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: Tue, 24 Nov 2009 20:17:37 +0100 [thread overview]
Message-ID: <873a43bulq.fsf@galatea.local> (raw)
In-Reply-To: heh27o$oht$1@colin2.muc.de
Alan Mackenzie <acm@muc.de> writes:
> Pascal J. Bourguignon <pjb@informatimago.com> wrote:
>> Alan Mackenzie <acm@muc.de> writes:
>>> What would this guru say about the macro which generates a
>>> font-lock-defaults structure?
>
>> If this structure cannot be passed to eval without signaling an error,
>> then I would say that it is bad style to use a macro to generate such
>> data.
>
> OK. So people who might have benefitted from a clever macro now have to
> do things more laboriously and more erroneously.
That's where you're wrong, using functions to generate data is
simplier and less erroneous than using macros to do the same.
> Another example you
> would call bad would be `define-minor-mode' which, in addition to
> creating a top level command also creates a keymap, a syntax table, and
> so on. Would you propose eliminating `define-minor-mode' from Emacs?
define-minor-mode generates a valid lisp form. You can execute a
define-minor-mode macro call without getting an error.
>>>> (defmacro ifdef (expr &rest body)
>>>> (and (eval expr) `(progn ,@body)))
>
>>> That version of ifdef is ugly because it contains an obtrusive
>>> `progn'.
>
>> In a way here, progn is the dual of list.
>
>> When you want to generate a list of data, you use list:
>
>> `(list ,expr1 ,expr2 ... ,exprN)
>
>> When you want to generate a list of code, you use progn:
>
>> `(progn ,form1 ,form2 ... ,formN)
>
>
>> It is not obtrusive in any way, it only shows that we are generating
>> code, not mere data.
>
> It makes the generated code more difficult to read, because it has no
> function, unless it has some specific function.
Why do you use variable names from a natural language dictionnary?
Names have no function, you should use gensyms...
>>> It seems this `progn' is there purely to satisfy
>>> the (as yet unsubstantiated) injunction to return only "perfectly good"
>>> lisp forms.
>
>> Not 'purely'. You could optimize it out when unnecessary with:
>
>> (defun progn-encapsulate (forms)
>> (if (= 1 (length forms))
>> (first forms)
>> (list* 'progn forms)))
>
> , or I could just have the macro return (form1 form2 .... formn), ready
> to be spliced into the caller.
No, because such a macro you couldn't use it at the REPL or in any
other function.
--
__Pascal Bourguignon__
next prev parent reply other threads:[~2009-11-24 19:17 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 [this message]
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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