GNU bug report logs - #62037
(proper-list-p '#1=(a #1#)) => 2. It should return nil.

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From: Alan Mackenzie <acm <at> muc.de>
To: Philip Kaludercic <philipk <at> posteo.net>
Cc: Ruijie Yu <ruijie <at> netyu.xyz>, 62037 <at> debbugs.gnu.org
Subject: bug#62037: (proper-list-p '#1=(a #1#)) => 2.  It should return nil.
Date: Sat, 18 Mar 2023 08:29:02 +0000

Hello, Philip.

On Sat, Mar 18, 2023 at 07:41:14 +0000, Philip Kaludercic wrote:
> Ruijie Yu <ruijie <at> netyu.xyz> writes:

> > Notice the distinction between these two snippets:

> >     (let ((lst-1 '#1=(a #1#)))
> >       (list lst-1 (proper-list-p lst-1)))
> >     ;; => ((a #1) 2)

> >     (let ((lst-2 '#1=(a . #1#)))
> >       (list lst-2 (proper-list-p lst-2)))
> >     ;; => ((a . #0) nil)

> Doesn't this point resolve the issue?

No, it doesn't.  A circular list is defined (Elisp manual page "Lists
and Cons Cells") as one where "some cons cell’s CDR could point to one
of the previous cons cells in the list".  A proper list (page
"List-related Predicates") is one which is neither dotted nor circular.

The list #1=(a . #1#) is clearly circular.  proper-list-p should return
nil for it.

The purpose of proper-list-p is surely to find out in advance whether an
algorithm one wishes to run on a list can proceed without taking special
precautions for dottedness or circularity.  proper-list-p fails here.

> Shouldn't the bug report be closed?

Only once it's been fixed.

> -- 
> Philip Kaludercic

-- 
Alan Mackenzie (Nuremberg, Germany).

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