From: David Kastrup <dak@gnu.org>
Cc: emacs-devel@gnu.org
Subject: Re: Macro vs Function
Date: Wed, 12 Apr 2006 13:19:17 +0200 [thread overview]
Message-ID: <85d5fn3utm.fsf@lola.goethe.zz> (raw)
In-Reply-To: <BAY112-F30837797609033FA038DA3DAC20@phx.gbl> (Herbert Euler's message of "Wed, 12 Apr 2006 18:39:19 +0800")
"Herbert Euler" <herberteuler@hotmail.com> writes:
>>From: David Kastrup <dak@gnu.org>
>>To: "Herbert Euler" <herberteuler@hotmail.com>
>>CC: emacs-devel@gnu.org
>>Subject: Re: Macro vs Function
>>Date: Wed, 12 Apr 2006 11:53:25 +0200
>>
>>(defmacro xxx (a) a)
>>(defun yyy (a) a)
>>
>>(xxx (+ 3 4)) => 7
>>(yyy '(+ 3 4)) => (+ 3 4)
>
> I'm confused now. Take a look at this:
>
> (defun f ()
> nil)
>
> (defmacro m (a)
> (if a
> t
> nil))
>
> (m (f)) => t
>
> Why does (f) in this example not get evaluated?
Uh, why should it get evaluated? Macro arguments are _never_
evaluated. They are passed quoted into the macro, the macro gets
executed, and the return value of the macro gets evaluated.
In the above, m is called with '(f) as its argument. (if '(a)
... obviously evaluates to t which the macro returns. Then t gets
evaluated, but since it is a self-quoting form, nothing else happens
with it.
Here is a more complex example:
(defmacro m (a)
(append a '(2 4)))
(m (+)) => 6
And here is what happens in compilation:
(defun xxx () (m (or)))
M-x disassemble RET xxx RET
byte code for xxx:
args: nil
0 constant 2
1 return
--
David Kastrup, Kriemhildstr. 15, 44793 Bochum
next prev parent reply other threads:[~2006-04-12 11:19 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-12 8:22 Macro vs Function Herbert Euler
2006-04-12 9:07 ` David Kastrup
2006-04-12 9:17 ` Herbert Euler
2006-04-12 9:24 ` David Kastrup
2006-04-12 9:39 ` Herbert Euler
2006-04-12 9:53 ` David Kastrup
2006-04-12 10:39 ` Herbert Euler
2006-04-12 11:14 ` Andreas Schwab
2006-04-12 11:56 ` Herbert Euler
2006-04-12 12:08 ` Herbert Euler
2006-04-12 12:19 ` David Kastrup
2006-04-12 13:37 ` Herbert Euler
2006-04-12 13:52 ` David Kastrup
2006-04-12 15:13 ` Herbert Euler
2006-04-12 15:26 ` Stuart D. Herring
2006-04-12 15:54 ` Herbert Euler
2006-04-12 15:29 ` David Kastrup
2006-04-12 15:56 ` Herbert Euler
2006-04-12 16:14 ` Herbert Euler
2006-04-12 16:59 ` David Kastrup
2006-04-13 2:17 ` Herbert Euler
2006-04-12 12:12 ` David Kastrup
2006-04-12 11:19 ` David Kastrup [this message]
-- strict thread matches above, loose matches on Subject: below --
2006-04-12 12:05 Herbert Euler
2006-04-12 12:13 ` David Kastrup
2006-04-12 12:17 ` Herbert Euler
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