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bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil.
From: |
Philip Kaludercic |
Subject: |
bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil. |
Date: |
Sat, 18 Mar 2023 13:48:40 +0000 |
Alan Mackenzie <acm@muc.de> writes:
> Hello, Philip.
>
> On Sat, Mar 18, 2023 at 07:41:14 +0000, Philip Kaludercic wrote:
>> Ruijie Yu <ruijie@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.
But (proper-list-p '#1=(a . #1#)) does return nil? And (proper-list-p
'#1=(a #1#)) does return 2,
+---+---+ +---+---+
| a | ----->| | | ------> nil
+---+---+ +-|-+---+
^ |
\_________/
because #1=(a #1#)) is not a circular list, the cadr only has a
reference back to a the beginning of the list, but #1=(a . #1#)) is
cyclical because as you say a cdr points back to a previous cons cell:
+---+---+
| a | | |
+---+-|-+
^ |
\__/
> 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
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Alan Mackenzie, 2023/03/07
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Ruijie Yu, 2023/03/07
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Philip Kaludercic, 2023/03/18
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Alan Mackenzie, 2023/03/18
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil.,
Philip Kaludercic <=
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Ruijie Yu, 2023/03/31
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Philip Kaludercic, 2023/03/31
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Ruijie Yu, 2023/03/31
- bug#62037: (proper-list-p '#1=(a #1#)) => 2. It should return nil., Eli Zaretskii, 2023/03/31