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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 <address@hidden> wrote:
> Alan Mackenzie <address@hidden> writes:

>> Pascal J. Bourguignon <address@hidden> wrote:
>>> "Drew Adams" <address@hidden> 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.


>>> 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.


Alan Mackenzie (Nuremberg, Germany).

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