From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Andreas Schwab Newsgroups: gmane.emacs.devel Subject: Re: inverse of add-to-list: remove-from-list Date: Tue, 13 Oct 2020 22:01:18 +0200 Message-ID: <871ri1x5dd.fsf@igel.home> References: <20201013193400.uqm2ng6rgfnsei2w@E15-2016.optimum.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="29682"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) Cc: Emacs-Devel List To: Boruch Baum Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Tue Oct 13 22:02:17 2020 Return-path: Envelope-to: ged-emacs-devel@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 1kSQUy-0007cg-MP for ged-emacs-devel@m.gmane-mx.org; Tue, 13 Oct 2020 22:02:16 +0200 Original-Received: from localhost ([::1]:50786 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kSQUx-0002gT-OJ for ged-emacs-devel@m.gmane-mx.org; Tue, 13 Oct 2020 16:02:15 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:47688) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kSQU8-000241-Kp for emacs-devel@gnu.org; Tue, 13 Oct 2020 16:01:24 -0400 Original-Received: from mail-out.m-online.net ([212.18.0.9]:50683) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kSQU6-0004lD-5Y for emacs-devel@gnu.org; Tue, 13 Oct 2020 16:01:24 -0400 Original-Received: from frontend01.mail.m-online.net (unknown [192.168.8.182]) by mail-out.m-online.net (Postfix) with ESMTP id 4C9mdW6PSjz1qs03; Tue, 13 Oct 2020 22:01:19 +0200 (CEST) Original-Received: from localhost (dynscan1.mnet-online.de [192.168.6.70]) by mail.m-online.net (Postfix) with ESMTP id 4C9mdW6CLLz1qrFC; Tue, 13 Oct 2020 22:01:19 +0200 (CEST) X-Virus-Scanned: amavisd-new at mnet-online.de Original-Received: from mail.mnet-online.de ([192.168.8.182]) by localhost (dynscan1.mail.m-online.net [192.168.6.70]) (amavisd-new, port 10024) with ESMTP id eNYgH0dVY9Ay; Tue, 13 Oct 2020 22:01:19 +0200 (CEST) X-Auth-Info: A6rF1Rf/pSvBFx0CgXT2TCeV1CFkizTtNLjpinVSVV/DkFdC+mnnF8iWK0fTKOwK Original-Received: from igel.home (ppp-46-244-162-164.dynamic.mnet-online.de [46.244.162.164]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.mnet-online.de (Postfix) with ESMTPSA; Tue, 13 Oct 2020 22:01:18 +0200 (CEST) Original-Received: by igel.home (Postfix, from userid 1000) id 78F2F2C28BD; Tue, 13 Oct 2020 22:01:18 +0200 (CEST) X-Yow: I feel better about world problems now! In-Reply-To: <20201013193400.uqm2ng6rgfnsei2w@E15-2016.optimum.net> (Boruch Baum's message of "Tue, 13 Oct 2020 15:34:00 -0400") Received-SPF: pass client-ip=212.18.0.9; envelope-from=whitebox@nefkom.net; helo=mail-out.m-online.net X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/13 16:01:20 X-ACL-Warn: Detected OS = Linux 3.11 and newer [fuzzy] X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.25, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.io gmane.emacs.devel:257574 Archived-At: On Okt 13 2020, Boruch Baum wrote: >> On Okt 13 2020, Uwe Brauer wrote: >> > But there is, as far as I know, no inverse functionality > > 1] Similar to what Andreas suggested, try this: > > (let ((aa '(1 2 3 1 4 1 2))) > (delete 2 aa) > aa) > > And note that the fine distinction made in the docstring for the > `delete' function: > > "If SEQ is not a list, deletion is never performed destructively; > instead this function creates and returns a new vector or string. > > Write ‘(setq foo (delete element foo))’ to be sure of correctly > changing the value of a sequence ‘foo’. > > So, in the case of SEQ being a list, there is no need to `setq foo'. This is not true. If the element to delete is the first element of the list, it returns the cdr of the list. Andreas. -- Andreas Schwab, schwab@linux-m68k.org GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1 "And now for something completely different."