From: Francis Moreau <francis.moro@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Basic questions about elisp
Date: Fri, 6 Nov 2009 08:03:36 -0800 (PST) [thread overview]
Message-ID: <4d5245de-4a71-4be7-a445-6d033be48490@g23g2000yqh.googlegroups.com> (raw)
In-Reply-To: 871vkdm2p1.fsf@lola.goethe.zz
On 5 nov, 16:06, David Kastrup <d...@gnu.org> wrote:
> p...@informatimago.com (Pascal J. Bourguignon) writes:
>
>
>
> > Francis Moreau <francis.m...@gmail.com> writes:
>
> >> Hello,
>
> >> I'm trying to learn elisp and have a couple of basic questions.
>
> >> I'm iterating over a list using dotimes, but in the body of dotimes,
> > dolist dolist
>
> >> the list can mutate. For example I have:
>
> >> (dolist (elt lst)
> >> ;; some codes
> >> (nconc lst '(2)))
>
> > This is an infinite loop. It will break when the program runs out of
> > memory.
>
> It was oversimplified. But it violates one basic principle of
> programming:
Basic principle of _elisp_ programming, I asssume...
>
> Only ever use destructive list operators like nconc on lists that have
> been consed together _entirely_ under your control.
>
> In this particular case, the cons '(2) has been consed together under
> control of the Lisp reader. The second time this code gets executed,
> the cons is destroyed.
eh ?
When I wrote '(2), I suppose the elisp interpreter to create a new
list. And doing (nconc lst '(2)), I assume that this new list is now
referenced by 'lst', hence can't be destroyed.
I initialy thought that this didn't work, but after retrying it, I got
the correct result:
(let ((lst (list 1 2 3)))
(dolist (elt lst)
(when (eq elt 1)
(nconc lst '(4))))
lst)
which evalutes to "(1 2 3 4)"
next prev parent reply other threads:[~2009-11-06 16:03 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-05 11:13 Basic questions about elisp Francis Moreau
2009-11-05 11:50 ` Lennart Borgman
[not found] ` <mailman.10118.1257421858.2239.help-gnu-emacs@gnu.org>
2009-11-05 12:07 ` Francis Moreau
2009-11-05 12:44 ` Lennart Borgman
2009-11-05 12:59 ` David Kastrup
2009-11-05 14:25 ` Francis Moreau
2009-11-05 14:37 ` Pascal J. Bourguignon
2009-11-05 14:58 ` Pascal J. Bourguignon
2009-11-05 12:57 ` tomas
[not found] ` <mailman.10122.1257425638.2239.help-gnu-emacs@gnu.org>
2009-11-05 14:29 ` Francis Moreau
2009-11-05 14:41 ` Pascal J. Bourguignon
2009-11-05 15:06 ` David Kastrup
2009-11-06 16:03 ` Francis Moreau [this message]
2009-11-06 16:49 ` David Kastrup
2009-11-06 20:53 ` Francis Moreau
2009-11-06 21:18 ` Pascal J. Bourguignon
2009-11-07 14:49 ` Francis Moreau
2009-11-07 17:50 ` Pascal J. Bourguignon
2009-11-08 9:46 ` tomas
[not found] ` <mailman.10266.1257674088.2239.help-gnu-emacs@gnu.org>
2009-11-09 20:51 ` Francis Moreau
2009-11-08 15:18 ` Francis Moreau
2009-11-08 16:58 ` tomas
2009-11-08 17:12 ` Pascal J. Bourguignon
2009-11-09 21:04 ` Francis Moreau
2009-11-10 12:11 ` Joost Kremers
2009-11-10 14:16 ` Francis Moreau
2009-11-10 18:53 ` David Kastrup
2009-11-06 5:06 ` Barry Margolin
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