From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from weis@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id WAA23783 for caml-red; Wed, 15 Nov 2000 22:38:14 +0100 (MET)
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id PAA20314 for <caml-list@pauillac.inria.fr>; Tue, 14 Nov 2000 15:41:32 +0100 (MET)
Received: from beaune.inria.fr (beaune.inria.fr [128.93.8.3])
	by nez-perce.inria.fr (8.11.1/8.10.0) with ESMTP id eAEEfW901461
	for <caml-list@inria.fr>; Tue, 14 Nov 2000 15:41:32 +0100 (MET)
Received: by beaune.inria.fr (8.8.8/1.1.22.3/14Sep99-0328PM)
	id PAA0000012997; Tue, 14 Nov 2000 15:41:32 +0100 (MET)
Date: Tue, 14 Nov 2000 15:41:32 +0100 (MET)
From: Damien Doligez <Damien.Doligez@inria.fr>
Message-Id: <200011141441.PAA0000012997@beaune.inria.fr>
To: caml-list@inria.fr
Subject: Re:  Is this use of Object.magic safe?
Sender: weis@pauillac.inria.fr

>From: Stephan Houben <stephan@pcrm.win.tue.nl>
>
>let value_of_int (i : int) : value =
>  Obj.magic i
>
>let value_of_allocated_value (av :allocated_value) : value=
>  Obj.magic av
> 
>let unwrap (int_func : int -> 'a) 
>                (aval_func : allocated_value -> 'a)
>                (v : value) : 'a =
>  if Obj.is_int (Obj.repr v)
>  then int_func (Obj.magic v)
>  else aval_func (Obj.magic v) 

>1. Is this safe w.r.t. the garbage collector? Or am I going to confuse the GC?

No problem with the GC.


>2. Can I safely marshall values?

No problem with marshalling either.


>3. Is it worth the trouble? Perhaps O'Caml already does something smart
>   when a constructor of the form `Int of int' is encountered?

It doesn't work as you expect because O'Caml is strongly typed, so it
can (and does) use a smart representation for your constant
constructors True and False  They are represented as integers, so your
unwrap function cannot tell the difference between a boolean and an
integer.  This is typical of the dangers of using Obj.magic.

-- Damien