From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via "Emacs development discussions." Newsgroups: gmane.emacs.devel Subject: Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? Date: Sat, 02 Sep 2023 19:12:19 -0400 Message-ID: References: <87zg2bosf3.fsf@dataswamp.org> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="23120"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) To: emacs-devel@gnu.org Cancel-Lock: sha1:xNzvHfPDwCUjvEWs3iq7zupeAc4= Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Sun Sep 03 01:15:44 2023 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 1qcZqC-0005qZ-2A for ged-emacs-devel@m.gmane-mx.org; Sun, 03 Sep 2023 01:15:44 +0200 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qcZpd-0003MA-ER; Sat, 02 Sep 2023 19:15:09 -0400 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 1qcZpc-0003KU-AW for emacs-devel@gnu.org; Sat, 02 Sep 2023 19:15:08 -0400 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 1qcZpZ-0007ap-5I for emacs-devel@gnu.org; Sat, 02 Sep 2023 19:15:07 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1qcZpW-0004xu-I5 for emacs-devel@gnu.org; Sun, 03 Sep 2023 01:15:02 +0200 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=ged-emacs-devel@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: emacs-devel@gnu.org X-Mailman-Version: 2.1.29 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-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.devel:309920 Archived-At: >> > In C, we have the very useful conditional compilation >> > directives introduced by #if or #ifdef, etc., which end at >> > #end. >> >> Those are directives to the C preprocessor and some people say >> it is a sign of weakness such a thing is even necessary to >> begin with. >> >> Let's see if they are right! >> >> What problem do you have? If we can solve it in Elisp, we >> cannot say those people saying that are wrong. >> > > FWIW, Common Lisp has reader macros to solve the problem: > http://clhs.lisp.se/Body/24_aba.htm I don't see why reader macros would be needed here. Reader macros are useful to introduce funny new shorthands or to avoid *reading* a chunk of code (e.g. because it uses some unsupported funny syntax) but in the present case we can read both branches of the `if` just fine, we just want to throw away one of the two before we macro-expand it. Stefan