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Re: Is it possible for a macro to expand to nothing?


From: Jeff Clough
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Mon, 23 Nov 2009 11:49:00 -0500 (EST)

From: Alan Mackenzie <address@hidden>
Date: Mon, 23 Nov 2009 14:56:47 +0000 (UTC)

> That is, not to a call to a null defun, but truly to nothing: i.e., the
> ensuing code (as least, when byte compiled) will be identical to what it
> would have been, had the macro invocation been omitted.
> 
> I want something like this:
> 
> (defmacro ifdef (condition &rest forms)
>   "If the compile time CONDITION is non-nil, dump the FORMS to the
> calling defun.  Otherwise do nothing."
>   ......)

I think the short answer to your question is no, you can't do what you
are asking with a macro like this.  You are looking for what you would
get from a C pre-processor #ifdef and that's not what macros in Lisp
do.  Lisp macros do more and different magic than the C pre-processor.

Jeff





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