From: Alan Mackenzie <acm@muc.de>
To: help-gnu-emacs@gnu.org
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Tue, 24 Nov 2009 00:55:57 +0000 (UTC) [thread overview]
Message-ID: <hefaut$1gui$1@colin2.muc.de> (raw)
In-Reply-To: 873a44dcf2.fsf@galatea.local
Pascal J. Bourguignon <pjb@informatimago.com> wrote:
> "Drew Adams" <drew.adams@oracle.com> 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. Fact is, though, it allows a simple tabular writing of constants
which vary between C, C++, java, .... Kudos to Martin Stjernholm, who
wrote it.
>> When the list resulting from macroexpansion is then EVALUATED, yes, of
>> course its car must be a defined function, macro, `lambda', etc. (or
>> the list must be nil). That is an entirely different matter. That is a
>> consideration for _any_ macro one writes.
>> In Alan's case, the resulting list has `defun' as its car. It is a
>> valid defun expression whose evaluation defines a function.
> I don't mind titi, I object to your use of ifdef in titi. That ifdef
> call returns a list that is not a valid form. This is not good style.
I was more than happy with it. The form was valid, for a reasonable
value of "form", and worked.
>> The only thing relevant here, AFAICT, is how to create the list Alan
>> wants: a list that conditionally contains some element. Backquote plus
>> ,@ is the answer.
> Yes, since you only want to create a list at that point, do not use a
> macro, use a function.
Sounds like a fair alternative. Though it's too late at night for my
head to work round whether or not a mere function could do the job in the
general case.
[ .... ]
>> Dunno why you do all that.
> 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.)
>> I'm sure you know what you're talking about, and I know what I'm
>> talking about ;-). The only real question is whether either of us
>> has actually helped Alan at all.
> Well, he has a lot of solutions to choose from now. Indeed, his
> situation may not be better :-)
Yes, I feel I have been helped. Thank you, both, and good night!
--
Alan Mackenzie (Nuremberg, Germany).
next prev parent reply other threads:[~2009-11-24 0:55 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-23 14:56 Is it possible for a macro to expand to nothing? Alan Mackenzie
2009-11-23 16:03 ` Drew Adams
[not found] ` <mailman.11344.1258992201.2239.help-gnu-emacs@gnu.org>
2009-11-23 16:31 ` Alan Mackenzie
2009-11-23 17:29 ` Drew Adams
2009-11-23 18:33 ` Pascal J. Bourguignon
2009-11-23 18:51 ` Drew Adams
[not found] ` <mailman.11354.1259004470.2239.help-gnu-emacs@gnu.org>
2009-11-23 20:08 ` Pascal J. Bourguignon
2009-11-23 20:24 ` Alan Mackenzie
2009-11-23 22:09 ` Drew Adams
[not found] ` <mailman.11367.1259014174.2239.help-gnu-emacs@gnu.org>
2009-11-23 23:55 ` Pascal J. Bourguignon
2009-11-24 0:55 ` Alan Mackenzie [this message]
2009-11-24 9:42 ` Pascal J. Bourguignon
2009-11-24 10:45 ` Alan Mackenzie
2009-11-24 11:14 ` Pascal J. Bourguignon
2009-11-24 16:39 ` Alan Mackenzie
2009-11-24 19:17 ` Pascal J. Bourguignon
2009-11-25 14:13 ` Jeff Clough
[not found] ` <mailman.11467.1259158369.2239.help-gnu-emacs@gnu.org>
2009-11-26 6:53 ` Alan Mackenzie
2009-11-26 11:11 ` Pascal J. Bourguignon
2009-11-26 11:52 ` Lennart Borgman
[not found] ` <mailman.11564.1259236392.2239.help-gnu-emacs@gnu.org>
2009-11-26 12:16 ` Pascal J. Bourguignon
2009-11-26 12:43 ` Lennart Borgman
2009-11-27 8:32 ` Kevin Rodgers
[not found] ` <mailman.11626.1259310779.2239.help-gnu-emacs@gnu.org>
2009-11-27 13:15 ` Alan Mackenzie
2009-11-27 13:52 ` Pascal J. Bourguignon
2009-11-27 16:57 ` Alan Mackenzie
2009-11-27 17:09 ` Pascal J. Bourguignon
2009-11-27 17:19 ` Helmut Eller
2009-11-27 17:45 ` Pascal J. Bourguignon
2009-11-27 23:17 ` Tim X
2009-11-28 0:06 ` Pascal J. Bourguignon
2009-11-28 8:29 ` Alan Mackenzie
2009-11-28 10:25 ` Pascal J. Bourguignon
2009-11-28 12:57 ` Thierry Volpiatto
[not found] ` <mailman.11699.1259413441.2239.help-gnu-emacs@gnu.org>
2009-11-29 0:54 ` Pascal J. Bourguignon
2009-11-24 11:56 ` Pascal J. Bourguignon
[not found] ` <mailman.11352.1258997403.2239.help-gnu-emacs@gnu.org>
2009-11-23 18:42 ` Pascal J. Bourguignon
2009-11-23 20:12 ` Drew Adams
[not found] ` <mailman.11356.1259007263.2239.help-gnu-emacs@gnu.org>
2009-11-23 20:21 ` Pascal J. Bourguignon
2009-11-23 22:09 ` Drew Adams
[not found] ` <mailman.11368.1259014177.2239.help-gnu-emacs@gnu.org>
2009-11-24 0:03 ` Pascal J. Bourguignon
2009-11-23 20:09 ` Alan Mackenzie
2009-11-23 16:49 ` Jeff Clough
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