GNU bug report logs - #47391
27.2; static list return from function and nconc

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Rajeev Narang <rajeev.jnk <at> sivalik.com>
To: bug-gnu-emacs <at> gnu.org, emacs-devel <at> gnu.org
Subject: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 10:54:20 -0400

(defun fn () '(1 2))

(nconc (fn) '(3))
-> (1 2 3)

(nconc (fn) '(3))
-> (1 2 3 3) ? should be (1 2 3)

If this is not a bug, then hfy-invisible, hfy-face-to-style-i may need to be fixed.


In GNU Emacs 27.2 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.24)
 of 2021-03-19 built on hp
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: emacs-27
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Recent messages:
fn
(1 2 3)
(1 2 3 3)
next-line: End of buffer [2 times]
Mark activated
next-line: End of buffer
Making completion list...
Quit
Mark activated
Making completion list...

Configured using:
 'configure --prefix=/home/rajeev/tmp/build/emacs'

Configured features:
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Message #8 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Philipp Stephani <p.stephani2 <at> gmail.com>
To: Rajeev Narang <rajeev.jnk <at> sivalik.com>
Cc: 47391 <at> debbugs.gnu.org, Emacs developers <emacs-devel <at> gnu.org>
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 16:28:26 +0100

Am Do., 25. März 2021 um 16:23 Uhr schrieb Rajeev Narang via Bug
reports for GNU Emacs, the Swiss army knife of text editors
<bug-gnu-emacs <at> gnu.org>:
>
> (defun fn () '(1 2))
>
> (nconc (fn) '(3))
> -> (1 2 3)

This is undefined behavior/unsupported. The value that (fn) evaluates
to is immutable, but nconc mutates it.

>
> (nconc (fn) '(3))
> -> (1 2 3 3) ? should be (1 2 3)
>
> If this is not a bug, then hfy-invisible, hfy-face-to-style-i may need to be fixed.

If they use this pattern, then they should indeed be fixed.

Message #11 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Philipp Stephani <p.stephani2 <at> gmail.com>
Cc: Rajeev Narang <rajeev.jnk <at> sivalik.com>, 47391 <at> debbugs.gnu.org,
 Emacs developers <emacs-devel <at> gnu.org>
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 11:43:20 -0400

>> (defun fn () '(1 2))
>>
>> (nconc (fn) '(3))
>> -> (1 2 3)
>
> This is undefined behavior/unsupported. The value that (fn) evaluates
> to is immutable, but nconc mutates it.
>
>>
>> (nconc (fn) '(3))
>> -> (1 2 3 3) ? should be (1 2 3)
>>
>> If this is not a bug, then hfy-invisible, hfy-face-to-style-i may need to be fixed.
>
> If they use this pattern, then they should indeed be fixed.

Note that in the code above, we usually consider that the bug is not in
`fn` but in the use of `nconc` on a list which the code doesn't know can
safely be mutated (it should hence use `append` instead).


        Stefan

Message #14 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Gregory Heytings <gregory <at> heytings.org>
To: Rajeev Narang <rajeev.jnk <at> sivalik.com>
Cc: 47391 <at> debbugs.gnu.org, emacs-devel <at> gnu.org
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 15:55:37 +0000

[Message part 1 (text/plain, inline)]

>
> (defun fn () '(1 2))
>
> (nconc (fn) '(3))
> -> (1 2 3)
>
> (nconc (fn) '(3))
> -> (1 2 3 3) ? should be (1 2 3)
>

Why do you think this is a bug?  The docstring of "nconc" says: 
"Concatenate any number of lists by _altering_ them.  Only the last 
argument is not altered, and need not be a list."  (append (fn) '(3)) 
always returns (1 2 3).

>
> If this is not a bug, then hfy-invisible, hfy-face-to-style-i may need 
> to be fixed.
>

Does the attached patch fix the bug you see?

[0001-lisp-htmlfontify.el-hfy-invisible-Return-a-new-list-.patch (text/x-diff, attachment)]

Message #17 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Gregory Heytings <gregory <at> heytings.org>
Cc: Rajeev Narang <rajeev.jnk <at> sivalik.com>, 47391 <at> debbugs.gnu.org,
 emacs-devel <at> gnu.org
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 11:58:48 -0400

> Does the attached patch fix the bug you see?

I pushed another one instead which avoids `nconc` instead of trying to
allow it.


        Stefan

Message #20 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Gregory Heytings <gregory <at> heytings.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Rajeev Narang <rajeev.jnk <at> sivalik.com>, 47391 <at> debbugs.gnu.org,
 emacs-devel <at> gnu.org
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Thu, 25 Mar 2021 16:03:23 +0000

>> Does the attached patch fix the bug you see?
>
> I pushed another one instead which avoids `nconc` instead of trying to 
> allow it.
>

Okay.  Apparently htmlfontify.el was written with the idea of mutating 
lists returned by functions (for efficiency reasons?), hfy-invisible was 
an exception.

Message #23 received at 47391 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Gregory Heytings <gregory <at> heytings.org>, 47391 <at> debbugs.gnu.org,
 Rajeev Narang <rajeev.jnk <at> sivalik.com>
Subject: Re: bug#47391: 27.2; static list return from function and nconc
Date: Sun, 28 Mar 2021 15:21:21 +0200

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

>> Does the attached patch fix the bug you see?
>
> I pushed another one instead which avoids `nconc` instead of trying to
> allow it.

I guess this means that this bug has been fixed?  So I'm closing this
report now.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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