From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <Xavier.Leroy@inria.fr>
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 5E903BC37
	for <caml-list@yquem.inria.fr>; Wed, 28 Oct 2009 17:52:28 +0100 (CET)
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AoMCAF0T6ErUGyoGkWdsb2JhbACbPgEBAQEJCwoHEwO+eYQ/BA
X-IronPort-AV: E=Sophos;i="4.44,640,1249250400"; 
   d="scan'208";a="39112340"
Received: from smtp6-g21.free.fr ([212.27.42.6])
  by mail1-smtp-roc.national.inria.fr with ESMTP; 28 Oct 2009 17:52:27 +0100
Received: from smtp6-g21.free.fr (localhost [127.0.0.1])
	by smtp6-g21.free.fr (Postfix) with ESMTP id 275F8E081FD;
	Wed, 28 Oct 2009 17:52:21 +0100 (CET)
Received: from [192.168.1.3] (che78-2-82-237-71-191.fbx.proxad.net [82.237.71.191])
	by smtp6-g21.free.fr (Postfix) with ESMTP id E293EE080F0;
	Wed, 28 Oct 2009 17:52:18 +0100 (CET)
Message-ID: <4AE876C2.8050707@inria.fr>
Date: Wed, 28 Oct 2009 17:52:18 +0100
From: Xavier Leroy <Xavier.Leroy@inria.fr>
User-Agent: Thunderbird 2.0.0.4 (X11/20070620)
MIME-Version: 1.0
To: 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>
In-Reply-To: <1256250121.4178.37.camel@MATHIAS-ENS>
X-Enigmail-Version: 0.95.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-Spam: no; 0.00; compiler:01 ocaml:01 rec':01 val:01 val:01 lukasz:01 ocaml:01 mathias:98 wrote:01 wrote:01 rec:01 rec:01 caml-list:01 cyclic:01 int:01 

Mathias Kende wrote:

> I need to write something like this :
> 
> 	let f f i = if i = 0 then 1 else i * f (i - 1)
> 	let rec g = f g
> 
> Of course the compiler won't let me write it (even if the OCaml type
> system is happy):
> 	"This kind of expression is not allowed as right-hand side of `let rec'"
In general, the best thing to do in this case is to switch to lazy
evaluation:

# let f f i = if i = 0 then 1 else i * Lazy.force f (i-1);;
val f : (int -> int) Lazy.t -> int -> int = <fun>
# let rec g' = lazy (f g');;
val g' : (int -> int) Lazy.t = <lazy>
# let g = Lazy.force g';;
val g : int -> int = <fun>
# g 10;;
- : int = 3628800

Lukasz Stafiniak wrote:

> While we are at it, what is the best way to convert a "straight" list
> into a cyclic list?
> 
> i.e. convert
> 
> let l = a::b::[]
> 
> into
> 
> let rec l = a::b::l
> 
> (for arbitrary length lists). (The answer I recall from the archives
> was using Obj.magic to mutate the [] in the original list).

Obj.magic is not part of the OCaml language :-)

Again, you can do that just fine using lazy lists instead of lists:

type 'a lazylist = 'a lazylist_content Lazy.t
and 'a lazylist_content = Nil | Cons of 'a * 'a lazylist

Hope this helps,

- Xavier Leroy