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: Why doesn't nconc change my variable? Date: Sun, 05 Oct 2014 03:59:39 +0200 Organization: Informatimago Message-ID: <87zjdbmg38.fsf@kuiper.lan.informatimago.com> References: NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Trace: ger.gmane.org 1412474725 6905 80.91.229.3 (5 Oct 2014 02:05:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 5 Oct 2014 02:05:25 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Sun Oct 05 04:05:21 2014 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 1XabCK-00030Q-Fn for geh-help-gnu-emacs@m.gmane.org; Sun, 05 Oct 2014 04:05:20 +0200 Original-Received: from localhost ([::1]:45860 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XabCK-0000OP-2p for geh-help-gnu-emacs@m.gmane.org; Sat, 04 Oct 2014 22:05:20 -0400 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 39 Original-X-Trace: individual.net F0ozDjGpdhCi1e0oXNtt7w9hdFAajfEvDbPBuHBTZKMQ+BAclc Cancel-Lock: sha1:N2UwZjIzZjllYjQ2NDg0NDVlODZjMTVkMTQ2NjU2YTljYzk1NDViMQ== sha1:ffzTgIwhRz8PDq5PqT5h6ZpU/m8= 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/24.3 (gnu/linux) Original-Xref: usenet.stanford.edu gnu.emacs.help:208002 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:100278 Archived-At: Marcin Borkowski 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