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

From: Alan Mackenzie
Subject: Re: Major modes using `widen' is a good, even essential, programming practice.
Date: Mon, 8 Aug 2022 10:38:36 +0000

Hello, Stefan.

On Sun, Aug 07, 2022 at 19:21:32 -0400, Stefan Monnier wrote:
> > Where, exactly are the terms of this supposed contract formulated?

> I'm not sure it's written anywhere.
> More specifically, for `jit-lock-functions`, the contract is not
> very constraining.

> For font-lock the contract is not still very explicit but is more
> constraining in that we expect major modes not to look before point-min
> or after point-max.  For that reason font-lock normally widens the
> buffer before it does anything else (unless `font-lock-dont-widen` is
> set).

> > And which part of this supposed contract has CC Mode broken?

> It calls `widen` within its font-lock code.

I can't remember exactly why CC Mode widens here (though I could surely
find it in my notes), but it should be largely harmless in normal

> Eli Zaretskii [2022-08-07 17:20:52] wrote:
> > jit-lock calls the functions with two arguments, BEG and END, and
> > expects them to work only on that chunk of text.

> That is not the case: it expects the function to "fontify" *at least*
> from BEG to END, but is quite happy to let it fontify more (and the
> function can return a value indicating which portion was actually
> returned in that case).  Furthermore, it's clear that fontification of
> BEG..END may need to look at text before BEG (and occasionally beyond
> END as well).

It's also worth pointing out that _looking_ at text, whatever that means
exactly, is an order of magnitude faster than _fontifying_ that text.

>         Stefan

Alan Mackenzie (Nuremberg, Germany).

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