From: Adam Spiers <orgmode@adamspiers.org>
To: Carsten Dominik <dominik@science.uva.nl>
Cc: org-mode mailing list <emacs-orgmode@gnu.org>
Subject: Re: proposal: defconst/defcustom org-tags-regexp
Date: Sat, 1 Sep 2007 12:01:23 +0100 [thread overview]
Message-ID: <20070901110123.GA32102@atlantic.linksys.moosehall> (raw)
In-Reply-To: <931245f7aaa24436377e537552f1beab@science.uva.nl>
Carsten Dominik (dominik@science.uva.nl) wrote:
> On Jul 20, 2007, at 17:05, Adam Spiers wrote:
> >On Wed, Jul 18, 2007 at 11:24:40PM +0200, Carsten Dominik wrote:
> >>On Jul 16, 2007, at 15:21, Adam Spiers wrote:
> >>>There seem to be a number of hardcoded regexps currently used for
> >>>matching heading tags, all very similar looking, and typically
> >>>something like:
> >>>
> >>> [ \t]*\\(:[[:alnum:]_@:]+:\\)?[ \t]*\\($\\|\r\\)
> >>>
> >>>Is there any reason why these shouldn't be factored out into a new
> >>>defcustom org-tags-regexp?
> >>
> >>Well, one reason is efficiency. When a regular expression is a constant,
> >>Emacs is able to cache the compiled version of the regular expression,
> >>and this can speed up code that does a lot of matching quite a bit.
> >>The token you show above is usually part of a larger string, so the full
> >>regular expression would have to be make with concat and will therefore
> >>be recompiled all the time.
> >
> >Right, point taken - like m//o in Perl. To be honest, it doesn't
> >matter too much to me if it's defconst rather than defcustom. The
> >main thing is that I can have tags starting with '<' :-)
>
> Hi Adam,
>
> this is not about defcustom or defconst, but about the question
> of the regexp is built each time with concat, or not.
Ah, I was assuming that the elisp interpreter was intelligent enough
that if you did a concat of two or more constants, it would only build
the regexp the first time, similar to m//o in Perl. Is that not the
case? Or maybe it only performs this optimisation if you
byte-compile? I found this in the elisp manual:
-- Special Form: eval-when-compile body...
This form marks BODY to be evaluated at compile time but not when
the compiled program is loaded. The result of evaluation by the
compiler becomes a constant which appears in the compiled program.
If you load the source file, rather than compiling it, BODY is
evaluated normally.
If you have a constant that needs some calculation to produce,
`eval-when-compile' can do that at compile-time. For example,
(defvar my-regexp
(eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))
Maybe I should practice what I preach and use mercurial to start an
experimental branch to look at the impact on performance of doing this
refactoring :-)
next prev parent reply other threads:[~2007-09-01 11:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-07-16 13:21 proposal: defconst/defcustom org-tags-regexp Adam Spiers
2007-07-18 21:24 ` Carsten Dominik
2007-07-19 13:08 ` Eddward DeVilla
2007-07-19 15:08 ` Carsten Dominik
2007-07-19 16:14 ` Eddward DeVilla
2007-07-20 15:05 ` Adam Spiers
[not found] ` <931245f7aaa24436377e537552f1beab@science.uva.nl>
2007-09-01 11:01 ` Adam Spiers [this message]
2007-09-02 7:30 ` Carsten Dominik
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