bug#47391: 27.2; static list return from function and nconc

bug#47391: 27.2; static list return from function and nconc

[Rajeev Narang via Bug reports for GNU Emacs, the Swiss army knife of text editors]

(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...

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Re: bug#47391: 27.2; static list return from function and nconc

[Philipp Stephani]

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@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.

Re: bug#47391: 27.2; static list return from function and nconc

[Stefan Monnier]

>> (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

Re: bug#47391: 27.2; static list return from function and nconc

[Gregory Heytings]

[-- Attachment #1: Type: text/plain, Size: 489 bytes --]


>
> (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?

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: Type: text/x-diff; name=0001-lisp-htmlfontify.el-hfy-invisible-Return-a-new-list-.patch, Size: 837 bytes --]

From 268e15ea8d94d4e258d576408b6026bbc5e1fda3 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Thu, 25 Mar 2021 15:51:05 +0000
Subject: [PATCH 2/2] * lisp/htmlfontify.el (hfy-invisible): Return a new list
 at each invocation (bug#47391)

---
 lisp/htmlfontify.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/htmlfontify.el b/lisp/htmlfontify.el
index bfbe0ee165..cd7df5dfdf 100644
--- a/lisp/htmlfontify.el
+++ b/lisp/htmlfontify.el
@@ -822,7 +822,7 @@ VAL is ignored."
   "This text should be invisible.
 Do something in CSS to make that happen.
 VAL is ignored here."
-  '(("display" . "none")))
+  (list '("display" . "none")))
 
 (defun hfy-combined-face-spec (face)
   "Return a `defface' style alist of possible specifications for FACE.
-- 
2.30.2

bug#47391: 27.2; static list return from function and nconc

[Stefan Monnier]

> Does the attached patch fix the bug you see?

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


        Stefan

Re: bug#47391: 27.2; static list return from function and nconc

[Gregory Heytings]

>> 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.