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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 00:55:57 +0000 (UTC)
User-agent: tin/1.6.2-20030910 ("Pabbay") (UNIX) (FreeBSD/4.11-RELEASE (i386))

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.  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).



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