From: Adam Porter <adam@alphapapa.net>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: emacs-devel <emacs-devel@gnu.org>
Subject: Re: Make peg.el a built-in library?
Date: Sat, 2 Oct 2021 02:32:22 -0500 [thread overview]
Message-ID: <CAO_b3FXRX-7h7yAE84z9rq-CCXYKmr8hk67on50F8wHcDd-zgg@mail.gmail.com> (raw)
In-Reply-To: <jwvee94d4if.fsf-monnier+emacs@gnu.org>
On Fri, Oct 1, 2021 at 1:05 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>
> > In org-ql, the PEX is redefined at load time and/or run time, being
> > derived from search keywords that are defined by the package and
> > possibly by the user. So the PEX can't be defined in advance, at
> > compile time. So having to use `with-peg-rules' means having to use
> > `eval'.
>
> If the grammar changes radically at run time, based on external/user
> data there's probably no better way than via `eval` or similar (`load`,
> `byte-compile`, you name it).
>
> But if the changes are sufficiently limited (e.g. have an (or "foo"
> "bar" ....) with a variable set of strings that can match), then we can
> do better.
In org-ql's case, it's the latter: the grammar doesn't fundamentally
change, only the list of strings that can be matched in a certain
expression:
https://github.com/alphapapa/org-ql/blob/31aeb0a2505acf8044c07824888ddec7f3e529c1/org-ql.el#L869
> E.g. we could have a PEX of the form (re FORM) where FORM can be any
> ELisp expression that returns a regular expression. Then `org-ql.el`
> could do
>
> (let ((predicate-re (regexp-opt predicate-names)))
> (peg-parse
> ((query (+ term
> (opt (+ (syntax-class whitespace) (any)))))
> [...]
> (predicate (re predicate-re))
> [...])))
That would be helpful, yes.
> PS: BTW, regarding your comment:
>
> ;; Sort the keywords longest-first to work around what seems to be an
> ;; obscure bug in `peg': when one keyword is a substring of another,
> ;; and the shorter one is listed first, the shorter one fails to match.
>
> The behavior you describe indeed seems like a bug, but maybe what you
> see is slightly different (and not a bug): if you have a PEX like
> (and (or "foo" "foobar") "X")
> the "foo" will match when faced with "foobarX" and the parser won't
> backtrack to try and match the "foobar" when the "X" fails to match.
Hmm, thanks. I think an example of the problem is that a predicate in
org-ql might have a shorter alias, e.g. "heading" is has the alias
"h", and predicates are followed by arguments, like "heading:foo", so
IIRC, without sorting them there, "heading:foo" would work, while
"h:foo" wouldn't. (Or maybe a better example is predicates that
optionally accept keyword-style arguments, like
"ts-active:from=2021-10-01", which has the alias "ts-a", and could
also be called without arguments, like "ts-a:".)
> It's one of those differences between BNF and PEG grammars.
> So indeed you do want to sort from longest to shortest to avoid
> this problem.
Thanks, I didn't realize that.
next prev parent reply other threads:[~2021-10-02 7:32 UTC|newest]
Thread overview: 100+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-25 18:52 Make peg.el a built-in library? Eric Abrahamsen
2021-08-26 6:17 ` Eli Zaretskii
2021-08-26 15:34 ` Eric Abrahamsen
2021-09-09 4:36 ` Eric Abrahamsen
2021-09-19 15:25 ` Eric Abrahamsen
2021-09-30 19:44 ` Stefan Monnier
2021-09-30 20:34 ` Adam Porter
2021-10-01 8:14 ` Augusto Stoffel
2021-10-01 18:05 ` Stefan Monnier
2021-10-01 18:40 ` Eric Abrahamsen
2021-10-02 3:57 ` Stefan Monnier
2021-10-02 7:32 ` Adam Porter [this message]
2021-10-02 14:45 ` Stefan Monnier
2021-10-02 15:13 ` Adam Porter
2021-08-26 17:02 ` Adam Porter
2021-08-26 17:25 ` Eric Abrahamsen
2021-08-27 3:17 ` Eric Abrahamsen
2021-08-27 6:41 ` Helmut Eller
2021-08-27 16:57 ` Eric Abrahamsen
2021-09-26 10:59 ` Augusto Stoffel
2021-09-26 15:06 ` Eric Abrahamsen
2021-09-26 18:36 ` Augusto Stoffel
2021-09-27 16:18 ` Eric Abrahamsen
2021-09-27 22:34 ` Richard Stallman
2021-09-28 3:52 ` Eric Abrahamsen
2021-09-28 8:09 ` tomas
2021-09-28 9:32 ` Helmut Eller
2021-09-28 10:45 ` tomas
2021-09-28 15:24 ` Augusto Stoffel
2021-09-30 6:04 ` Richard Stallman
2021-10-01 3:27 ` Eric Abrahamsen
2021-10-09 1:31 ` Michael Heerdegen
2021-10-09 5:28 ` Michael Heerdegen
2021-10-09 8:12 ` Helmut Eller
2021-10-09 12:52 ` Stefan Monnier
2021-10-10 5:49 ` Helmut Eller
2021-10-14 10:25 ` Michael Heerdegen
2021-10-09 12:54 ` Stefan Monnier
2021-10-09 16:47 ` Eric Abrahamsen
2021-10-10 4:20 ` Michael Heerdegen
2021-10-10 21:40 ` Eric Abrahamsen
2021-10-13 2:58 ` Michael Heerdegen
2021-10-09 16:49 ` Eric Abrahamsen
2021-10-10 3:43 ` Stefan Monnier
2021-10-10 4:46 ` Michael Heerdegen
2021-10-10 5:58 ` Helmut Eller
2021-10-10 13:56 ` Stefan Monnier
2021-10-22 16:33 ` Michael Heerdegen
2021-10-31 23:43 ` Michael Heerdegen
2021-11-15 23:16 ` Michael Heerdegen
2022-11-07 3:33 ` Ihor Radchenko
2022-11-07 19:46 ` Eric Abrahamsen
2022-11-08 6:57 ` Helmut Eller
2022-11-08 8:51 ` Ihor Radchenko
2022-11-10 4:04 ` Richard Stallman
2022-11-10 5:25 ` tomas
2022-11-10 8:15 ` Eli Zaretskii
2022-11-10 8:29 ` tomas
2022-11-11 4:36 ` Richard Stallman
2022-11-08 8:47 ` Ihor Radchenko
2022-11-08 16:18 ` Eric Abrahamsen
2022-11-08 19:08 ` tomas
2022-11-08 19:42 ` Eric Abrahamsen
2022-11-16 4:27 ` [PATCH] " Eric Abrahamsen
2022-11-16 5:07 ` tomas
2022-11-16 5:39 ` Eric Abrahamsen
2022-11-16 15:53 ` tomas
2022-11-16 6:24 ` Ihor Radchenko
2022-11-16 18:15 ` Eric Abrahamsen
2022-11-17 12:21 ` Ihor Radchenko
2022-11-27 1:46 ` Eric Abrahamsen
2022-11-27 8:57 ` Eli Zaretskii
2022-11-28 1:09 ` Eric Abrahamsen
2022-11-28 12:16 ` Eli Zaretskii
2023-09-25 1:30 ` Eric Abrahamsen
2023-09-25 2:27 ` Adam Porter
2023-09-25 13:00 ` Alexander Adolf
2024-03-24 14:19 ` Ihor Radchenko
2024-03-24 15:32 ` Eli Zaretskii
2024-03-25 1:45 ` Eric Abrahamsen
2023-01-11 7:39 ` Michael Heerdegen
2023-01-11 8:04 ` Ihor Radchenko
2023-01-11 11:01 ` Michael Heerdegen
2023-01-11 11:32 ` tomas
2023-02-05 12:10 ` Ihor Radchenko
2023-02-05 15:41 ` Eduardo Ochs
2023-02-05 15:45 ` Ihor Radchenko
2023-02-05 16:19 ` Eduardo Ochs
2023-02-05 16:50 ` Ihor Radchenko
2023-02-09 5:44 ` Jean Louis
2023-02-06 0:33 ` Michael Heerdegen
2022-11-08 14:01 ` Stefan Monnier
2022-11-08 14:42 ` tomas
2022-11-08 15:08 ` Visuwesh
2022-11-08 16:29 ` Juanma Barranquero
2022-12-02 20:20 ` Augusto Stoffel
2022-11-08 16:10 ` Eric Abrahamsen
2022-11-08 18:59 ` tomas
2022-11-08 19:42 ` Eric Abrahamsen
2022-11-08 22:03 ` Tim Cross
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