From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Alan Mackenzie Newsgroups: gmane.emacs.devel Subject: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp? Date: Mon, 28 Aug 2023 19:37:57 +0000 Message-ID: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="36953"; mail-complaints-to="usenet@ciao.gmane.io" To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Mon Aug 28 21:39:23 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 1qai55-0009OJ-Ea for ged-emacs-devel@m.gmane-mx.org; Mon, 28 Aug 2023 21:39:23 +0200 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qai44-0005ex-3B; Mon, 28 Aug 2023 15:38:20 -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 1qai42-0005eo-0f for emacs-devel@gnu.org; Mon, 28 Aug 2023 15:38:18 -0400 Original-Received: from mail.muc.de ([193.149.48.3]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qai3z-0001bY-1b for emacs-devel@gnu.org; Mon, 28 Aug 2023 15:38:17 -0400 Original-Received: (qmail 55482 invoked by uid 3782); 28 Aug 2023 21:37:59 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=muc.de; i=@muc.de; q=dns/txt; s=default; t=1693251478; h=date : to : subject : message-id : mime-version : content-type : from : from; bh=4dCBAwKTBBL7WzOmoGl3r4muy04iM4+rIuqzfbFgav4=; b=VE3WPngaoYU+utWB80DCO8G75Aei+RlNVuLNqxSTPk+gK8dvbZERFgah+s1zLmA99pYbw rg9Wu6VrhBjTMgattiIW55xH7pY2TFOgw94w995oyN9CTG21g30+wPVuuEwRgTbYUkWX5OB qApfFzXJwV4edxPAAhClDW9VY4/dlcEG7bur5fMZ+BYOGWN/IGjvAJDwG1HXVmMM9hJnqQ7 ocnhlKumRiUbDjX+pL9wd/iC8GQ0er9FYpFAEysEKqB9O4xQWjUzyNK7xC/p4CsrZP1MChI +8+b9xynYnmbDMD9k1IvKYiL0kV74ZhTwkN1QDD8ao8J2dPMq3zSRHKCs84w== Original-Received: from acm.muc.de (pd953a405.dip0.t-ipconnect.de [217.83.164.5]) (using STARTTLS) by colin.muc.de (tmda-ofmipd) with ESMTP; Mon, 28 Aug 2023 21:37:57 +0200 Original-Received: (qmail 4810 invoked by uid 1000); 28 Aug 2023 19:37:57 -0000 Content-Disposition: inline X-Submission-Agent: TMDA/1.3.x (Ph3nix) X-Primary-Address: acm@muc.de Received-SPF: pass client-ip=193.149.48.3; envelope-from=acm@muc.de; helo=mail.muc.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: 2.1.29 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:309449 Archived-At: Hello, Emacs. In C, we have the very useful conditional compilation directives introduced by #if or #ifdef, etc., which end at #end. In Emacs Lisp we have no such construct. This is a Bad Thing. More and more, especially recently, irritating warning messages are occurring for, for example, obsolete variables and functions inside conditionals which ensure they aren't used. For example: (when (< emacs-major-version 24) (defadvice .....)) produces the warning about defadvice being obsolete. (I haven't actually tested this example). What we really want here is for the defadvice only to be _compiled_ when (< emacs-major-version 24), rather than compiled unconditionally and not run. I propose a new function, hash-if, which would do what we want. The above example could then be written something like: (hash-if (< emacs-major-version 24) (defadvice .....) (advice-add .....)) .. This is not actually all that difficult to write. My first attempt uses a compiler-macro, and looks like this: (defun hash-if (condition if-part &rest else-part) "A compiler macro analogous to C's #if. CONDITION is evaluated at compile time. If it is non-nil, IF-PART gets compiled. Otherwise ELSE-PART (enclosed in a `progn') gets compiled." (declare (indent 2)) (error "hash-if has been called directly")) (put 'hash-if 'compiler-macro (lambda (form condition if-part &rest else-part) (if (eval condition lexical-binding) if-part (cons 'progn else-part)))) .. I propose adding it to subr.el, just before (defmacro when ....). What do people think about this? -- Alan Mackenzie (Nuremberg, Germany).