Re: Major modes using `widen' is a good, even essential, programming practice.

[Dmitry Gutov]

On 09.08.2022 14:30, Eli Zaretskii wrote:
> > Date: Tue, 9 Aug 2022 00:16:23 +0300
> > Cc: acm@muc.de, gregory@heytings.org, emacs-devel@gnu.org
> > From: Dmitry Gutov <dgutov@yandex.ru>
> >
> > On 08.08.2022 14:30, Eli Zaretskii wrote:
> > > So if you dislike the current solution of locked narrowing, how about
> > > making syntax-ppss work in chunks (perhaps from an idle timer?), after
> > > initially scanning only the first small portion of the file.  The goal
> > > is to have the file displayed quickly enough, and thereafter complete
> > > the scan when possible.
> >
> > The file is already displayed "quickly enough".
>
> What do you mean by "quickly enough"?  With this recipe:
>
>    emacs -Q
>    M-: (setq long-line-threshold nil) RET
>    M-: (setq syntax-wholeline-max most-positive-fixnum) RET
>
> visiting dictionary.json, a 19MB single-line file, takes "forever" (I
> killed it after 20 minutes) before it shows anything in the window.
> And since both variables use "arbitrary restrictions", and both can
> cause inaccurate/incorrect/wrong/buggy/<your euphemism here>
> fontifications, my proposal above was to do something smarter.

I never recommended you to change any of those vars.

Doing that brings back pathological slowdowns that don't have anything 
to do with the speed of parse-partial-sexp.

> > What's going to happen then, if the timer hasn't fired yet?
>
> We should process a relatively small portion of the buffer around the
> new position of point.

To speed up the jump to a far distant part of the buffer after doing an 
edit "here", the timer would have to parse the whole buffer between here 
and there. Or most of it.

> Not surprisingly, this is precisely how jit-lock is supposed to work,
> if only the stuff called through fontification-functions obeyed the
> region which it was told to process.

If you concerned with the speed of font-lock itself (and not with the 
speed of syntax-ppss cache maintenance which we've talked about before), 
and in your case it might be justified, given the unoptimized build, 
then using something like stealth fontifications could indeed speed up 
C-v/M-v. Not M->, though.

> > And for the timer's work to be useful, it has to had happened
> > between the last edit and the subsequent navigation. A lot of idle
> > timers like that = a lot of discarded work.
>
> Not if the user will subsequently visit the place where the "discarded
> work" was invested.
>
> Full disclosure: I'm a long-time and very happy user of
> jit-lock stealth fontifications.