Re: font-lock-syntactic-keywords obsolet?

[Dmitry Gutov]

On 06/21/2016 06:26 PM, Alan Mackenzie wrote:

> > If Emacs is supposed to keep the syntax-table value on the last
> > character up to date using after-change-functions, it will have to scan
> > the whole 300000 line buffer after every keypress.
>
> Could it not restrict the scanning to cases where a "`" is inserted or
> deleted?

Even if we could, it would be more like "cases where `, /, " or % are 
inserted or deleted" (with actually more cases, and complex logic for 
the cases' detection).

But "auuugh Ruby Mode is slow when I type /" would be a problem anyway, 
just look at http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22884.

> Do you not have to do the scanning anyway when you type in "`"
> at the end of the "b = :`" line?

I don't. ` has the syntax "string delimiter" in ruby-mode-syntax-table.

When ruby-syntax-propertize is called on the last line, it just checks 
what (syntax-ppss POSITION-OF-THE-LAST-`) returns. If the return value 
says "inside a string", we do not apply syntax-table value "symbol" to 
the last `. Otherwise, we do.

This way, the full scan is replaced with a call to (syntax-ppss), which 
uses cache. In more complex cases, we can go look at the actual 
character that begins the string (or a string-like literal) that we're 
currently inside, parse the text around it locally a bit, and so on, but 
this is always faster that scanning the whole buffer.