From: Stefan Monnier <monnier@iro.umontreal.ca>
To: daanturo <daanturo@gmail.com>
Cc: emacs-devel@gnu.org
Subject: Re: A function to take the regexp-matched subsring directly
Date: Sun, 30 Oct 2022 18:01:43 -0400 [thread overview]
Message-ID: <jwvleox2d1i.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <47fff48c-90d4-7c6b-7b92-8a99d9453f3f@gmail.com> (daanturo@gmail.com's message of "Mon, 31 Oct 2022 00:16:03 +0700")
>> `save-match-data` is costly and extremely rarely needed.
>
> I committed a change that now makes inhibit-modify optional (though `(declare
> (pure t) (side-effect-free t))` is lost in the process).
Optional makes no sense: those who need the match data to be saved can
use `save-match-data` around the call just as easily as passing an
optional argument.
> Although I think intuitively, when running those kind of functions, we
> naturally expect them not to cause any side-effects from a high-level
> perspective so `save-match-data` should be the default.
That's not how it works: your intuition should say "oh, it uses
a regexp, so it most assuredly messes with the match data". Only very
few primitive operations like `car/cdr` preserve the match data.
Everything else should be presumed to mess with the match data.
`save-match-data` should almost never be used at the top-level of
a function.
It should only be used in cases such as:
...
(string-match ..)
...
(save-match-data
...do something that may mess with the match data...)
...
(match-string ..)
I tried to explain that in the docstring as follow:
NOTE: The convention in Elisp is that any function, except for a few
exceptions like car/assoc/+/goto-char, can clobber the match data,
so `save-match-data' should normally be used to save *your* match data
rather than your caller's match data."
-- Stefan
next prev parent reply other threads:[~2022-10-30 22:01 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-30 15:17 A function to take the regexp-matched subsring directly daanturo
2022-10-30 15:45 ` Philip Kaludercic
2022-10-30 16:46 ` daanturo
2022-10-30 17:26 ` Philip Kaludercic
2022-10-30 15:52 ` Stefan Monnier
2022-10-30 17:16 ` daanturo
2022-10-30 22:01 ` Stefan Monnier [this message]
2022-10-31 3:47 ` daanturo
2022-10-30 17:29 ` Philip Kaludercic
2022-10-30 22:07 ` Stefan Monnier
2022-10-31 8:56 ` Mattias Engdegård
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=jwvleox2d1i.fsf-monnier+emacs@gnu.org \
--to=monnier@iro.umontreal.ca \
--cc=daanturo@gmail.com \
--cc=emacs-devel@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.