From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: "Pascal J. Bourguignon" Newsgroups: gmane.emacs.help Subject: Re: About Circular Objects Date: Tue, 25 Dec 2012 15:51:48 +0100 Organization: Informatimago Message-ID: <87ip7qxjzv.fsf@kuiper.lan.informatimago.com> References: NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: ger.gmane.org 1356447315 26487 80.91.229.3 (25 Dec 2012 14:55:15 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 25 Dec 2012 14:55:15 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Tue Dec 25 15:55:28 2012 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1TnVuh-00045P-R1 for geh-help-gnu-emacs@m.gmane.org; Tue, 25 Dec 2012 15:55:27 +0100 Original-Received: from localhost ([::1]:39658 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TnVuT-0000sK-Dw for geh-help-gnu-emacs@m.gmane.org; Tue, 25 Dec 2012 09:55:13 -0500 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 79 Original-X-Trace: individual.net MYyA7zkBAaC4UAts8h+zBgojwLRFhzAifmbWq4UVvfwF7oixL2KIJGqwgf9lbbRz7B Cancel-Lock: sha1:MTNjOTQwOTMxOTYwODZlMjEzMTQ0MzIzZDYyY2MxYjAzYjY0ZTg2MA== sha1:4bvO5GCLD7cSQpQtKKLct4YiVis= Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwAQMAAABtzGvEAAAABlBMVEUAAAD///+l2Z/dAAAA oElEQVR4nK3OsRHCMAwF0O8YQufUNIQRGIAja9CxSA55AxZgFO4coMgYrEDDQZWPIlNAjwq9 033pbOBPtbXuB6PKNBn5gZkhGa86Z4x2wE67O+06WxGD/HCOGR0deY3f9Ijwwt7rNGNf6Oac l/GuZTF1wFGKiYYHKSFAkjIo1b6sCYS1sVmFhhhahKQssRjRT90ITWUk6vvK3RsPGs+M1RuR mV+hO/VvFAAAAABJRU5ErkJggg== X-Accept-Language: fr, es, en User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux) Original-Xref: usenet.stanford.edu gnu.emacs.help:196000 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:88316 Archived-At: Xue Fuqiao writes: > I have a question about the #N# read syntax and #N= syntax. Look at this code: > > (progn > (setq x '#1=(a #1#)) > (eq x (cdr x))) > > Why does this code return nil? Isn't the second element the list itself? No. Try it. (use M-x ielm). *** Welcome to IELM *** Type (describe-mode) for help. ELISP> (setq x '#1=(a #1#)) #1=(a #1#) ELISP> x #1=(a #1#) ELISP> (cdr x) #1=((a . #1#)) ELISP> (eq x (second x)) t ELISP> (eq x (cadr x)) t ELISP> (eq x (cdr x)) nil ELISP> If you want (eq x (cdr x)), then write: (setq x '#1=(a . #1#)) instead of: (setq x '#1=(a #1#)) ELISP> (setq x '#1=(a . #1#)) #1=(a . #1#) ELISP> (eq x (cdr x)) t ELISP> Not all circular structures are circular lists. Even circular lists may have a stem: ELISP> (let ((circle '#1=(a . #1#))) (setq x (list* 'a 'a 'a circle))) (a a a . #1=(a . #1#)) ELISP> x (a a a . #1=(a . #1#)) ELISP> (eq (car x) (cadr x)) t ELISP> (eq (cadr x) (caddr x)) t ELISP> (eq (caddr x) (cadddr x)) t and so on ad libitum, but: ELISP> (eq x (cdr x)) nil ELISP> -- __Pascal Bourguignon__ http://www.informatimago.com/ A bad day in () is better than a good day in {}.