From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: =?utf-8?Q?=C3=93scar_Fuentes?= Newsgroups: gmane.emacs.help Subject: Re: (*) -> 1 Date: Wed, 18 Jan 2023 15:37:26 +0100 Message-ID: <874jsnaoyh.fsf@telefonica.net> References: <87sfg9kuya.fsf@web.de> <87bkmxkpzg.fsf@web.de> <878ri1av5j.fsf@telefonica.net> <874jspaso5.fsf@telefonica.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="36957"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:M4PZPpiuHWfJu6+J5mWWQhhsVsA= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Wed Jan 18 15:38:21 2023 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 1pI9a1-0009PU-4z for geh-help-gnu-emacs@m.gmane-mx.org; Wed, 18 Jan 2023 15:38:21 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pI9ZV-0007g3-Bt; Wed, 18 Jan 2023 09:37:49 -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 1pI9ZK-0007fk-WF for help-gnu-emacs@gnu.org; Wed, 18 Jan 2023 09:37:40 -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 1pI9ZJ-00035J-2W for help-gnu-emacs@gnu.org; Wed, 18 Jan 2023 09:37:38 -0500 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1pI9ZH-0008Km-5Q for help-gnu-emacs@gnu.org; Wed, 18 Jan 2023 15:37:35 +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: -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 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:142362 Archived-At: Jean Louis writes: > * Óscar Fuentes [2023-01-17 22:06]: >> I think your confusion comes from an assumption that everybody else on >> this conversation is blind to, in the sense that nobody (I didn't read >> most of the thread, though) didn't explicitly stated it: >> >> + in Elisp is not the "plus" operation that we all know (the same C uses >> and school children use.) + in Elisp is not the binary operation, it is >> the summatory operator, which takes a list of arguments and returns the >> sum of them all. >> >> In that sense, maybe you can see more naturally that "the sum of nothing >> is zero." This makes possible to apply the + (summatory!) operator to >> any list of numbers, including the empty list. > > Do you mean with "C uses" that "C language uses"? Yes. Please keep in mind that * in Elisp is not the same as * in C. Elisp provides a variadic function, while C provides a binary operator. > We speak of absence of any elements. One less important question is > why author of Lisp decided to yield 1 for (*), more important > questions is of the use of it. > > Practically I found one use during sketching stage of programs, just > to write it there as preliminary placeholders, like: > > (* (*) (*)) then in few minutes it is then expanded to: > > (* (* specific-gravity cubic-meter) (* )) > > then to be expanded to: > > (* (* specific-gravity cubic-meter) (* excavator-cycles hour)) > > Apart from few references on Internet we talked about, I cannot find > uses for it, or purpose, it seem difficult. So let's borrow an example from a previous post from Michael. Let's suppose you want to write a function that takes a list with the factors that represent a sequence of annual interest rates (so 1.05 is 5%, etc.) You want that function to return the equivalent rate for the period comprising those years: R1 * R2 * R3... A possible implementation would be: (defun equiv-rate (yearly-rates) (apply '* yearly-rates)) You use that value to calculate how much money you get from a deposit after those years. (defun money-after-years (initial-deposit yearly-rates) (* initial-deposit (equiv-rate yearly-rates))) But what happens if yearly-rates contains less than 2 elements? Let's see: If it contains just one element (one year) the result is the rate of that year. If it contains zero elements (zero years) the result shall be 1, otherwise money-after-years would be wrong: if you make a deposit, it must be the same after 0 years! So you adapt equiv-rate: (defun equiv-rate (yearly-rates) (case (length yearly-rates) (0 1) (1 (car yearly-rates)) (otherwise (apply '* (yearly-rates))))) But as it happens in Elisp, you can use the first, simple version of equiv-rate just fine, because the * variadic function already does the right thing for you.