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From: Ihor Radchenko <yantar92@posteo.net>
To: Eric Abrahamsen <eric@ericabrahamsen.net>
Cc: emacs-devel@gnu.org,  Stefan Monnier <monnier@iro.umontreal.ca>
Subject: Re: Make peg.el a built-in library?
Date: Tue, 08 Nov 2022 08:47:44 +0000	[thread overview]
Message-ID: <87edud25ov.fsf@localhost> (raw)
In-Reply-To: <87tu3asg2r.fsf@ericabrahamsen.net>

Eric Abrahamsen <eric@ericabrahamsen.net> writes:

>> Is there any progress merging peg.el to Emacs?
>> I do not see any obvious blockers in the discussion, but the merge never
>> happened?
>
> I will say that I tried to use PEG to resolve some gruesome text-parsing
> issues in EBDB very recently, and failed to make it work in the hour or
> two I'd allotted to the problem. The file-comment docs are pretty good,
> but I think they would need to be expanded in a few crucial ways,
> particularly to help those who don't necessarily know how PEGs work.
>
> Specifically, it is not obvious (to me) the ways in which PEGs (or maybe
> just peg.el) are not fully declarative. It doesn't backtrack, and I
> suspect it won't ever backtrack or isn't even supposed to, which means
> users should be made explicitly aware of the ways in which their rules
> can fail, and the ways in which declaration order matter. The comment
> for the `or' construct reads:
>
> Prioritized Choice
>
> And that's about the only hint you get.

As the comment in peg.el states, the definitions are adapted from the
original PEG paper. There is even a link to paper and also to
presentation explaining how peg works. I strongly advice you to read
that. Prioritized Choice is explained there.

> I was trying to parse a
> multiword name like
>
> Eric Edwin Abrahamsen
>
> into the structure
>
> (("Eric" "Edwin") "Abrahamsen")
>
> using rules like
>
> (plain-name (substring (+ [word])) (* [space]))
> (full-name (list (+ plain-name) plain-name)
>     `(names -- (list (butlast names) (car (last names)))))
>
> Which always fails to match because (+ plain-name) is greedy and eats up
> all the words. It doesn't ever try leaving out the last word in an
> attempt to make the rule match.

One way is

(with-peg-rules
    ((name (substring (+ [word])) (* [blank]))
     (given-name name (not (eol)))
     (last-name name (and (eol)))
     (full-name (list (+ given-name) last-name) `(names -- (list (butlast names) (car (last names))))))
  (peg-run (peg full-name)))

A simple-minded non-greedy version would be ambiguous. You must
necessarily indicate end of input.

A more appropriate non-ambiguous non-greedy statement would involve or
(which you admittedly did not understand):

(with-peg-rules
    ((name (substring (+ [word])) (* [blank]))
     (given-name name)
     (last-name name (and (eol)))
     (full-name (list (+ (or last-name given-name)) (and (eol))) `(names -- (list (butlast names) (car (last names))))))
     ;;;;;;;;;;;;;;;;;;;;;^^
  (peg-run (peg full-name)))

> I'm happy to write the docs (should it have its own info manual
> section?), if we really think there are no other necessary
> fixes/improvements.

I find PEG to be a nice addition when regexps do not cut the necessary
parsing, while using Bovine or tree-sitter is an overkill.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>



  parent reply	other threads:[~2022-11-08  8:47 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
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 [this message]
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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