Re: Bug #22983 (syntax-ppss returns wrong result) is still open. Could we fix it before the release, please.

[Dmitry Gutov]

On 06/12/2016 12:34 PM, Alan Mackenzie wrote:

> I can't reason about hard-widen very much, because I haven't read its
> specification.

The change to the narrow-to-region's docstring in the patch contains the 
specification. Here's a better link to the message:

http://lists.gnu.org/archive/html/emacs-devel/2016-03/msg01576.html

> No.  A careful consideration of its proposed working before
> implementation (let's call it a design ;-) can reveal a lot about how
> well it could work.

...and time, if we're being realistic.

> `widen' makes the entire buffer accessible.  `narrow-to-region' makes
> the specified region the accessible portion.  And that's it!  What could
> be simpler?

If you take a shovel and remove its blade, the result will look very 
simple. That doesn't mean it's appropriate for any interesting task.

Nowhere does narrow-to-region's documentation say it should only be used 
in major modes.

> I have encountered no such problems in over 15 years of hacking Emacs.
> I think you're seeing problems somewhere and blaming
> narrow-to-region/widen, when the real problem is somewhere else.

It's been a known problem for a while, and it has come up in multiple 
discussions over the last years.

The "real problem" is a matter of perspective. Some might argue that 
trying to use narrowing at all, for any purpose, is a bad idea, and 
then, of course, the right choice would be to use something else.

> > How else would you apply each major mode's fontifications only within
> > its subregions?
>
> By adding the appropriate structures as buffer local variables (or
> perhaps as text properties) which would delimit the subregions, then
> enhance Font Lock to respect those boundaries.

Please go ahead and write up this proposal in detail. It doesn't seem as 
simple as you make it sound. And I wonder if it ends up to be different 
from the "islands" proposal.

Further, font-lock is not the only facility we want here. Making 
syntax-ppss obey region boundaries is another. There's also indentation 
code, which is more difficult to handle, but limiting visibility there 
somehow also seems essential.

The super mode can also adapt different simpler facilities, like Imenu, 
using the same approach (go over the chunks, narrow to each, scan using 
the language-appropriate tools, and concatenate the results).

> > See
> > https://github.com/purcell/mmm-mode/blob/c9a857a638701482931ffaaee262b61ce53489f3/mmm-region.el#L789-L816
>
> That's rather a lot to take in before breakfast!

Did you have the breakfast yet? It's just 30 lines.

> > super modes are currently implemented as minor modes. major modes
> > shouldn't override the choices made by minor modes, if at all possible.
>
> What????

What I said. To the extent the facilities allow, of course.

> The use/non-use of narrowing is NOT a
> setting, any more than the use/non-use of cdr is.  They're just general
> purpose tools.

Turns out, they are not general purpose enough. That's the problem.

> > There have been several discussions.
>
> They're not coherent, and they're not very useful.

If there's anything in them that makes you confused, I've yet to hear 
about it. As far as I can see, you're not interested in making even that 
effort in learning the subject.

> Could you please write this document.  Given the massive change you want
> to make in a fundamental Emacs tool, this is not unreasonable to ask.

I don't enjoy rehashing others' arguments when the original messages are 
readable enough.

It's a small change, by itself. Please read the discussion, starting 
with http://lists.gnu.org/archive/html/emacs-devel/2016-03/msg01129.html

And http://lists.gnu.org/archive/html/emacs-devel/2016-03/msg01182.html 
in particular. After that, please let me know if you have any specific 
questions.

> This is, after all, what I did for the "islands" concept a couple of
> months ago.

And I have spent considerable time contributing to that discussion as well.

> > The main thing CC Mode would have to worry about from then on, is that
> > it won't always be able to goto-char to positions beyond the hard
> > narrowing, even if they exist in the buffer (and are pointed to by some
> > buffer-local data structure maintained by CC Mode's parser).
>
> That sounds like something worthwhile sorting out in advance.

Sure. Please go ahead and present your thoughts on the subject.

I realize it might be rather complicated, actually, to use the narrowing 
approach with CC Mode as submode. But I don't see any alternative 
solutions that we're likely to implement. So it may be that some modes 
will still refuse to work in multi-mode context.