Re: forward-comment and syntax-ppss

[Alan Mackenzie]

Hello, Stefan.

On Wed, Dec 07, 2016 at 09:32:50PM -0500, Stefan Monnier wrote:
> > Read up on bug #22983.  Basically, syntax-ppss is fundamentally broken.
> > It won't work with narrowing of buffers, and delivers inconsistent
> > results which aren't in accordance with what the function's doc says it
> > does.  This breakage isn't anything that can be fixed.

> > The only way to deal with it seems to be to remove syntax-ppss and
> > replace it with a new function.

> And how would the new function not face the same breakage that can't be
> fixed?

It would have a sensible definition.  "Returns the equivalent of
(parse-partial-sexp (point-min) POS)" is a silly definition which cannot
be coded up in any reasonable fashion.  I think it should be replaced by
"returns the equivalent of (parse-partial-sexp 1 POS)", which can be
readily implemented.  I propose, additionally, a new buffer local
variable which, if non-nil, will contain a buffer position to use in
place of the "1".

> As you should know, your comment-cache code suffers from the exact same
> problem.

Rubbish.  Modulo any remaining bugs, it does what it says it does.

> It's a fundamental problem in caching parsing data in the face of
> narrowing, given the fact that our narrowing primitive don't say what
> is the intention behind it.

Nor should it.  Our primitives `car', `cdr', `list', etc. also don't say
what the intention behind them is; they just work.  Narrowing is what it
is.  Simple and austere.   Complexifying narrowing by introducing a
notion of "intention" would surely make it more difficult to use and
possibly foul things up massively.

> If you think your comment-cache works well (i.e. you consider its
> failures ....

What failures?  If you've found any, please tell me specifically what, so
that I can fix it/them.  Please stop spreading FUD like this.

> .... to be caused by inappropriate use of narrow-to-region), then it's
> trivial to "fix" syntax-ppss in the same way: just put a `widen` in it
> and voilà: "fundamentally broken" solved.

Rubbish again.  "Returns the equivalent of (parse-partial-sexp
(point-min) POS)" cannot be implemented in any reasonable fashion, and
just putting a `widen' into the current code won't change that one bit.

As I said above, I think syntax-ppss should be retired and replaced by a
function which does parse-partial-sexp starting at 1 rather than
(point-min).

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).