GNU bug report logs - #76472
nconc should fail for dotted lists

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Pip Cet <pipcet <at> protonmail.com>
Subject: bug#76472: closed (Re: bug#76472: nconc should fail for dotted lists)
Date: Fri, 21 Feb 2025 18:04:03 +0000

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Your bug report

#76472: nconc should fail for dotted lists

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 76472 <at> debbugs.gnu.org.

-- 
76472: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=76472
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Pip Cet <pipcet <at> protonmail.com>
To: Mattias Engdegård <mattias.engdegard <at> gmail.com>
Cc: 76472-done <at> debbugs.gnu.org
Subject: Re: bug#76472: nconc should fail for dotted lists
Date: Fri, 21 Feb 2025 18:03:19 +0000

Mattias Engdegård <mattias.engdegard <at> gmail.com> writes:

>> The current behavior is: (nconc '(1 . 2) '(3 4)) => (1 3 4)
>
> Actually that is probably intended and completely fine. It's what
> Common Lisp defines for this operation, and I don't think adding a nil
> check would be an improvement.

Sorry for the noise, closing.

Pip

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From: Pip Cet <pipcet <at> protonmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: nconc should fail for dotted lists
Date: Fri, 21 Feb 2025 14:14:44 +0000

The current behavior is:

(nconc '(1 . 2) '(3 4)) => (1 3 4)

This doesn't seem to me to match the documentation, because the cdr of
the first argument, 2, is discarded.  'append' signals in the equivalent
situation, and I think nconc should, too:

From 285a30392273552503fa6d66bf7a9bdb472f6a8e Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet <at> protonmail.com>
Subject: [PATCH] Make 'nconc' signal an error for dotted lists (bug#)

* src/fns.c (Fnconc): Call 'CHECK_LIST_END' for all but the last
element.
---
 src/fns.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/fns.c b/src/fns.c
index 0ba8e42a60f..720ddb666ca 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -3354,6 +3354,7 @@ DEFUN ("nconc", Fnconc, Snconc, 0, MANY, 0,
       Lisp_Object tail UNINIT;
       FOR_EACH_TAIL (tem)
 	tail = tem;
+      CHECK_LIST_END (tem, args[argnum]);
 
       tem = args[argnum + 1];
       Fsetcdr (tail, tem);
-- 
2.48.1

No indication in the git log that this behavior is intentional.

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