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From: Jesper Harder <harder@ifa.au.dk>
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Subject: Re: how to turn this Gnus function into a macro?
Date: Thu, 08 Jan 2004 05:13:37 +0100
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Ted Zlatanov <tzz@lifelogs.com> writes:

> On Wed, 07 Jan 2004, harder@ifa.au.dk wrote:
>
>> Why do you want it as a macro?  If it's just to save a function
>> call, then `inline' or `defsubst' is better.
>
> Because it can be much simpler, in the form
>
> ;; field is 'from or 'message-id for instance
> (eval (format "mail-header-%s" (symbol-name field)))
>
> Only a few lines, and it could be used with anything as the field,
> instead of the hard-coded field names I have now.

Yeah, I see what you mean.  

`mail-header-*' should really be defsubsts rather than macros, but
unfortunately the Emacs byte compiler isn't good enough to make them
as efficient (it should be possible for a sufficiently smart
compiler).

> It also seemed like an interesting challenge, but I failed it pretty
> badly :)

You can sometimes cheat and funcall a macro by wrapping a lambda
around it:

       (funcall (lambda (x) (macro x)) y)

But if you think about when macro expansion happens, it's obvious that
it can only work in a few cases.