Re: [Emacs-diffs] scratch/widen-less a4ba846: Replace prog-widen with consolidating widen calls

[Alan Mackenzie <acm@muc.de>]

Hello, Dmitry.

Just to be clear, in the following, by "narrowing" I mean all operations
of narrowing and widening regarded generically.

On Thu, Nov 30, 2017 at 23:09:00 +0000, Dmitry Gutov wrote:
> Hi Alan,

> On 11/30/17 9:46 PM, Alan Mackenzie wrote:

> > Using narrowing for marking the bounds of a sub-mode is a bad thing,
> > since it is likely to cause contention with other uses of narrowing.

> Please give an example.

A low level function which as an essential part of its functionality
(for example, to make sure point-min isn't within a comment or string)
widens.

> > It's not clear what is meant here, 

> To rephrase, don't widen in indent-line-function or 
> beginning-of-defun-function.

This is an intolerable restriction.  The low level function mentioned
above cannot, should not, must not know whether it's being called
(indirectly) from indent-line-function or b-o-d-function.  It will have
to widen in all cases or none.  Therefore there will be failures whilst
being called either from one of the two noted functions or from
elsewhere.

> > but mandating maintainers of major
> > modes to use narrowing in a particular way is at best controversial, and
> > probably will render many major modes non-functional.

> I don't see a reason why. Even more, it should be compatible with all 
> uses of narrowing that I know about.

See above.

> > Narrowing belongs to users and major modes. 

> It can serve all.

Indeed it can, and it must.  A super-mode thus may not "reserve"
narrowing for its own purposes to the exclusion of other uses.

[ .... ]

> > You can also understand me being a bit concerned at the reference to CC
> > Mode.  ;-)

> You shouldn't be: CC Mode can just ignore this new rule, as long as it's 
> too hard to support embedding it in multi-mode buffers.

The multi-mode mechanism should be designed to be usable with any major
mode.  There's nothing particularly hard about supporting CC Mode in a
well designed multi-mode scheme.

> >> - Multi-mode packages have been using narrowing for this purpose for
> >> years, so they won't have to change much.

> > They have used narrowing because that is the only tool they have had.

> The only other tool we have is also narrowing, semantically.

We need better tools.  I have already proposed and offered to implement
such tools.

-- 
Alan Mackenzie (Nuremberg, Germany).