Re: Bug #25608 and the comment-cache branch

[Alan Mackenzie <acm@muc.de>]

Hello, John.

On Sat, Feb 11, 2017 at 18:53:58 -0800, John Wiegley wrote:
> >>>>> "DG" == Dmitry Gutov <dgutov@yandex.ru> writes:

> GD> One normally adds an alternative source of truth (i.e. a "cache") to fix a
> DG> significant performance problem, when one really can't do so otherwise.

> DG> It seems we agree now that comment-cache's existence can't be justified by
> GD> performance considerations.

> DG> Cache invalidation is a known hard problem in CS, so we generally don't
> GD> want to have extra caches.

> This argument right here is why I would vote against comment-cache: I'd rather
> have parens-in-comments-at-column-0 parsed incorrectly -- at least, until
> syntax-ppss is fixed -- than to add another cache just to fix this problem.
> Unless I've missed something...

What you've missed is that the cache invalidation in comment-cache is
rock solid - with the exception that it doesn't watch
parse-sexp-lookup-properties and parse-sexp-ignore-comments, variables
that are typically used only in initialisation.  If this were deemed a
flaw, it could be fixed very easily.

The other thing is that syntax-ppss doesn't look like getting fixed.
Bug #22983 has been open for almost a year, despite requests to have it
fixed.  Also syntax-ppss's cache invalidation is less than rigorous.

Yet another thing is that it is me that is having to field the
open-paren-in-column-0-in-comment bugs, which typically happen in CC
Mode, and it is a demoralising waste of time each time it happens.
Recently, I've been telling the bug raisers that there's a fix which
should hopefully appear in Emacs 26.  With all honesty, I don't think I
can say that any more.

> -- 
> John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
> http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2

-- 
Alan Mackenzie (Nuremberg, Germany).