From: "Drew Adams" <drew.adams@oracle.com>
To: "'Pascal J. Bourguignon'" <pjb@informatimago.com>,
<help-gnu-emacs@gnu.org>
Subject: RE: Is it possible for a macro to expand to nothing?
Date: Mon, 23 Nov 2009 14:09:11 -0800 [thread overview]
Message-ID: <24AB7C55C06E45238BC3FC5FFD983E97@us.oracle.com> (raw)
In-Reply-To: <87my2dc8d7.fsf@galatea.local>
> > Sorry, but it works just fine. I do this all the time.
>
> No, it doesn't:
> (defmacro ifdef (expr &rest body)
> (and (eval expr) `(progn ,@body)))
> (ifdef t ((setq bar 2)))
> Debugger entered--Lisp error: (invalid-function (setq bar 2))
You seem to be complaining about yourself. I never suggested using such a macro.
All I suggested was to use backquote with ,@ to splice in a list with the
element that Alan conditionally wants in the macroexpansion. That's the way to
add something or nothing to a list: put the something or nothing in a list, then
splice in that list.
The list to be built here is a defun expression. It is then evaluated without
problem.
> I said that it was a bad idea to take the habit of giving
> an invalid form to a macro to get an invalid form from it.
Sorry, dunno what you're saying. There wasn't anything invalid in what I
suggested.
As far as I'm concerned, all that's really involved is writing a macro that
expands to a list that contains or doesn't contain the element in question. Any
such list is a valid _list_.
When the list resulting from macroexpansion is then EVALUATED, yes, of course
its car must be a defined function, macro, `lambda', etc. (or the list must be
nil). That is an entirely different matter. That is a consideration for _any_
macro one writes.
In Alan's case, the resulting list has `defun' as its car. It is a valid defun
expression whose evaluation defines a function.
The only thing relevant here, AFAICT, is how to create the list Alan wants: a
list that conditionally contains some element. Backquote plus ,@ is the answer.
> Notice also my alternative solution uses macroexpand. This is a clue
> that if you want to go that way, you should use a function rather than
> a macro:
> (defun %parenthesized-ifdef (expr forms)
> (if expr
> '()
> `((progn ,@forms))))
> (defmacro titi (fn)
> `(defun ,fn ()
> (setq bar 1)
> ,@(%parenthesized-ifdef baz '((setq bar 2)))))
>
> (macroexpand '(titi foo))
> --> (defun foo nil (setq bar 1) (progn (setq bar 2)))
Dunno why you do all that. Just `,@forms is as useful here as `((progn ,@forms))
- a defun body is an implicit progn. (And there is no need to quote nil.)
Again, all of that code boils down to just this (which is what I wrote earlier):
(defmacro titi (fn)
`(defun ,fn ()
(setq bar 1)
,@(and baz '((setq bar 2)))))
I'm sure you know what you're talking about, and I know what I'm talking about
;-). The only real question is whether either of us has actually helped Alan at
all.
next prev parent reply other threads:[~2009-11-23 22:09 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-23 14:56 Is it possible for a macro to expand to nothing? Alan Mackenzie
2009-11-23 16:03 ` Drew Adams
[not found] ` <mailman.11344.1258992201.2239.help-gnu-emacs@gnu.org>
2009-11-23 16:31 ` Alan Mackenzie
2009-11-23 17:29 ` Drew Adams
2009-11-23 18:33 ` Pascal J. Bourguignon
2009-11-23 18:51 ` Drew Adams
[not found] ` <mailman.11354.1259004470.2239.help-gnu-emacs@gnu.org>
2009-11-23 20:08 ` Pascal J. Bourguignon
2009-11-23 20:24 ` Alan Mackenzie
2009-11-23 22:09 ` Drew Adams [this message]
[not found] ` <mailman.11367.1259014174.2239.help-gnu-emacs@gnu.org>
2009-11-23 23:55 ` Pascal J. Bourguignon
2009-11-24 0:55 ` Alan Mackenzie
2009-11-24 9:42 ` Pascal J. Bourguignon
2009-11-24 10:45 ` Alan Mackenzie
2009-11-24 11:14 ` Pascal J. Bourguignon
2009-11-24 16:39 ` Alan Mackenzie
2009-11-24 19:17 ` Pascal J. Bourguignon
2009-11-25 14:13 ` Jeff Clough
[not found] ` <mailman.11467.1259158369.2239.help-gnu-emacs@gnu.org>
2009-11-26 6:53 ` Alan Mackenzie
2009-11-26 11:11 ` Pascal J. Bourguignon
2009-11-26 11:52 ` Lennart Borgman
[not found] ` <mailman.11564.1259236392.2239.help-gnu-emacs@gnu.org>
2009-11-26 12:16 ` Pascal J. Bourguignon
2009-11-26 12:43 ` Lennart Borgman
2009-11-27 8:32 ` Kevin Rodgers
[not found] ` <mailman.11626.1259310779.2239.help-gnu-emacs@gnu.org>
2009-11-27 13:15 ` Alan Mackenzie
2009-11-27 13:52 ` Pascal J. Bourguignon
2009-11-27 16:57 ` Alan Mackenzie
2009-11-27 17:09 ` Pascal J. Bourguignon
2009-11-27 17:19 ` Helmut Eller
2009-11-27 17:45 ` Pascal J. Bourguignon
2009-11-27 23:17 ` Tim X
2009-11-28 0:06 ` Pascal J. Bourguignon
2009-11-28 8:29 ` Alan Mackenzie
2009-11-28 10:25 ` Pascal J. Bourguignon
2009-11-28 12:57 ` Thierry Volpiatto
[not found] ` <mailman.11699.1259413441.2239.help-gnu-emacs@gnu.org>
2009-11-29 0:54 ` Pascal J. Bourguignon
2009-11-24 11:56 ` Pascal J. Bourguignon
[not found] ` <mailman.11352.1258997403.2239.help-gnu-emacs@gnu.org>
2009-11-23 18:42 ` Pascal J. Bourguignon
2009-11-23 20:12 ` Drew Adams
[not found] ` <mailman.11356.1259007263.2239.help-gnu-emacs@gnu.org>
2009-11-23 20:21 ` Pascal J. Bourguignon
2009-11-23 22:09 ` Drew Adams
[not found] ` <mailman.11368.1259014177.2239.help-gnu-emacs@gnu.org>
2009-11-24 0:03 ` Pascal J. Bourguignon
2009-11-23 20:09 ` Alan Mackenzie
2009-11-23 16:49 ` Jeff Clough
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