From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Tomas Hlavaty Newsgroups: gmane.emacs.devel Subject: RE: lengths and stuff Date: Sun, 27 Dec 2020 21:52:36 +0100 Message-ID: <87sg7rq8dn.fsf@logand.com> References: <87o8ivumn5.fsf@telefonica.net>> <86wnx8otoj.fsf@163.com>> <834kkbp9vr.fsf@gnu.org>> <87czyxuxw6.fsf@db48x.net>> <87y2hlt82w.fsf@db48x.net>> <87lfdlvsw4.fsf@logand.com>> <83h7o8ncly.fsf@gnu.org>> <87pn2wudab.fsf@db48x.net>> <87mty0c3m1.fsf@gnus.org>> <83czywnb86.fsf@gnu.org>> <87im8ob707.fsf@gnus.org>> <87eejcb6nx.fsf@gnus.org>> > <875z4ob5c9.fsf@gnus.org>> > <83mtxzly4f.fsf@gnu.org>> <87czyvsv3h.fsf_-_@db48x.net> <87v9cnqgjr.fsf@logand.com> <40d9718a-adcc-4ac8-a7dd-8b66270f40d3@default> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="18491"; mail-complaints-to="usenet@ciao.gmane.io" To: Drew Adams , emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Sun Dec 27 21:53:25 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 1ktd2a-0004he-KV for ged-emacs-devel@m.gmane-mx.org; Sun, 27 Dec 2020 21:53:24 +0100 Original-Received: from localhost ([::1]:58780 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ktd2Z-0001gb-Jd for ged-emacs-devel@m.gmane-mx.org; Sun, 27 Dec 2020 15:53:23 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:49634) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ktd1u-0001Ht-Gk for emacs-devel@gnu.org; Sun, 27 Dec 2020 15:52:42 -0500 Original-Received: from logand.com ([37.48.87.44]:59236) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ktd1s-0002Tb-Lx for emacs-devel@gnu.org; Sun, 27 Dec 2020 15:52:42 -0500 Original-Received: by logand.com (Postfix, from userid 1001) id 9590619F65A; Sun, 27 Dec 2020 21:52:38 +0100 (CET) X-Mailer: emacs 26.3 (via feedmail 11-beta-1 I) In-Reply-To: <40d9718a-adcc-4ac8-a7dd-8b66270f40d3@default> Received-SPF: pass client-ip=37.48.87.44; envelope-from=tom@logand.com; helo=logand.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, SPF_HELO_PASS=-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:261916 Archived-At: On Sun 27 Dec 2020 at 10:52, Drew Adams wrote: >> Using length in a predicate is yet completely different and most >> likely bad because to answer the predicate, traversing the whole list >> is wasted time and energy. > > (lambda (xs) (= (length xs) 25)) ; need to count elts Is that a joke? The predicates were proposed to help programers avoid writing such bad code. And if somebody writes such bad code, it is almost trivial to fix it with search and replace: "(= (length" -> "(length=" "(< (length" -> "(length<" "(> (length" -> "(length>" "(/= (length" -> "(length/=" "(<= (length" -> "(length<=" "(>= (length" -> "(length>=" Also the "symmetry" (or exhaustiveness?) here is to assist in easily fixing bad code with minimum changes. Nothing of course helps to those who are determined to unneccessarily count _all_ the elements of lists.