Re: PATCH: Explicitly show how let works on global-variables

[Phil Sainty]

On 2022-10-04 21:09, Pedro Andres Aranda Gutierrez wrote:
> I understood as local variable a 'value that was stored in the
> function's stack' to be used in the scope of the let. That implied
> (once again in my understanding) that the global system-time-locale
> would not be affected and hence format-time-string would not see the
> change in the value within the let.

Since the addition of lexical binding to Emacs Lisp in Emacs 24.1,
both results are possible depending on whether you are dealing with
a dynamic or a lexical variable.

I.e. given:

 (defun myfunc () foo)
 (let ((foo 'bar)) (myfunc))

If foo is a dynamic variable then the let form will return 'bar.

If foo is a lexical variable, then you'd get this error:
"let: Symbol’s value as variable is void: foo".

Eli quoted the manual:

     Local variables created by a ‘let’ expression retain their value
  _only_ within the ‘let’ expression itself (and within expressions 
called
  within the ‘let’ expression); the local variables have no effect 
outside
  the ‘let’ expression.

That "(and within expressions called within the ‘let’ expression)" is
pretty ambiguous wrt dynamic vs lexical binding, and a few lines later
it comments very briefly on this:

  in Emacs Lisp, the default scoping is dynamic, not lexical.
  (The non-default lexical binding is not discussed in this manual.)

Which keeps the rest of the text accurate, yet in an almost-entirely
unexplained manner.

I suggest that at this point it has become pretty necessary for lexical
binding to be discussed in this manual...

* The *scratch* buffer, in which users will perform many if not most of
  their experiments, now uses lexical binding by default.

* If enabled, auto-insert-mode adds lexical-binding: t to new elisp 
files
  by default.

* IIRC most elisp files in Emacs core are now using lexical binding.

* The emacs-lisp-mode mode-name treats dynamic binding as a warning.

So while it's as true as ever that dynamic binding is the default, the
fact that so many things nowadays default to *enabling* lexical binding
really blurs this line, to the point where I think it's unreasonable to
avoid discussing lexical binding in the introduction to emacs lisp, as
the user will almost unavoidably be exposed to it.

I think examples would be hugely helpful in explaining the difference
between the two types of binding.


-Phil