Re: Several Major Modes. [Was: master 7362554: Widen around c-font-lock-fontify-region. This fixes bug #38049.]

[Dmitry Gutov]

On 14.11.2019 23:24, Alan Mackenzie wrote:
> Hello, Dmitry.
>
> On Thu, Nov 14, 2019 at 00:33:33 +0200, Dmitry Gutov wrote:
>
> [ .... ]
>
> > > > In mmm-mode context, however, we apply definite boundaries to the
> > > > region chunks. Here's an example of some Noweb code:
> > > > https://en.wikipedia.org/wiki/Noweb#Example_of_a_simple_noweb_program
>
> > > > The inside of hello.c block would be narrowed to.
>
> > > I think I've said this before, but I don't think narrowing is the right
> > > tool for that task.  I don't think there is a suitable tool in Emacs at
> > > the moment.
>
> > *shrug* We do the best with what we have.
>
> Why can we not formulate something better, an enhancement to the Emacs
> core which would support several major modes properly?  I have made
> proposals in this area before, but I think they were to grandiose to be
> implementable.
>
> What seems to be needed is a way of partioning a buffer into several
> sub-buffers (which I have called "islands" in the past), and having a new
> type of local variable, one valid in exactly one sub-buffer.  More or
> less.

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 or, if we absolutely 
can't make it work, add a new element to prog-indentation-context which 
will be a function that would return the bounds of the current chunk.

Regarding "new type of local variable", mmm-mode already tracks 
something like that.

> > > > Now, I have remembered that CC Mode calls widen from many places
> > > > already, so it already is problematic for using in a context like
> > > > that.
>
> > > It does, yes.  Users also use widening and narrowing.
>
> I believe these problems won't go away, and there will always be
> conflicts between CC Mode (as it is) and mmm-mode (as it is).

I think we should also try to understand whether making CC Mode play 
nice to doable/feasible, and for what uses. Like, I think it can work 
(more or less) already when it's the primary major mode (meaning the 
buffer starts and ends with it), so the embedded chunks are all other 
modes. 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)? By answering these questions we can temper our 
expectations and come up with a practical plan.

Doing nothing is also a valid choice, BTW, since for many uses replacing 
c-mode with js-mode works pretty okay. I've been recommending it to 
MMM-mode users.