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

[Stefan Monnier]

>> (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.

If we don't add such a thing in the common API, how could individual
packages know the boundary of the current chunk without adding
MMM-specific hacks?

> (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.

But just `widen` will do thew wrong thing (it'll widen too much) when
we're inside an MMM chunk.

>> [ 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?

Having it as a function means you don't have to compute the value of
prog-first-column every time to "enter" a chunk, but instead we only
compute it if/when the submode decides it needs to know what is the
value of prog-first-column.

> And when else will we need this value?

Here are some examples I can think of:
- auto-fill-mode may like to compute the hypothetical indentation that
  would result from inserting a newline somewhere (and do that before
  touching the buffer).
- some package may like to highlight lines which aren't currently
  indented right (so, it won't call indent-according-to-mode, but
  it will need to compute the "desired" indentation).
I'm sure there can be other cases.


        Stefan