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

From: Tim Cross
Subject: Re: PATCH: Explicitly show how let works on global-variables
Date: Wed, 05 Oct 2022 09:22:02 +1100
User-agent: mu4e 1.9.0; emacs 29.0.50

Phil Sainty <psainty@orcon.net.nz> writes:

> 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.

+1. I think this has become quite important.

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