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Re: possible bug in anchored font-lock functions

From: Sam Halliday
Subject: Re: possible bug in anchored font-lock functions
Date: Sun, 11 Nov 2018 15:43:02 +0000

Further to this, I think I've tracked the issue to
`font-lock-fontify-anchored-keywords` which is called with a `limit`
parameter, but then `limit` is mutated very intentionally to the limit
generated by the outer matcher... so I think this is NOT a bug, but
just different behaviour to what I expected. i.e. the first pattern
resets the limit for the anchored highlighters.
On Sun, 11 Nov 2018 at 14:26, Sam Halliday <address@hidden> wrote:
> Dear all,
> I think I might have found a bug in GNU Emacs but I would like to check
> my understanding first, before filing a report or (ideally) fixing it.
> In Search Based Fontification[1] it is possible to specify a function as a
> matcher. It must obey the following contract:
> > it receives one argument, the limit of the search; it should begin
> > searching at point, and not search beyond the limit. It should return
> > non-nil if it succeeds, and set the match data to describe the match
> > that was found. Returning nil indicates failure of the search.
> In addition, it is possible to extend the region to be fontified by
> adding a routine to `font-lock-extend-region-functions` that in-place
> updates the `font-lock-beg` or `font-lock-end` variables (these
> variables are not visible in a function matcher). Let's ignore
> `font-lock-multiline` property approaches, I'm not using them.
> Indeed, I have confirmed that if I extend the region in a
> `font-lock-extend-region-functions` then the `limit` does increase for
> my function matcher!
> However, if I use an an `anchored` matcher, having the form `(matcher .
> anchored-highlighter)`, where the `anchored-highlighter` is a
> `function`, my custom `font-lock-{beg,end}` regions are ignored and
> `limit` is much reduced!
> Is there something I need to do so that anchored matchers receive the
> calculated regions or are they only designed to extend to the end of the
> current line by default?
> If I had to guess I'd say the anchored matcher is forgetting to use
> `font-lock-{beg,end}` and is instead calculating a new limit or using a
> cached version of the limit from before
> `font-lock-extend-region-functions` ran.
> I would greatly appreciate it if somebody could please point me to the
> source code in GNU Emacs where the `font-lock-keywords` are called for
> anchored matchers. I suspect the limit is also broken for `regexp`
> matchers, not just `function`, but I have no way of printing out `limit`
> in that case.
> A final note, this is the first time I've written a syntax-table and
> font-lock for a programming language, and I have found the experience to
> be much more pleasant than I expected! The font-lock-keyword API is
> lovely to work with and I've been using the rx macro to avoid writing
> regexps by hand... my code reads like a simplified BNF description!
> [1] 
> https://www.gnu.org/software/emacs/manual/html_mono/elisp.html#Search_002dbased-Fontification
> --
> Best regards,
> Sam

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