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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 <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. 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).
- 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?, 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 <=
- 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?, 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