Re: Several Major Modes.

[Dmitry Gutov]

Hi Alan,

On 15.11.2019 22:10, Alan Mackenzie wrote:

> > There are options. We'd have to decide on a suitable model, calling them
> > islands or whatever, but I think the first approximation is to either
> > make sure narrowing is available for this purpose ....
>
> You also need to make sure narrowing is available for any purpose
> required by a major mode.

Eh, I think it's "available" already, but I'd have to see specific examples.

The problem which triggered this discussion is that *something* called 
font-lock rules from a narrowed buffer directly. But that's not a 
"purpose required by a major mode".

> > Regarding "new type of local variable", mmm-mode already tracks
> > something like that.
>
> I was envisaging something at the C level, where different regions of a
> buffer would have different values of variables, without needing the
> continual swapping at the Lisp level.  Maybe such a thing isn't needed.

I'd been told that even a C-based implementation is unlikely to make 
things much faster. Anyway, it would be a perf optimization, and we 
could get to it later.

> It can't work if any external Lisp corrupts its syntax-table text
> properties.  This is what syntax-ppss-flush-cache (on
> before-change-functions for many modes) would do if there were a non-nil
> syntax-propertize-function at the time.  This may be the biggest problem
> to getting CC Mode integrated into MMM Mode.

mmm-mode sets its own syntax-propertize-function that calls major mode 
specific syntax-propertize-function's over their respective 
chunks/subregions. So, in principle, that should work fine. As long as 
nobody calls 'widen' unexpectedly.

> > Is it feasible to support embedded chunks? To support chunks with
> > incomplete pieces of code (which are e.g. included conditionally by the
> > surrounding template)?
>
> Well CC Mode already supports preprocessor macros and (for C++) raw
> strings, which are syntactically somewhat and very different from the
> enclosing code.

I'm not sure it's the same. Like, would CC Mode cope with a region 
starting with closing brackets, etc. This might not be a frequent 
situation, but at least it shouldn't blow up.