From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Why doesn't nconc change my variable?
Date: Sun, 05 Oct 2014 03:59:39 +0200 [thread overview]
Message-ID: <87zjdbmg38.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.10460.1412473030.1147.help-gnu-emacs@gnu.org
Marcin Borkowski <mbork@wmi.amu.edu.pl> writes:
> Hi list,
>
> I don't get it.
>
> (setq my-list ())
> (nconc my-list '("wtf"))
>
> and my-list is still nil.
>
> If, OTOH, I do
>
> (setq my-list ())
> (setq my-list (nconc my-list '("wtf")))
>
> my-list is ("wtf").
>
> Why is that so? I though nconc is supposed to change all its arguments
> but the last one. Is the latter construct a correct way of adding an
> element at the end of the list?
() is actually read as the symbol nil.
Symbols are symbols, and not cons cells.
You cannot transform a symbol into a cons cell.
It is impossible for any function to transform nil into a cons cell.
And vice-versa, it is impossible to transform a cons cell into a symbol
such as nil. This is the reason why you should also use setf when
deleting elements from a list:
(setf list (delete e list))
--
__Pascal Bourguignon__ http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk
next parent reply other threads:[~2014-10-05 1:59 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.10460.1412473030.1147.help-gnu-emacs@gnu.org>
2014-10-05 1:59 ` Pascal J. Bourguignon [this message]
2014-10-05 9:35 ` Why doesn't nconc change my variable? Marcin Borkowski
[not found] ` <mailman.10474.1412501757.1147.help-gnu-emacs@gnu.org>
2014-10-05 17:50 ` Pascal J. Bourguignon
2014-10-05 18:04 ` Pascal J. Bourguignon
2014-10-05 1:36 Marcin Borkowski
2014-10-05 1:58 ` Drew Adams
2014-10-05 9:54 ` Marcin Borkowski
2014-10-05 10:52 ` Thorsten Jolitz
2014-10-05 16:29 ` Drew Adams
2014-10-05 16:38 ` Drew Adams
[not found] ` <mailman.10475.1412502895.1147.help-gnu-emacs@gnu.org>
2014-10-05 18:12 ` Pascal J. Bourguignon
2014-10-05 18:14 ` Pascal J. Bourguignon
2014-10-05 18:31 ` Pascal J. Bourguignon
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87zjdbmg38.fsf@kuiper.lan.informatimago.com \
--to=pjb@informatimago.com \
--cc=help-gnu-emacs@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.