From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via Users list for the GNU Emacs text editor Newsgroups: gmane.emacs.help Subject: Re: Easy to add with push but not to the end of a list Date: Tue, 06 Dec 2022 08:56:09 -0500 Message-ID: References: <877czcid1d.fsf@web.de> <87a648kzov.fsf@dataswamp.org> <87cz93noqc.fsf@web.de> <87zgc76npf.fsf@dataswamp.org> <87k038npls.fsf@web.de> <87sfht8luq.fsf@dataswamp.org> <87h6y91ffg.fsf@web.de> <878rjlgvdd.fsf@dataswamp.org> <87bkoh1dix.fsf@web.de> <871qpdgsql.fsf@dataswamp.org> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="9840"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:TJBxNeaVIJH+gGizdqVOfQeKLXc= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Tue Dec 06 14:56:55 2022 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1p2YRL-0002MZ-DI for geh-help-gnu-emacs@m.gmane-mx.org; Tue, 06 Dec 2022 14:56:55 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1p2YQm-00071G-29; Tue, 06 Dec 2022 08:56:20 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1p2YQl-000712-2q for help-gnu-emacs@gnu.org; Tue, 06 Dec 2022 08:56:19 -0500 Original-Received: from ciao.gmane.io ([116.202.254.214]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1p2YQj-00012d-IF for help-gnu-emacs@gnu.org; Tue, 06 Dec 2022 08:56:18 -0500 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1p2YQg-0001XI-Ei for help-gnu-emacs@gnu.org; Tue, 06 Dec 2022 14:56:14 +0100 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=geh-help-gnu-emacs@m.gmane-mx.org; helo=ciao.gmane.io X-Spam_score_int: -15 X-Spam_score: -1.6 X-Spam_bar: - X-Spam_report: (-1.6 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.25, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.29 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-mx.org@gnu.org Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.help:141436 Archived-At: >> That supports generalized variables; you don't need to do >> that (unless you want to). > > Just wonder why mine looks so different ... Because when you want to do: (push V (aref A (cl-incf I))) the naive expansion gives: (setf (aref A (cl-incf I)) (cons V (aref A (cl-incf I)))) which will increment I twice :-( So the expansion need to look like: (let ((tmp (cl-incf I))) (setf (aref A tmp) (cons V (aref A tmp)))) And of course, the same holds if A is not just a variable reference but say a call to function that's costly or that is not pure. Furthermore, the above performs the `cl-incf` before computing V, so if V is an expression that uses I it changes the expected result compared to the usual left-to-right evaluation order provided by ELisp. So in the end you need something like: (let ((tmp1 V) (tmp2 A) (tmp3 (cl-incf I))) (setf (aref tmp2 tmp3) (cons tmp1 (aref tmp2 tmp3)))) -- Stefan