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Re: Problems with syntax-ppss

From: Stefan Monnier
Subject: Re: Problems with syntax-ppss
Date: Mon, 07 Apr 2008 10:57:08 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux)

>> > Will calling s-ppss on a narrowed buffer corrupt the cache at all,
>> > for example?
>> Yes it can.  Similarly the cache does not keep track of the
>> syntax-table so if you switch syntax-table between calls you may get
>> unexpected results.

> That's also not good.

> Would it not be a good idea to (i) redefine syntax-ppss as calculating
> the syntax from BOB (as opposed to (point-min));

That's already what it does, when it can.  Maybe it could be improved as
follows: signal an error if it needs to recompute from (point-min) and
(point-min) is not the beginning of the buffer, unless the caller sets
a `syntax-ppss-narrowed-is-ok' variable (for use in modes like Info and
Rmail).  Maybe that variable should be shared with font-lock-dont-widen.

> (ii) have several caches, each associated with a particular syntax
> table (how many modes are going to use more than 2 or, perhaps, 3?);

This may incur a significant cost.  I much prefer to let the caller
decide when a change of syntax-table requires flushing the cache.

Another approach might be to introduce a `syntax-ppss-syntax-table'.

> (iii) put a `save-match-data' round the function?

This is a definite "no": it is much better to let the caller do it in
the very few cases where it's needed, than to pay the cost needlessly
for all the other cases.


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