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Re: Syntax ambiguities in narrowed buffers and multiple major modes: a p

From: Stefan Monnier
Subject: Re: Syntax ambiguities in narrowed buffers and multiple major modes: a proposed solution.
Date: Mon, 27 Feb 2017 15:52:19 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

>> In order to design a solution, we need to think about the problem somehow.
> That's all very well, but we could spend weeks (or months) talking about
> the problem without getting anywhere.

You think whichever way you want.  All I said was:

    [ I like to consider that strings and comments are also a form of
      "island", although we're probably better off supporting them in
      a special way like we do now.  ]

which I wrote mostly so as to give some background in how I think about
the problem so you can hopefully understand better my position.

You then started to argue that it's wrong to think this way.

>> >> But the issue is that the syntax beginning in the above example should be
>> >> point-min, not 1.
> Incidentally, a parse state (like a voltage) is always a _difference_
> between two points.

Here I disagree.  When you say "difference" it makes it sound like we
could take two differences and combine them.  That would be great: we
could cache every N chars the cumulated difference applied by those
N chars and then efficiently compute the state at any position by only
combining those pre-computed cumulated diffs.

But we can't do that, because the "parse state" is really a *state*.

> It is determined by (parse-partial-sexp start end ...).

Yes, the state depends on where we start parsing.  But there is
a privileged state which is the one rendered visible via font-lock, and
that's the one syntax-ppss intends to cache.

> It is an error in thinking to think that there is any a priori
> syntax beginning in any buffer situation.

I'm glad we agree about this.

> Here's that code fragment again, for reference:

>     (save-restriction
>       (narrow-to-region ...)
>       (with-syntax-table ...
>         (backward-sexp 1)))

>> It's not really inherent in the code, but it's how it currently behaves
>> (mostly), and in some cases that is what the author wants.  In other
>> cases, the author wants something else.

> That code appears to be from .../lisp/obsolete/complete.el, function
> PC-lisp-complete-symbol.

If so, it's a complete accident.  The fragment came straight out of
my imagination.  The situations I have in mind are more like in
perl-mode's syntax-propertize function where we need to find the
matching braces in regexp operations (where the matching rules are
slightly different from the ones in normal code) or in sgml-mode where
we jump from < to > and vice-versa using a specialized syntax-table, or
in sm-c-mode where I parse the C code within a CPP directive (itself
treated from the outside as a kind of comment).

> :-)  There will be situations where things like backward-sexp will call
> back_comment (which is why it is important that back_comment be fast)
> but that code fragment isn't one of them.  And even if it did (which
> will be rare), it is not doing it inside a tight loop.

I'm saying that the code fragment can be inside a tight loop (e.g. as
part of a backward lexer used for indentation purposes).

> I think you've chosen a bad example for making that point.  The
> syntax-ppss cache in the above code will need flushing anyway due to the
> with-syntax-table.  The comment-cache cache might or might not need
> flushing for that reason (depending on the differences between the two
> syntax tables).

Alan, I'm not trying to pimp syntax-ppss here or to put down
comment-cache, here.  I'm just pointing out a real existing problem
which I think the new design should take into account.  Indeed the
current syntax-ppss treatment is incorrect (and in sm-c-mode
I work around it by meddling in syntax-ppss's internals, which is
clearly a bad idea).

IOW, instead of trying to come up with ad-hoc ways to treat
narrow-to-region as something that places island markers, which are then
somehow removed by something else (presumably at the end of
save-restriction, tho that makes for ugly semantics, IOM) and then
additionally handle the with-syntax-table thingy, I think we should
design a new macro specifically for that kind of "temporarily work on
a region as if it was its own buffer, with its own syntax"
(i.e. combining the kind of effect usually obtained with

Then we can implement it by adding island markers (and flush the cache)
if we want, and if that proves inefficient later on, we can change it to
use another implementation strategy.


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