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.