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

[Dmitry Gutov]

On 12/1/17 4:26 PM, Stefan Monnier wrote:
> The way your above situation would be handled is to say "the low level
> function mentioned above assumes that the buffer has already been
> properly widened".  This way, it doesn't need to know whether it's being
> called (indirectly) from indent-line-function or b-o-d-function.

That one way, I'd call it "clean". But the change might take quite a bit 
of effort, depending on the size of the code base.

> But some of its callers may need to be changed to do the widening, and
> indeed, in order to know how far to widen, they'll need the help from
> the multi-major-mode package.

That would be the "dirty" way.

> So maybe we need something like:
>
>      (defvar prog-widen-function #'widen)
>      (defun prog-widen () (funcall prog-widen-function))

I don't know if we need that in the common API, but individual packages 
can define such shims if they find that convenient.

E.g.

(defvar cc-mode-inside-indent-line nil)

(defun cc-mode-maybe-widen ()
  ;; You can also throw in an Emacs version check here,
  ;; for good measure.
  (unless cc-mode-inside-indent-line (widen)))

And then the "low-level primitives" can call cc-mode-maybe-widen.

> [ And maybe for the same reason, we need a function (rather than
>    a variable) that returns the `first-column`, so we can get the info
>    anywhere/anytime rather than only during indentation.  ]

Please elaborate.

How does using a function instead of a variable change the duration when 
prog-first-column is set?

And when else will we need this value?