Re: proposal: defconst/defcustom org-tags-regexp

[Adam Spiers <orgmode@adamspiers.org>]

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 :-)