From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: pjb@informatimago.com (Pascal J. Bourguignon) Newsgroups: gmane.emacs.help Subject: Re: Is it possible for a macro to expand to nothing? Date: Tue, 24 Nov 2009 10:42:50 +0100 Organization: Informatimago Message-ID: <87pr78b6n9.fsf@galatea.local> References: <87vdh1ccra.fsf@galatea.local> <87my2dc8d7.fsf@galatea.local> <873a44dcf2.fsf@galatea.local> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1259059424 6979 80.91.229.12 (24 Nov 2009 10:43:44 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 24 Nov 2009 10:43:44 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Tue Nov 24 11:43:37 2009 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.50) id 1NCsrw-0003hx-7a for geh-help-gnu-emacs@m.gmane.org; Tue, 24 Nov 2009 11:43:36 +0100 Original-Received: from localhost ([127.0.0.1]:37555 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NCsrv-0005mv-GN for geh-help-gnu-emacs@m.gmane.org; Tue, 24 Nov 2009 05:43:35 -0500 Original-Path: news.stanford.edu!usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 73 Original-X-Trace: individual.net xoRDgiXJl7a44RzHUOJ07wyQBe6kjHmYLKekYh4Y1wrOsc6ae7 Cancel-Lock: sha1:NjIzNGNmNTQxZDI5NTFhZTkyMTc2ZDYyNDVhZDM1NzZmZmQ0OWQ2Zg== sha1:nQ/to8Mw+PtCrKAe5HYS+Kc/f/o= Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwAQMAAABtzGvEAAAABlBMVEUAAAD///+l2Z/dAAAA oElEQVR4nK3OsRHCMAwF0O8YQufUNIQRGIAja9CxSA55AxZgFO4coMgYrEDDQZWPIlNAjwq9 033pbOBPtbXuB6PKNBn5gZkhGa86Z4x2wE67O+06WxGD/HCOGR0deY3f9Ijwwt7rNGNf6Oac l/GuZTF1wFGKiYYHKSFAkjIo1b6sCYS1sVmFhhhahKQssRjRT90ITWUk6vvK3RsPGs+M1RuR mV+hO/VvFAAAAABJRU5ErkJggg== X-Accept-Language: fr, es, en X-Disabled: X-No-Archive: no User-Agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.3 (darwin) Original-Xref: news.stanford.edu gnu.emacs.help:175010 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:70081 Archived-At: Alan Mackenzie writes: > Pascal J. Bourguignon wrote: >> "Drew Adams" writes: > >> This is the problem! Macros shouldn't return a _list_, they should >> return a _form_. If you write a macro that returns a list, or you use >> it so that it returns a list, that is not a valid form, then it is not >> good style, even if you catch up. > > Is that right? I think you should be required to justify this assertion > of "good style". If that "good style" really is good style, then the > whole of cc-langs.el (which uses intense macro magic to generate data > structures with both compile-time and run-time behaviour) is attrocious > style. If that was the case, yes, I would think so. Macros are designed to generate code, not other data. If you are generating general data, then using functions will be easier and clearer. But cc-langs.el only defines four macros and they all generate perfectly good lisp code. > Fact is, though, it allows a simple tabular writing of constants > which vary between C, C++, java, .... Kudos to Martin Stjernholm, who > wrote it. Unfortunately, most of emacs lisp code is bad code. Functions one kilometer long, chained with one or more others one kilometer long. Copy-and-pasted chunks instead of abstracting it away. Etc. I cannot say that I've read a significant percentage of it, but the code I had to peek at in my 20 years of use of emacs, I cannot say I was impressionned by its good quality... Now of course, I had a peek at code that had bugs or missing features in the first place. Perhaps the good quality emacs lisp code I hadn't a peek at because it worked well enough so I didn't need to. >> Because it is a better style. It avoids abusing the ifdef macro. > > Where does this notion of "abuse" come from? What is its rationale? > (This is a genuine question.) The general contract of a macro is that it returns valid forms. In all fairness, ifdef does return valid forms, when provided valid forms as argument. (defmacro ifdef (expr &rest body) (and (eval expr) `(progn ,@body))) (ifdef t (setq bar 2)) --> 2 It's only when provided invalid argu-ment that it returns an invalid form: (ifdef t ((setq bar 2))) --> Debugger entered--Lisp error: (invalid-function (setq bar 2)) The fact that such a macro call embedded in another form building form that processes it properly doesn't mean that it is not bad style: it has to do something special to the result of ifdef to make it work. If you extract that ifdef call to run it at the repl, it just cannot work. -- __Pascal Bourguignon__