From: pjb@informatimago.com (Pascal J. Bourguignon)
To: help-gnu-emacs@gnu.org
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Tue, 24 Nov 2009 10:42:50 +0100 [thread overview]
Message-ID: <87pr78b6n9.fsf@galatea.local> (raw)
In-Reply-To: hefaut$1gui$1@colin2.muc.de
Alan Mackenzie <acm@muc.de> writes:
> 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.
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__
next prev parent reply other threads:[~2009-11-24 9:42 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 [this message]
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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