Re: problem with macro definitions

[Michael Heerdegen <michael_heerdegen@web.de>]

hector <hectorlahoz@gmail.com> writes:

> I did it. This is what I meant:
>
> (defmacro idx (list needle)
>   (let (found
> 	 (haystack (eval list))
> 	 (idx 0))
>     (while (not found)
>       (when (eq (car haystack) needle)
> 	  (setq found idx))
>       (setq idx (1+ idx))
>       (setq haystack (cdr haystack)))
>     found))
>
> The macro expansion is just a number.

Yes, that's true now.

> I'm not quite sure what the "(let ((haystack (eval list))))" does but
> it works.

A macro doesn't evaluate its arguments.  Normally, arguments are
evaluated when the expanded code (with argument expressions "pasted in")
is run.  You want to do the whole calculation at compile time, and you
want to allow LIST to be an expression (known at compile time), so you
need to evaluate it explicitly to get a list value.

The same applies to the NEEDLE argument btw.


Michael.