From: David Kastrup <dak@gnu.org>
To: help-gnu-emacs@gnu.org
Subject: Re: Basic questions about elisp
Date: Fri, 06 Nov 2009 17:49:11 +0100 [thread overview]
Message-ID: <87zl6ziooo.fsf@lola.goethe.zz> (raw)
In-Reply-To: 4d5245de-4a71-4be7-a445-6d033be48490@g23g2000yqh.googlegroups.com
Francis Moreau <francis.moro@gmail.com> writes:
> 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...
Of Lisp programming.
>> 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 ?
Did you try my example?
> When I wrote '(2), I suppose the elisp interpreter to create a new
> list.
It does so, but at read time. Not execution time.
> And doing (nconc lst '(2)), I assume that this new list is now
> referenced by 'lst', hence can't be destroyed.
Garbage collection is the least of your problems.
> 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)"
And
(let ((lst (list 1 2 3)))
(dolist (elt lst)
(when (< elt 3)
(nconc lst '(4))))
lst)
just crashes.
--
David Kastrup
next prev parent reply other threads:[~2009-11-06 16:49 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
2009-11-06 16:49 ` David Kastrup [this message]
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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