From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Marcin Borkowski Newsgroups: gmane.emacs.help Subject: Re: Closures in Emacs and their usage scenarios. Date: Sun, 10 Oct 2021 20:25:15 +0200 Message-ID: <87mtngu48k.fsf@mbork.pl> References: <87r1ct3qzf.fsf@zoho.eu> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="5099"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: mu4e 1.1.0; emacs 28.0.50 Cc: help-gnu-emacs@gnu.org To: Emanuel Berg Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Sun Oct 10 20:26:16 2021 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 1mZdWa-00018o-8q for geh-help-gnu-emacs@m.gmane-mx.org; Sun, 10 Oct 2021 20:26:16 +0200 Original-Received: from localhost ([::1]:39484 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mZdWY-0003XI-M3 for geh-help-gnu-emacs@m.gmane-mx.org; Sun, 10 Oct 2021 14:26:14 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:55616) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mZdW0-0003XA-IL for help-gnu-emacs@gnu.org; Sun, 10 Oct 2021 14:25:40 -0400 Original-Received: from mail.mojserwer.eu ([195.110.48.8]:52652) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mZdVw-0005jX-Dw for help-gnu-emacs@gnu.org; Sun, 10 Oct 2021 14:25:40 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by mail.mojserwer.eu (Postfix) with ESMTP id 17A3EE67FE; Sun, 10 Oct 2021 20:25:24 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at mail.mojserwer.eu Original-Received: from mail.mojserwer.eu ([127.0.0.1]) by localhost (mail.mojserwer.eu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id QmpygAU3Yhct; Sun, 10 Oct 2021 20:25:16 +0200 (CEST) Original-Received: from localhost (178235147138.dynamic-3-poz-k-0-1-0.vectranet.pl [178.235.147.138]) by mail.mojserwer.eu (Postfix) with ESMTPSA id 8FE8DE6047; Sun, 10 Oct 2021 20:25:16 +0200 (CEST) In-reply-to: <87r1ct3qzf.fsf@zoho.eu> Received-SPF: pass client-ip=195.110.48.8; envelope-from=mbork@mbork.pl; helo=mail.mojserwer.eu X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, 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: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.23 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:133687 Archived-At: On 2021-10-10, at 16:16, Emanuel Berg via Users list for the GNU Emacs text editor wrote: > Hongyi Zhao wrote: > >> I'm learning "Advising Emacs Lisp Functions" now. >> According to my current superficial understanding, it seems >> that both closure and advice function are intended to >> provide a clean and concise method to patch/repair/adapt the >> existing function/macros with a most consistent way. > > That should be one of many use cases for advising functions, > I don't know how one does that with closure tho ... > > I've still only seen two use cases for closures, one is the > persistent variable (in C you'd use a static variable, in > Python just a global one) and the other one is the sharing of > one "almost global" variable between two or more functions (in > both C and Python, that would be a real global variable > instead). > > And the second use case is a version of the first, or > extension perhaps, since that variable (or set of variables) > would also be persistent. It looks a lot like OOP to me - > I say it in that order because I learned the OOP basics/theory > before I heard of closures, but I expect closures were > actually first, right? - and it is even the very core of OOP > (the coupling/enclosure of data and functions/methods that > operate that data) - so we can say not without reason that > Lisp is the original OOP - with the core stuff implemented in > such as simple way - but without all the other stuff that no > one uses anyway :) Yet another use (which of course - technically - is again a variant of the same thing) is generating a closure whose behavior depends on the argument of the function that defines it. A simple example from my book: (defun negate (fun) "Return a function returning the logical opposite of FUN." (lambda (&rest args) (not (apply fun args)))) so that (negate #'zerop) behaves like a function testing its argument for "non-zeroness" (i.e., returning t unless its argument is 0, when it returns nil). As a homework, try to use it under dynamic binding and see why it won't work. (See also this thread: https://lists.gnu.org/archive/html/help-gnu-emacs/2021-09/msg00267.html .) Best, -- Marcin Borkowski http://mbork.pl