Re: Make peg.el a built-in library?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Bumping this up in case it slid off radars: I'd like to at least push
the documentation patch to peg.el...

On 09/08/21 21:36 PM, Eric Abrahamsen wrote:
> On 08/26/21 08:34 AM, Eric Abrahamsen wrote:
>> Eli Zaretskii <eliz@gnu.org> writes:
>>
>>>> From: Eric Abrahamsen <eric@ericabrahamsen.net>
>>>> Date: Wed, 25 Aug 2021 11:52:00 -0700
>>>> Cc: Stefan Monnier <monnier@iro.umontreal.ca>
>>>> 
>>>> In my on-again-off-again quest to not have to write text parsers myself,
>>>> I was pointed towards the PEG library (in ELPA), which does pretty much
>>>> exactly what I want (Parsing Expression Grammars).
>>>> 
>>>> Would the maintainers consider moving this into Emacs proper? I ask
>>>> mostly because this would be very useful to have in Gnus, both to
>>>> replace the home-made parser in gnus-search.el, and I would hope to
>>>> parse eg IMAP server responses more fully and reliably.
>>>
>>> Fine with me, but please update the (outdated) Wiki page to say where
>>> the latest peg.el is, when it is imported.
>>
>> Will do. Stefan also asked me to make sure the library actually does
>> what I expect it to do, before making this move, so I'll write the code
>> first.
>
> Okay, I wrote some code: the "use-peg-in-gnus-search.diff" attachment is
> the result of that. It works really well! A net removal of ~100 LOC
> (obviously we're still in deficit with the addition of peg.el), it
> already fixes some wrong behavior of the old parser, and it's much
> easier to reason about and add new behavior to. It's the shiny
> declarative future I was looking forward to.
>
> Whether or not PEG gets added to core I'd like to propose some patches.
> The "peg-doc-patches.diff" attachment adds some documentation to the
> Commentary section, including an example grammar based on a
> much-simplified version of what gnus-search does.
>
> The peg-allow-symbols patch is more tentative. The issue is that _all_
> of the entry-points to peg code are macros, meaning you can't build your
> grammar up in a variable, and then pass that variable to any of
> `peg-run', `peg-parse', `with-peg-rules', etc. Nobody will evaluate the
> variable; you have to literally write the rules inside the
> `with-peg-rules' form. It seems like a fairly plausible use-case to
> store the rules in a variable or an option, even if you're not doing
> run-time manipulation of them. The only solution, as Adam found with
> org-ql, is to `eval' one of the macros.
>
> This doesn't seem necessary! The patch has `with-peg-rules' check if the
> rules are a symbol, and take the `symbol-value' if so. But I wonder if
> it wouldn't be nicer to break some of the code out: `peg-normalize'
> seems to be the entry-point for "compile this grammar", and that could
> be modified to work the way that some languages provide for pre-compiled
> regexps: a way to let the developer build and compile the grammar at
> load-time or launch-time, then feed the stored compiled version to
> parsing routines.
>
> `peg-parse' could be a function, or maybe it also could also just check
> if its argument is a symbol.
>
> I hope someone will have some thoughts on this!
>
> Eric