From: Stephen Berman <stephen.berman@gmx.net>
To: Jean-Christophe Helary <brandelune@gmail.com>
Cc: help-gnu-emacs <help-gnu-emacs@gnu.org>
Subject: Re: using setq to create lists based on other lists...
Date: Sun, 02 Dec 2018 14:08:34 +0100 [thread overview]
Message-ID: <87zhto2i0d.fsf@gmx.net> (raw)
In-Reply-To: <D1EAF121-4457-4C76-9233-70F96A591838@gmail.com> (Jean-Christophe Helary's message of "Sun, 2 Dec 2018 21:22:54 +0900")
On Sun, 2 Dec 2018 21:22:54 +0900 Jean-Christophe Helary <brandelune@gmail.com> wrote:
> Stephen, thank you for the additional explanations.
>
>> On Dec 2, 2018, at 20:51, Stephen Berman <stephen.berman@gmx.net> wrote:
>
>>>> I don't suppose that's a bug, but really it ought the be very clearly
>>>> documented in the reference. Also, I'd like to know why that's happening.
>>
>> To expand of this, since Jean-Christophe didn't find an explanation of
>> this behavior of setq in the Lisp reference, but it is in fact
>> documented:
>
> Well, yes and no.
>
>> Special Form: setq [symbol form]...
>> This special form is the most common method of changing a
>> variable’s value. Each SYMBOL is given a new value, which is the
>> result of evaluating the corresponding FORM. The current binding
>> of the symbol is changed.
>>
>> In the above case, the symbol `list1' is given the result of evaluating
>> `list0', which is the list `'(1 2)'. So now both `list0' and `list1'
>> refer to this list,
>
> That's not clear at all from the paragraph you quote. Because for all
> practical purposes, when I evaluate list0 I get (1 2) and not "a pointer to an
> object that is the list (1 2)".
>
> In fact, I just found the explanation, it is in the Introduction to Emacs Lisp
> and it says:
>
>> When a variable is set to a list with a function such as setq, it stores the
>> address of the first box in the variable.
>
> So, setq has a specific behavior when it applies to lists: it evaluates the
> form as a pointer to an object and not as a value.
I don't think setq is behaving any different with lists than with other
Lisp objects. Lists are defined this way in Lisp, cf. (info "(elisp)
Cons Cell Type"):
A “list” is a series of cons cells, linked together so that the CDR
slot of each cons cell holds either the next cons cell or the empty
list.
[...]
A note to C programmers: a Lisp list thus works as a “linked list”
built up of cons cells. Because pointers in Lisp are implicit, we
do not distinguish between a cons cell slot holding a value versus
pointing to the value.
See also the diagrams and explanations in (info "(elisp) Box Diagrams")
and in the discussion of `append' in (info "(elisp) Building Lists").
Steve Berman
next prev parent reply other threads:[~2018-12-02 13:08 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.5010.1543748027.1284.help-gnu-emacs@gnu.org>
2018-12-02 11:21 ` using setq to create lists based on other lists Barry Margolin
2018-12-02 11:51 ` Stephen Berman
2018-12-02 12:22 ` Jean-Christophe Helary
2018-12-02 13:08 ` Stephen Berman [this message]
2018-12-02 13:28 ` Jean-Christophe Helary
2018-12-02 14:40 ` Michael Heerdegen
2018-12-02 15:34 ` Jean-Christophe Helary
2018-12-02 15:44 ` Michael Heerdegen
2018-12-02 15:57 ` Jean-Christophe Helary
2018-12-02 15:00 ` Stephen Berman
2018-12-02 15:30 ` Jean-Christophe Helary
[not found] ` <mailman.5026.1543764670.1284.help-gnu-emacs@gnu.org>
2018-12-04 9:00 ` Barry Margolin
2018-12-02 12:03 ` Jean-Christophe Helary
[not found] <mailman.5042.1543777897.1284.help-gnu-emacs@gnu.org>
2018-12-04 9:04 ` Barry Margolin
[not found] ` <(message>
[not found] ` <from>
[not found] ` <Barry>
[not found] ` <Margolin>
[not found] ` <on>
[not found] ` <Tue>
[not found] ` <04>
[not found] ` <Dec>
[not found] ` <2018>
[not found] ` <04:04:52>
2018-12-04 13:56 ` Stefan Monnier
2018-12-05 1:07 ` Robert Thorpe
2018-12-05 2:32 ` Drew Adams
2018-12-05 6:45 ` Jean-Christophe Helary
2018-12-05 8:00 ` Marcin Borkowski
2018-12-05 8:11 ` Jean-Christophe Helary
2018-12-05 14:57 ` Drew Adams
[not found] ` <mailman.5218.1544021892.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:59 ` Barry Margolin
[not found] ` <mailman.5186.1543978155.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:50 ` Barry Margolin
[not found] ` <mailman.5145.1543931778.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:47 ` Barry Margolin
2018-12-02 10:53 Jean-Christophe Helary
2018-12-02 15:07 ` Stefan Monnier
2018-12-02 15:41 ` Jean-Christophe Helary
2018-12-02 16:05 ` Stefan Monnier
2018-12-02 16:23 ` Jean-Christophe Helary
2018-12-02 17:02 ` Stefan Monnier
2018-12-02 17:21 ` Jean-Christophe Helary
2018-12-02 19:11 ` Robert Thorpe
2018-12-02 23:44 ` Jean-Christophe Helary
[not found] ` <mailman.5028.1543765273.1284.help-gnu-emacs@gnu.org>
2018-12-03 13:43 ` Rusi
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