From: Jeff Clough <jeff@chaosphere.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Wed, 25 Nov 2009 09:13:16 -0500 (EST) [thread overview]
Message-ID: <20091125.091316.197573325.jeff@chaosphere.com> (raw)
In-Reply-To: <heh27o$oht$1@colin2.muc.de>
From: Alan Mackenzie <acm@muc.de>
Date: Tue, 24 Nov 2009 16:39:20 +0000 (UTC)
>> It comes directly from the definition of defmacro,
>
>> defmacro is a special form in `src/eval.c'.
>> (defmacro name arglist [docstring] [decl] body...)
>
>> Define name as a macro.
>> The actual definition looks like
>> (macro lambda arglist [docstring] [decl] body...).
>> When the macro is called, as in (name ARGS...),
>> the function (lambda arglist body...) is applied to
>> the list ARGS... as it appears in the expression,
>> and the result should be a form to be evaluated instead of the original.
>
> Oh, come on! That last sentence is a tutorial, motivating one, expressed
> in the slightly loose, colloquial language of the hacker. "a form to be
> evaluated" is here shorthand for something like "some sort of atom or
> list structure which fits into the slot where the invocation is housed"
This is incorrect. The term "form" has a very specific definition in
Lisp. It is a sexp that can be evaluated. While I'm new to Lisp, I
have been knee deep in books and documents for the last couple of
weeks and everything (including the GNU Emacs Lisp Manual) presents
macros as a way to generate *forms*, not just sexps.
If you have a macro that expands to (1 2 3), it's not going to break
the world, but that macro isn't doing what every hacker is going to
expect. I'd call that a style problem.
If, for some reason, you absolutely have to use a macro to generate
something like that above, at least have it expand to (list 1 2 3),
that way the macro can still be evaluated and no one gets shafted.
You get the behavior you want and the macro is still expanding to a
valid form.
Jeff
next prev parent reply other threads:[~2009-11-25 14:13 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
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 [this message]
[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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