From mboxrd@z Thu Jan 1 00:00:00 1970
Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail
From: Philipp
Newsgroups: gmane.emacs.bugs
Subject: bug#43557: 28.0.50;
Please document which objects are mutable and which are not
Date: Mon, 5 Jul 2021 20:55:05 +0200
Message-ID:
References: <87mu0nv6y6.fsf@gnus.org>
<78C74FCB-EAEF-4E9F-9C79-3A2178FA60FE@gmail.com>
<6F14B065-611C-4ACE-BBD5-50B34512C00B@gmail.com>
Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.100.0.2.22\))
Content-Type: text/plain;
charset=us-ascii
Content-Transfer-Encoding: quoted-printable
Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214";
logging-data="25321"; mail-complaints-to="usenet@ciao.gmane.io"
Cc: Lars Ingebrigtsen , 43557@debbugs.gnu.org
To: Stefan Monnier
Original-X-From: bug-gnu-emacs-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org Mon Jul 05 20:56:21 2021
Return-path:
Envelope-to: geb-bug-gnu-emacs@m.gmane-mx.org
Original-Received: from lists.gnu.org ([209.51.188.17])
by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256)
(Exim 4.92)
(envelope-from )
id 1m0TlV-0006LB-J3
for geb-bug-gnu-emacs@m.gmane-mx.org; Mon, 05 Jul 2021 20:56:21 +0200
Original-Received: from localhost ([::1]:48252 helo=lists1p.gnu.org)
by lists.gnu.org with esmtp (Exim 4.90_1)
(envelope-from )
id 1m0TlU-0003Tj-Jj
for geb-bug-gnu-emacs@m.gmane-mx.org; Mon, 05 Jul 2021 14:56:20 -0400
Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:53012)
by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256)
(Exim 4.90_1) (envelope-from )
id 1m0TlD-0003Si-A9
for bug-gnu-emacs@gnu.org; Mon, 05 Jul 2021 14:56:04 -0400
Original-Received: from debbugs.gnu.org ([209.51.188.43]:35112)
by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128)
(Exim 4.90_1) (envelope-from )
id 1m0TlB-0005BG-Tf
for bug-gnu-emacs@gnu.org; Mon, 05 Jul 2021 14:56:03 -0400
Original-Received: from Debian-debbugs by debbugs.gnu.org with local (Exim 4.84_2)
(envelope-from ) id 1m0TlB-0003L9-M7
for bug-gnu-emacs@gnu.org; Mon, 05 Jul 2021 14:56:01 -0400
X-Loop: help-debbugs@gnu.org
Resent-From: Philipp
Original-Sender: "Debbugs-submit"
Resent-CC: bug-gnu-emacs@gnu.org
Resent-Date: Mon, 05 Jul 2021 18:56:01 +0000
Resent-Message-ID:
Resent-Sender: help-debbugs@gnu.org
X-GNU-PR-Message: followup 43557
X-GNU-PR-Package: emacs
Original-Received: via spool by 43557-submit@debbugs.gnu.org id=B43557.162551131512789
(code B ref 43557); Mon, 05 Jul 2021 18:56:01 +0000
Original-Received: (at 43557) by debbugs.gnu.org; 5 Jul 2021 18:55:15 +0000
Original-Received: from localhost ([127.0.0.1]:46658 helo=debbugs.gnu.org)
by debbugs.gnu.org with esmtp (Exim 4.84_2)
(envelope-from )
id 1m0TkQ-0003KD-NM
for submit@debbugs.gnu.org; Mon, 05 Jul 2021 14:55:15 -0400
Original-Received: from mail-wr1-f52.google.com ([209.85.221.52]:40836)
by debbugs.gnu.org with esmtp (Exim 4.84_2)
(envelope-from ) id 1m0TkO-0003Jv-N0
for 43557@debbugs.gnu.org; Mon, 05 Jul 2021 14:55:13 -0400
Original-Received: by mail-wr1-f52.google.com with SMTP id l5so6431937wrv.7
for <43557@debbugs.gnu.org>; Mon, 05 Jul 2021 11:55:12 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025;
h=mime-version:subject:from:in-reply-to:date:cc
:content-transfer-encoding:message-id:references:to;
bh=uyx6vHQmXAag8dSu1LufL7WQy69euPzRs6SclYJq5Xg=;
b=rGWDsZ16T0dCwUzmRGYYpUpI3lFYXmDk+5ak8tYVEJo5iQCaqxI4zjiKdHHbrmBGBs
mLfjs2qsmqdMJkWEKEM+/eE/jXyT5CVrFDlLsVmoFF5cDp66y7ddBaUl6sZiViAbTjrz
OjabwOleg7lJyJecGPlv5R953DuczfYkznmZFVtXhXnZBCaNi/8lPltz/Q/Jo1mtnFPo
5+A9YDkwQJSfVXp26n6pCmP6Awb+ZRPtIgSH2Ul913//6RqDDb9+jvoaddQAzFMYg2oi
xwJAOZESFqAGBtweGIAHEgPP0gHEDOhSpzkYBfIeUtP20aJCAZekelDgly5dkkBBq67h
Qjzg==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=1e100.net; s=20161025;
h=x-gm-message-state:mime-version:subject:from:in-reply-to:date:cc
:content-transfer-encoding:message-id:references:to;
bh=uyx6vHQmXAag8dSu1LufL7WQy69euPzRs6SclYJq5Xg=;
b=Il2BphXkwBbJf2gLVX+yNPYSE0TaqOKgBZLJR8I0MCSxW/itGWHpsDTviNrU/gBFLe
FLy1j3lEeIwt0h1cuktEp5o+iG/xCoUmxDRvwIPwCx8Vrc6cZ1my4QLVP/YnwB5h41MR
6fSYd6ksxEEZlaB/PCPFUq4GAcZpu/buIzjUVXJCn+5VcMioSh/tajmBnT9p6UDc2eAc
kwVtY85bpFczdOyMijoMvDQQfiRT5JMzh4r0bf/W5fWBpa83Bu4Z5QcQ9XaZMuFlA9rC
CDjuhnACMiopF9/1go7VAWLnQUJTn9NlqRL6iXDWBv1Y0ImzMB/EHdRBQdpHuDUHOJN3
KLRw==
X-Gm-Message-State: AOAM530mBkmcmsPP5JBTaaAS3Icnb16Typ7gzwdvTrIUzVHNK+iSMEBM
ApheQlRckBlECXrkTu+wRTw=
X-Google-Smtp-Source: ABdhPJzVN8M85h5XFEPawKeppNBasbiuozu8x4/hEsvnWCF+PEr53n1hZoeMf3XlOuWf+BBmhYfHqw==
X-Received: by 2002:a05:6000:1acb:: with SMTP id
i11mr16987241wry.120.1625511306693;
Mon, 05 Jul 2021 11:55:06 -0700 (PDT)
Original-Received: from smtpclient.apple ([46.128.198.100])
by smtp.gmail.com with ESMTPSA id k5sm14377528wmk.11.2021.07.05.11.55.05
(version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128);
Mon, 05 Jul 2021 11:55:06 -0700 (PDT)
In-Reply-To:
X-Mailer: Apple Mail (2.3654.100.0.2.22)
X-BeenThere: debbugs-submit@debbugs.gnu.org
X-Mailman-Version: 2.1.18
Precedence: list
X-BeenThere: bug-gnu-emacs@gnu.org
List-Id: "Bug reports for GNU Emacs,
the Swiss army knife of text editors"
List-Unsubscribe: ,
List-Archive:
List-Post:
List-Help:
List-Subscribe: ,
Errors-To: bug-gnu-emacs-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org
Original-Sender: "bug-gnu-emacs"
Xref: news.gmane.io gmane.emacs.bugs:209484
Archived-At:
> Am 05.06.2021 um 17:15 schrieb Stefan Monnier =
:
>=20
>> Assume you have the following (nonsense) function, with unknown =
implementation:
>>=20
>> (defun my-cons ()
>> "Return a cons cell consisting of the integers 1 and 2."
>> ...)
>>=20
>> I. Given only that information and the manual, is the following code =
valid
>> (i.e. can't trigger undefined behavior in any case)?
>>=20
>> (setcar (my-cons) 5)
>=20
> I don't think we want to labels this as "undefined" or "invalid"
> (Emacs and Emacs Lisp tries hard to avoid enforcing abstraction
> boundaries, and relies instead on softer forms of discipline),
I'm counting almost 100 occurrences of the words "undefined" or =
"unspecified" in the reference manual, so this isn't really new =
terminology. It's even used in the manual directly for this purpose:
If a program attempts to change objects that should not be
changed, the resulting behavior is undefined [...]
The remaining weirdness here is saying "objects that should not be =
changed" instead of "immutable objects".
Most programming languages have some notion of "undefined" or =
"unspecified", often to allow optimizations or multiple implementations. =
While there are downsides to introducing such behavior, often it's a =
reasonable compromise between underspecifying and overspecifying the =
language.
In this case, the manual already explains in a footnote that the current =
behavior is undesirable and Emacs Lisp should move into a direction =
where attempting to mutate immutable objects signals an error (thereby =
removing the undefined behavior).
> but I'd
> say that using `setcar` above is risky because the user has no =
guarantee
> about what it may impact.
I think that a word like "risky" has no place in a reference manual. =
What does that even mean? What are the risks? Why introduce such vague =
and confusing terminology to begin with? How would this help Emacs Lisp =
programmers?
>=20
> I think the rule is basically, that you should only ever use =
`setc[ad]r`
> on cons cells you yourself created.
What does "should" mean here? What happens if users don't follow this =
"recommendation"? How do they identify "cons cells you yourself =
created"?
Again, I think such terminology brings up more questions than it =
answers.
> But indeed the manual fails to
> document which functions guarantee to return "fresh" new cells, which
> makes it hard to know which cells "you yourself created".
Then let's document that (while avoiding terms such as "you yourself =
created").
>=20
>> II. Which of the following implementations of `my-cons' is correct
>> (i.e. follows the rules of Emacs Lisp as described in the manual)?
>=20
> All of them.
Good. Then let's document that!
>=20
>> =46rom what I can see there are four options:
>>=20
>> 1. Unless otherwise specified, objects are mutable. Then the =
`setcar' form
>> is valid, and only implementation (b) is correct.
>> 2. Unless otherwise specified, objects are immutable. Then the =
`setcar'
>> form always triggers undefined behavior, and only implementation (a)
>> is correct.
>> 3. Unless otherwise specified, the objects that forms return are of
>> unspecified mutability (i.e. they can be mutable or immutable). Then =
the
>> `setcar' form is invalid because the caller of `my-cons' can't assume =
that
>> its return value is mutable, and all three implementations of =
`my-cons'
>> are correct.
>> 4. Mutability of the return object must be specified in all cases.
>> Then none of the implementations is correct, since none of them =
specifies
>> the mutability of the returned cons object.
>=20
> I think we have (3).
>=20
Can we find a wording that we agree on and put it into the manual?=