bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Chunyang Xu]

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

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Andreas Schwab]

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."

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Drew Adams]

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.

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Lars Ingebrigtsen]

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

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Lars Ingebrigtsen]

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

bug#24875: 26.0.50; In Dotted Pair Notation, the read function returns CDR if CAR is absent

[Drew Adams]

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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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