
OpenPGP: id=49881AD3
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
X-Spam: no; 0.00; intuitively:01 cheers:01 phane:98 steph:98 mathias:98 phane:98 rec:01 rec:01 assert:01 caml-list:01 construct:02 let:03 let:03 tmp:05 tmp:05 

Mathias Kende a =C3=A9crit :
> [...] I also want to write :
> 	let rec h =3D t (f h)
> (with t : ('a -> 'b) -> 'a -> 'b) but here, I can't afford to use
> 	let rec h x =3D t (f h) x
> because t as some side effects and I need it to be evaluated only once.=


Then what about:

let h =3D
  let tmp =3D ref (fun x -> assert false) in
  let res x =3D !tmp x in
  tmp :=3D
    (fun x ->
       let y =3D t (f res) in
       tmp :=3D y;
       y x);
  res

Intuitively, the "tmp" reference caches the call to "t (f h)", but this
is otherwise the same technique as I gave earlier.


Cheers,

--=20
St=C3=A9phane