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Re: Is it possible for a macro to expand to nothing?
From: |
Alan Mackenzie |
Subject: |
Re: Is it possible for a macro to expand to nothing? |
Date: |
Tue, 24 Nov 2009 10:45:32 +0000 (UTC) |
User-agent: |
tin/1.6.2-20030910 ("Pabbay") (UNIX) (FreeBSD/4.11-RELEASE (i386)) |
Pascal J. Bourguignon <pjb@informatimago.com> wrote:
> 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.
I'm no lisp guru, but I must disagree strongly with this. What is code,
what is data? Surely they are essentiallty the same, particularly in
lisp. You would hold that a macro which generates a font-lock-defaults
structure (so as to reduce the tedium of doing it by hand) is an abuse of
the macro idea, would you?
> If you are generating general data, then using functions will be easier
> and clearer.
If it's possible. But if this were the case, using functions to generate
"code" would be easier and clearer, too.
> But cc-langs.el only defines four macros and they all generate
> perfectly good lisp code.
Any macro, once debugged, generates "perfectly good" lisp code. I don't
understand where this notion of "perfectly good" comes from.
>> 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 can't disagree with that, sadly. However I think Emacs's code base is
better than a typical 25 yo application still under development (if there
is such a beast).
> 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.
Perhaps.
>>> 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.
Sorry, Pascal, you're just restating the same thing again, not answering
my question. Why should I accept this "general contract of a macro"? I
haven't signed it. ;-) Is there some respected Lisp guru who says this?
What would this guru say about the macro which generates a
font-lock-defaults structure?
> In all fairness, ifdef does return valid forms, when provided valid
> forms as argument.
> (defmacro ifdef (expr &rest body)
> (and (eval expr) `(progn ,@body)))
That version of ifdef is ugly because it contains an obtrusive `progn'.
The version I used doesn't. There is no guarantee that a lisp compiler,
particularly the Emacs lisp byte compiler, is going to optimise away this
unnecessary artifact. It seems this `progn' is there purely to satisfy
the (as yet unsubstantiated) injunction to return only "perfectly good"
lisp forms.
> 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.
Yes.
--
Alan Mackenzie (Nuremberg, Germany).
- Re: Is it possible for a macro to expand to nothing?, (continued)
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?,
Alan Mackenzie <=
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Jeff Clough, 2009/11/25
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Lennart Borgman, 2009/11/26
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Lennart Borgman, 2009/11/26
- Re: Is it possible for a macro to expand to nothing?, Kevin Rodgers, 2009/11/27