Re: Syntax tables for multiple modes [was: bug#22983: syntax-ppss returns wrong result.]

[Dmitry Gutov <dgutov@yandex.ru>]

Hi Alan,

On 03/14/2016 05:16 PM, Alan Mackenzie wrote:

> How about an extension to the parse-partial-sexp (etc.) code?  For
> example, a feature I would call an "island", where a character could be
> marked with the "island start" syntax-table property, and another
> character lower down could be marked with the "island end" character.

Something like that might help, although I hesitate asking for that 
change because it's a relatively big one, and it would still solve only 
one multiple-mode-related issue.

What it would help with, is fool the "outer" major mode into ignoring 
the preceding submode regions, in the return value of syntax-ppss. But 
we could have pretty much that already by advising syntax-ppss. That 
leaves out parse-partial-sexp, but it's not used that often directly in 
major mode code (though sgml-mode uses it).

> parse-partial-sexp, on encountering an island start syntax would somehow
> stack the current parse state and start a new one with a different syntax
> table.  The parse state, instead of consisting of the 10 elements
> currently, would in general have 10n elements, where n is the depth of
> nesting.

To be able to parse across different regions, it would need to know the 
syntax table for each one (using the syntax-table text property?), as 
well as to be able to apply the appropriate syntax-propertize-function 
in each region. The latter is handled by mmm-mode, though, in a 
seemingly adequate fashion (it installs a composite function that knows 
how to dispatch to mode-specific ones).

Maybe it's worth a try. Though I don't know how Stefan uses narrowing in 
sm-c-mode, and whether this proposal is appropriate to replace narrowing 
in syntax-ppss for this use case.