* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
@ 2021-05-18 15:04 Drew Adams
0 siblings, 0 replies; 7+ messages in thread
From: Drew Adams @ 2021-05-18 15:04 UTC (permalink / raw)
To: 'Drew Adams', Chunyang Xu, 24875@debbugs.gnu.org
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> 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?
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^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
@ 2016-11-04 13:36 Chunyang Xu
2016-11-04 16:00 ` Andreas Schwab
` (2 more replies)
0 siblings, 3 replies; 7+ messages in thread
From: Chunyang Xu @ 2016-11-04 13:36 UTC (permalink / raw)
To: 24875
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
^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
2016-11-04 13:36 Chunyang Xu
@ 2016-11-04 16:00 ` Andreas Schwab
2016-11-04 16:00 ` Drew Adams
2021-07-06 15:32 ` Lars Ingebrigtsen
2 siblings, 0 replies; 7+ messages in thread
From: Andreas Schwab @ 2016-11-04 16:00 UTC (permalink / raw)
To: Chunyang Xu; +Cc: 24875
On Nov 04 2016, Chunyang Xu <mail@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@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
"And now for something completely different."
^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
2016-11-04 13:36 Chunyang Xu
2016-11-04 16:00 ` Andreas Schwab
@ 2016-11-04 16:00 ` Drew Adams
2021-07-06 15:32 ` Lars Ingebrigtsen
2 siblings, 0 replies; 7+ messages in thread
From: Drew Adams @ 2016-11-04 16:00 UTC (permalink / raw)
To: Chunyang Xu, 24875
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
2016-11-04 13:36 Chunyang Xu
2016-11-04 16:00 ` Andreas Schwab
2016-11-04 16:00 ` Drew Adams
@ 2021-07-06 15:32 ` Lars Ingebrigtsen
2021-07-06 16:43 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2 siblings, 1 reply; 7+ messages in thread
From: Lars Ingebrigtsen @ 2021-07-06 15:32 UTC (permalink / raw)
To: Chunyang Xu; +Cc: 24875, Stefan Monnier
Chunyang Xu <mail@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
^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
2021-07-06 15:32 ` Lars Ingebrigtsen
@ 2021-07-06 16:43 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2021-07-06 17:15 ` Lars Ingebrigtsen
0 siblings, 1 reply; 7+ messages in thread
From: Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors @ 2021-07-06 16:43 UTC (permalink / raw)
To: Lars Ingebrigtsen; +Cc: Chunyang Xu, 24875
Lars Ingebrigtsen [2021-07-06 17:32:26] wrote:
> Chunyang Xu <mail@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
^ permalink raw reply [flat|nested] 7+ messages in thread
* bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent
2021-07-06 16:43 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
@ 2021-07-06 17:15 ` Lars Ingebrigtsen
0 siblings, 0 replies; 7+ messages in thread
From: Lars Ingebrigtsen @ 2021-07-06 17:15 UTC (permalink / raw)
To: Stefan Monnier; +Cc: Chunyang Xu, 24875
Stefan Monnier <monnier@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
^ permalink raw reply [flat|nested] 7+ messages in thread
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2021-05-18 15:04 bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent Drew Adams
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2016-11-04 13:36 Chunyang Xu
2016-11-04 16:00 ` Andreas Schwab
2016-11-04 16:00 ` Drew Adams
2021-07-06 15:32 ` Lars Ingebrigtsen
2021-07-06 16:43 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2021-07-06 17:15 ` Lars Ingebrigtsen
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