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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: Mon, 23 Nov 2009 20:09:34 +0000 (UTC)
User-agent: tin/1.6.2-20030910 ("Pabbay") (UNIX) (FreeBSD/4.11-RELEASE (i386))

Hi, Drew!

Drew Adams <address@hidden> wrote:
>> I think I want to be able to do this:

>>     (defun foo ()
>>       (setq bar 1)
>>       (ifdef baz (setq bar 2)))

>> , and if baz is nil at compile time, this function should be 
>> identical to

>>     (defun foo ()
>>       (setq bar 1))

> (defmacro titi (fn)
>  `(defun ,fn ()
>     (setq bar 1)
>     ,@(ifdef baz '((setq bar 2))))))

> Assuming that ifdef returns nil if baz is nil, that should give you
> (defun foo () (setq bar 1)). If baz is not nil, it should give you
> this:

> (defun foo ()
> (setq bar 1)
> (setq bar 2))

> Or something like that.

Wow, you're a genius, Drew!  It's a little bit ugly having to write a
defmacro around a defun, but only a very very little.

The way you've described actually looks fairly obviousi now, but these
things always do after somebody bright has just shown you them.

Thanks again!

-- 
Alan Mackenzie (Nuremberg, Germany).



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