From: pjb@informatimago.com (Pascal J. Bourguignon)
To: help-gnu-emacs@gnu.org
Subject: Re: Is it possible for a macro to expand to nothing?
Date: Sun, 29 Nov 2009 01:54:26 +0100 [thread overview]
Message-ID: <87ocmm40cd.fsf@galatea.local> (raw)
In-Reply-To: mailman.11699.1259413441.2239.help-gnu-emacs@gnu.org
Thierry Volpiatto <thierry.volpiatto@gmail.com> writes:
> pjb@informatimago.com (Pascal J. Bourguignon) writes:
>
>> Alan Mackenzie <acm@muc.de> writes:
>>
>>> Pascal J. Bourguignon <pjb@informatimago.com> wrote:
>>>> Tim X <timx@nospam.dev.null> writes:
>>>
>>>>> On the other hand, Alan's arguments also have merit. If a macro can be
>>>>> useful in generating something other than a form that can be
>>>>> evaluated, such as a data structure and can do so in a way that is
>>>>> cleaner/simpler or just easier to understand than doing the same using
>>>>> functions, then it would make sense. His examples from the C modes
>>>>> seem to be a case in point.
>>>
>>>
>>>> Perhaps Alan's problem with functions comes from the confusion between
>>>> backquote and macros. Since backquote (and , and ,@) are often used in
>>>> macros, some people believe they can be used only in macros, and that
>>>> they ARE what macros are.
>>>
>>> Er, do I actually have a problem with functions? But no, I don't suffer
>>> that particular confusion between backquotes and macros, and have indeed
>>> used backquote when there hasn't been a macro within zeptoparsecs.
>>>
>>>> Far from it! I don't know any language more orthogonal than lisp.
>>>
>>> Maybe not, but even lisp only gets to about 89 degrees. It is missing an
>>> operator which does the same as ,@ outside backquote. This is one of the
>>> lacks which makes it so difficult to write an equivalent of C's #if.
>>
>> Yes, emacs lisp is missing reader macros. In Common Lisp there's #+,
>> #- and #., and reader macros may return 0 object.
>
> A little note about #., it have emacs equivalent `eval-when-compile':
But #. is not eval when compile, it's eval when reading!
Compilation occurs much later after reading...
'#.(+ 1 2) --> 3
The result 3 is computed when reading the quote form.
What is read is (quote 3):
(read-from-string "'#.(+ 1 2)") --> (quote 3) ; 10
> I don't know the #- and #+ macros, what are they for?
#+KEY THING
is equivalent to cpp:
#ifdef KEY
THING
#endif
only it's not textual. #- <=> #ifndef
What is tested is the presence of the keyword :KEY in the *FEATURES* list.
(when (zerop (random 2)) (push :test *features*))
'(#+test present #-test absent) --> (present) or (absent)
--
__Pascal Bourguignon__
next prev parent reply other threads:[~2009-11-29 0:54 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
[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 [this message]
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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