bug#46374: 28.0.50; Ask me to save buffers only if they are under callers dir

[Tino Calancha]

On Thu, 29 Apr 2021, Eli Zaretskii wrote:

> > From: Tino Calancha <tino.calancha@gmail.com>
> > Date: Wed, 28 Apr 2021 21:31:25 +0200
> > Cc: 46374@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>,
> >  Quách Mỹ Uyên Nhi
> >  <uyennhi.qm@gmail.com>
> >
> > Juri Linkov <juri@linkov.net> writes:
> >
> > +(defcustom save-some-buffers-default-predicate save-some-buffers-default-fun
> > +  "Generator function of the default predicate for `save-some-buffers'.
> > +
> > +It must be a function with no arguments that returns a predicate.
> > +This predicate is called (with no parameters) from the buffer to be
> > +saved.
>
> Reading this, I'm confused: a function that returns a predicate?  If
> this is literally so, why do we need this two-step approach? why not
> have the value _be_ a predicate function?

The point of this two-step approach is to get access to the caller 
environment:

- We have a function, generate-foo, that builds a predicate foo.
- By calling generate-foo inside same-some-buffers, we build a foo with
  access to the enviroment, i.e. foo is a closure.

That makes the trick to restrict the prompt to saving buffers only inside 
the current project.