From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Emanuel Berg Newsgroups: gmane.emacs.help Subject: Re: [External] : Re: Setting up abbrev Date: Mon, 01 Aug 2022 20:36:48 +0200 Message-ID: <87mtcnbykf.fsf@dataswamp.org> References: <87k07sfywz.fsf@dataswamp.org> <87tu6wchw5.fsf@dataswamp.org> <87zggoatct.fsf@dataswamp.org> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="12262"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:2E9PsLcXiv+z5dPFAc06r2RzZMM= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Mon Aug 01 20:40:06 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 1oIaKj-00030r-0v for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 01 Aug 2022 20:40:05 +0200 Original-Received: from localhost ([::1]:59064 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1oIaKh-0002ZS-Jk for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 01 Aug 2022 14:40:03 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:51956) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oIaHk-0002ST-I5 for help-gnu-emacs@gnu.org; Mon, 01 Aug 2022 14:37:04 -0400 Original-Received: from ciao.gmane.io ([116.202.254.214]:34654) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oIaHi-0003jc-BX for help-gnu-emacs@gnu.org; Mon, 01 Aug 2022 14:37:00 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1oIaHf-0009mP-Hu for help-gnu-emacs@gnu.org; Mon, 01 Aug 2022 20:36:55 +0200 X-Injected-Via-Gmane: http://gmane.org/ Mail-Followup-To: help-gnu-emacs@gnu.org Mail-Copies-To: never 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: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.249, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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" Xref: news.gmane.io gmane.emacs.help:138730 Archived-At: Drew Adams wrote: >> Okay, but I still don't understand the benefit of using it >> in practice compared to the list (a b)? > > Do you mean use the _notation_? I guess you instead mean use > a dotted list: a list whose last cdr is a non-nil atom. > > If so, the answer is that in general you do want to use > a true/proper list (last cdr is nil), and NOT a dotted list. > For many reasons, not least of which is using a function > that needs to traverse all list elements (e.g. `length', > mapping/sequence functions). > > You can use a dotted list in these cases: > > 1. You want to save conses (not create so many), in > a context where you're not going to be needing to use the > cons as a list (mapping etc.), or a context where you > know you'll only ever want an atom cdr. > > 2. You want/need, in effect, to have a backwards key-value > pair, (list-value . atom-key). You have a list to use for > most purposes (ignoring the last cdr), and you have an > atom (the cdr) for a few rare purposes, as a kind of > label for the list. > > #2 is usually the result of trying to adapt to legacy code > that expects to treat just a list (and doesn't need to > follow it to its end). IOW, #2 is typically an ugly hack. > For new code you'd instead just put the atom first: > (atom-key . list-value). Why? What data can (a . b) hold that (list a b) cannot? -- underground experts united https://dataswamp.org/~incal