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: `append' vs. `nconc' Date: Thu, 31 Dec 2015 06:36:08 +0100 Organization: Informatimago Message-ID: <87vb7fch87.fsf@kuiper.lan.informatimago.com> References: <568164D8.6050700@ojkastl.de> <87io3iyr7t.fsf@debian.uxu> <87si2kezg1.fsf@mithlond.arda> <87a8osrlj4.fsf_-_@debian.uxu> <87lh8bgafa.fsf@mithlond.arda> <877fjve0x5.fsf@kuiper.lan.informatimago.com> 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 1451540428 18786 80.91.229.3 (31 Dec 2015 05:40:28 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 31 Dec 2015 05:40:28 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Thu Dec 31 06:40:22 2015 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 1aEVyI-0007ZZ-1v for geh-help-gnu-emacs@m.gmane.org; Thu, 31 Dec 2015 06:40:22 +0100 Original-Received: from localhost ([::1]:54727 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aEVyH-0000hx-B3 for geh-help-gnu-emacs@m.gmane.org; Thu, 31 Dec 2015 00:40:21 -0500 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 62 Original-X-Trace: individual.net byrDZIMKINFYiSMh5wV6wA6AaBqHZjkjszwkfpliNqwIRTPlGv Cancel-Lock: sha1:MjVlZmE4NjViNWU3YWViNGI5MTJiY2E5YzU4Y2ZjMThmMzk0ODc5OA== sha1:g42sV+OKEVcqonGphcM+Z8SD9y8= 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:216254 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:108544 Archived-At: Emanuel Berg writes: > "Pascal J. Bourguignon" > writes: > >>> OK, so for example this: (setq >>> completion-ignored-extensions (append >>> completion-ignored-extensions '(".bcf" ".run.xml")) >>> ) Is better as: (nconc completion-ignored-extensions >>> (list ".bcf" ".run.xml")) ? >> >> No. You should never use nconc only for effect (just >> like delete, etc). > > What do you mean by "effect"? Side effect, same thing. (nconc nil '(1 2 3)) has no side effect! > The reason I thought it fitting was the definitions of > `append' and `ncons'. Here I want to change > `completion-ignored-extensions'. `append' doesn't > change it, which is why the assignment with `setq' was > there. If it can't be removed (well, in the form of > `setf'), there is still the gain of not creating the > list unnecessarily, which in this case is a matter of > principle but in other cases (with long lists) is > a performance bottleneck? > >> Always re-assign the modified list: >> >> (setf completion-ignored-extensions (nconc >> completion-ignored-extensions (list ".bcf" >> ".run.xml"))) >> >> otherwise it will break when the place is bound to >> nil. (nil is a symbol and cannot be mutated into >> a cons cell!). > > ? > > What place is bound to nil, when? Any place you want, when it is. (let ((mylist '())) (nconc mylist (list 1 2 3)) mylist) --> nil (setf completion-ignored-extensions '()) (nconc completion-ignored-extensions (list ".o")) completion-ignored-extensions --> nil -- __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