GNU bug report logs - #22983
syntax-ppss returns wrong result.

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Message #135 received at 22983 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Alan Mackenzie <acm <at> muc.de>
Cc: John Wiegley <jwiegley <at> gmail.com>, Philipp Stephani <p.stephani2 <at> gmail.com>,
 22983 <at> debbugs.gnu.org, Dmitry Gutov <dgutov <at> yandex.ru>
Subject: Re: bug#22983: [ Patch ] Re: bug#22983: syntax-ppss returns wrong
 result.
Date: Sun, 10 Sep 2017 18:53:53 -0400

> +;; Several caches.
> +;; Because `syntax-ppss' is equivalent to (parse-partial-sexp
> +;; (POINT-MIN) x), we need either to empty the cache when we narrow
> +;; the buffer, which is suboptimal, or we need to use several caches.

I think that (parse-partial-sexp 1 x) is more often what the caller
wants than (parse-partial-sexp (point-min) x), but if you're happy with
the behavior described by the docstring, then that's fine.

> +;; The implementation which follows uses three caches, the current one
> +;; (in `syntax-ppss-cache' and `syntax-ppss-last') and two inactive
> +;; ones (in `syntax-ppss-{cache,last}-{wide,narrow}'), which store the
> +;; former state of the active cache as it was used in widened and
> +;; narrowed buffers respectively.

Earlier in the thread, I suggested to use a single cache indexed by the
position of point-min (or by the position and point-min and by the
current syntax-table, so as to also handle changes in the syntax-table),
i.e. a list of (POINT-MIN-POS . CACHE-DATA) or
((POINT-MIN-POS . SYNTAX-TABLE) . CACHE-DATA).  I think it would lead to
less code duplication than your patch which only handles 2 different
POINT-MIN-POS (and one of the two has to be equal to 1), but existing
code trumps hypothetical designs.

So, I have no objections to the patch.  But I think (parse-partial-sexp
(point-min) x) is a design bug in syntax-ppss which we will need to fix
sooner or later, which is why I never bothered to implement something
like your patch, which only makes the code do what its doc says rather
than what the caller needs.


        Stefan

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