GNU bug report logs - #24875
26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

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

From: Chunyang Xu <mail <at> xuchunyang.me>
To: bug-gnu-emacs <at> gnu.org
Subject: 26.0.50;
 In Dotted Pair Notation, the read function returns CDR if CAR is
 absent
Date: Fri, 04 Nov 2016 21:36:38 +0800

For example, I eval this in the *scratch* buffer:

(read "( . 123)")
     ⇒ 123

(read "(1 . ( . (3 . nil)))")
     ⇒ (1 3)

I can't find explanation about this behavior in elisp manual. Is it
expected?

According to an answer [1] by wasamasa on Emacs StackExchange,
Guile behaves the same, but other Lisp interpreters (csi, pil and sbcl)
and MIT-Scheme don't permit reading this.


[1] http://emacs.stackexchange.com/a/28410/3889

Message #8 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Andreas Schwab <schwab <at> linux-m68k.org>
To: Chunyang Xu <mail <at> xuchunyang.me>
Cc: 24875 <at> debbugs.gnu.org
Subject: Re: bug#24875: 26.0.50;
 In Dotted Pair Notation, the read function returns CDR if CAR is
 absent
Date: Fri, 04 Nov 2016 17:00:27 +0100

On Nov 04 2016, Chunyang Xu <mail <at> xuchunyang.me> wrote:

> For example, I eval this in the *scratch* buffer:
>
> (read "( . 123)")
>      ⇒ 123
>
> (read "(1 . ( . (3 . nil)))")
>      ⇒ (1 3)
>
> I can't find explanation about this behavior in elisp manual. Is it
> expected?

The Emacs Lisp reader has always worked that way.

Andreas.

-- 
Andreas Schwab, schwab <at> linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Message #11 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Drew Adams <drew.adams <at> oracle.com>
To: Chunyang Xu <mail <at> xuchunyang.me>, 24875 <at> debbugs.gnu.org
Subject: RE: bug#24875: 26.0.50; In Dotted Pair Notation, the read function
 returns CDR if CAR is absent
Date: Fri, 4 Nov 2016 09:00:52 -0700 (PDT)

Indeed.  This is not "normal" (usual) Lisp behavior.
It seems quite bizarre - unexpected, to say the least.

If this is intentional for Emacs Lisp then the Emacs-Lisp
doc should both (1) describe this behavior and (2) give the
rationale behind it.

Message #14 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Drew Adams <drew.adams <at> oracle.com>
To: 'Drew Adams'
 <IMCEAORCL-drew+2Eadams+40oracle+2Ecom <at> namprd10.prod.outlook.com>, Chunyang
 Xu <mail <at> xuchunyang.me>, "24875 <at> debbugs.gnu.org" <24875 <at> debbugs.gnu.org>
Subject: RE: bug#24875: 26.0.50; In Dotted Pair Notation, the read function
 returns CDR if CAR is absent
Date: Tue, 18 May 2021 15:04:03 +0000

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

> Indeed.  This is not "normal" (usual) Lisp behavior.
> It seems quite bizarre - unexpected, to say the least.
> 
> If this is intentional for Emacs Lisp then the Emacs-Lisp
> doc should both (1) describe this behavior and (2) give the
> rationale behind it.

ping.  Can this please be addressed?

[winmail.dat (application/ms-tnef, attachment)]

Message #17 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Chunyang Xu <mail <at> xuchunyang.me>
Cc: 24875 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#24875: 26.0.50; In Dotted Pair Notation, the read function
 returns CDR if CAR is absent
Date: Tue, 06 Jul 2021 17:32:26 +0200

Chunyang Xu <mail <at> xuchunyang.me> writes:

> For example, I eval this in the *scratch* buffer:
>
> (read "( . 123)")
>      ⇒ 123
>
> (read "(1 . ( . (3 . nil)))")
>      ⇒ (1 3)
>
> I can't find explanation about this behavior in elisp manual. Is it
> expected?

That is indeed very eccentric behaviour, so we should probably mention
it somewhere?  (I guess we can't change it.)

Anybody got an opinion here, or a way that we can describe it in the
manual except pointing at the examples and then going ¯\_(ツ)_/¯?

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

Message #22 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: Chunyang Xu <mail <at> xuchunyang.me>, 24875 <at> debbugs.gnu.org
Subject: Re: bug#24875: 26.0.50; In Dotted Pair Notation, the read function
 returns CDR if CAR is absent
Date: Tue, 06 Jul 2021 12:43:58 -0400

Lars Ingebrigtsen [2021-07-06 17:32:26] wrote:

> Chunyang Xu <mail <at> xuchunyang.me> writes:
>
>> For example, I eval this in the *scratch* buffer:
>>
>> (read "( . 123)")
>>      ⇒ 123
>>
>> (read "(1 . ( . (3 . nil)))")
>>      ⇒ (1 3)
>>
>> I can't find explanation about this behavior in elisp manual. Is it
>> expected?
>
> That is indeed very eccentric behaviour, so we should probably mention
> it somewhere?  (I guess we can't change it.)
>
> Anybody got an opinion here, or a way that we can describe it in the
> manual except pointing at the examples and then going ¯\_(ツ)_/¯?

It's the "natural" behavior if you start from

    (Ea . Ed)

as the notation for cons cells and extend it to

    (E1 E2 E3 .. . Ed)

such that

    (E1 {foo} . Ed) = (E1 . ({foo} . Ed))

because then you replace `{foo}` with the empty sequence and you get:

    (E1 . Ed) = (E1 . ( . Ed))

and hence

    Ed = ( . Ed)

It's not used very often, so in theory we could potentially change it,
but I haven't seen any good alternative interpretation for it, and
I don't see the benefit of signaling an error.


        Stefan

Message #25 received at 24875 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Chunyang Xu <mail <at> xuchunyang.me>, 24875 <at> debbugs.gnu.org
Subject: Re: bug#24875: 26.0.50; In Dotted Pair Notation, the read function
 returns CDR if CAR is absent
Date: Tue, 06 Jul 2021 19:15:09 +0200

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

> It's the "natural" behavior if you start from
>
>     (Ea . Ed)
>
> as the notation for cons cells and extend it to
>
>     (E1 E2 E3 .. . Ed)
>
> such that
>
>     (E1 {foo} . Ed) = (E1 . ({foo} . Ed))
>
> because then you replace `{foo}` with the empty sequence and you get:
>
>     (E1 . Ed) = (E1 . ( . Ed))
>
> and hence
>
>     Ed = ( . Ed)
>
> It's not used very often, so in theory we could potentially change it,
> but I haven't seen any good alternative interpretation for it, and
> I don't see the benefit of signaling an error.

Ah, I see!  Thanks for the explanation -- I now taken a stab at finding
a place in the Lisp manual to explain this (and explaining it slightly
shorter).  Feel free to adjust as required.

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

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