[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Is it possible for a macro to expand to nothing?
From: |
Alan Mackenzie |
Subject: |
Re: Is it possible for a macro to expand to nothing? |
Date: |
Mon, 23 Nov 2009 20:24:47 +0000 (UTC) |
User-agent: |
tin/1.6.2-20030910 ("Pabbay") (UNIX) (FreeBSD/4.11-RELEASE (i386)) |
Pascal J. Bourguignon <pjb@informatimago.com> wrote:
> "Drew Adams" <drew.adams@oracle.com> writes:
>>> >> you can do that by producing nil and then using ,@ inside a
>>> >> backquote. IOW, instead of inserting nil, you splice it in,
>>> >> which means inserting nothing.
>>> That wouldn't work, backquote is a quote.
>> Sorry, but it works just fine. I do this all the time.
> No, it doesn't:
It worked fine for me.
> (defmacro ifdef (expr &rest body)
> (and (eval expr) `(progn ,@body)))
My definition of `ifdef' is:
(defmacro ifdef (condition form)
(if (eval condition)
`,form))
, which though lacking an &rest argument, could easily be enhanced to
take one.
[ .... ]
> I didn't say that it didn't work inside titi. 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.
If the form doesn't cause a runtime error, it's not invalid. ;-) The
reason I want this functionality is to optimise a VERY VERY tight loop.
I believe that the optimisation isn't feasible under XEmacs, so I want to
be able to put things for XEmacs inside Lisp equivalents of "#ifdef".
Even inserting an invocation of `(ignore)' seems to make a difference.
As far as I remember, anyway. With Drew's mechanism, one doesn't have to
rely on the byte-compiler optimising an empty `progn' or a frivolous
`progn' away.
> 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)))
Yes, but that `progn' is ugly.
--
Alan Mackenzie (Nuremberg, Germany).
- Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?,
Alan Mackenzie <=
- RE: Is it possible for a macro to expand to nothing?, Drew Adams, 2009/11/23
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/23
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Pascal J. Bourguignon, 2009/11/24
- Re: Is it possible for a macro to expand to nothing?, Jeff Clough, 2009/11/25
- Message not available
- Re: Is it possible for a macro to expand to nothing?, Alan Mackenzie, 2009/11/26