bug#66756: 30.0.50; [PATCH] Improve discussion of 'let' in Elisp Introduction manual

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 26 Oct 2023 11:30:46 -0700
> From: Jim Porter <jporterbugs@gmail.com>
> 
> On 10/26/2023 12:09 AM, Eli Zaretskii wrote:
>  > Thanks.
>  >
>  > The challenge in updating the Lisp Introduction manual is to try to
>  > keep its informal and reader-friendly style as much as possible.  It
>  > is not just another ELisp Reference manual!  So please try to keep
>  > that in mind when you write the text, and in particular try not to
>  > modify the existing text that is still accurate -- it was written by a
>  > master, and each word there counts, even if it looks at first sight as
>  > not important.
> 
> Ok, here's a second attempt. I've tried to avoid changing anything that 
> I don't think is truly necessary. I did alter a bit of the original 
> wording to emphasize that under lexical binding, 'let' isn't about time, 
> but about place. For example, that's why I changed this:
> 
> > This is like understanding that whenever your host refers to ``the house'', he means his house, not yours.
> 
> to this:
> 
> > This is like understanding that in your host's home, whenever he refers to ``the house'', he means his house, not yours.
> 
> My previous concern about the "lexical binding" digression still applies 
> though. However, I'm not sure how to get around that at present; if we 
> want to talk about lexical binding in the manual, we need to get users 
> to enable it, so I think it's unavoidable that we at least mention it. I 
> tried to introduce the jargon as gently as I could (by first introducing 
> the term "binding" on its own before mentioning "lexical/dynamic 
> binding"), but it's still a bit intimidating. On the positive side, when 
> lexical binding is the default, we could remove that entire digression.
> 
> There's also an argument that the example I added is in the wrong spot, 
> since we haven't actually introduced the 'let' syntax yet. However, I 
> personally find the example to be pretty useful since it shows off one 
> of the key differences between lexical and dynamic binding, and helps 
> show one of the boundaries of the 'let' form's scope. I myself tend to 
> learn best by seeing examples of that sort. Fixing the order so we 
> introduce the syntax first would require more extensive changes to this 
> section...

Richard, could you please review these changes?

> From 12847e0d59b2de3791efa090addc0169e713e6d1 Mon Sep 17 00:00:00 2001
> From: Jim Porter <jporterbugs@gmail.com>
> Date: Wed, 25 Oct 2023 20:43:57 -0700
> Subject: [PATCH] Introduce 'let' using lexical binding in the Lisp
>  Introduction
> 
> * doc/lispintro/emacs-lisp-intro.texi (Prevent confusion): Rework the
> explanation to discuss how things work under lexical binding
> (including how to enable it).
> ---
>  doc/lispintro/emacs-lisp-intro.texi | 67 ++++++++++++++++++++++-------
>  1 file changed, 51 insertions(+), 16 deletions(-)
> 
> diff --git a/doc/lispintro/emacs-lisp-intro.texi b/doc/lispintro/emacs-lisp-intro.texi
> index fce7583fe91..e805833f979 100644
> --- a/doc/lispintro/emacs-lisp-intro.texi
> +++ b/doc/lispintro/emacs-lisp-intro.texi
> @@ -3602,24 +3602,59 @@ Prevent confusion
>  @cindex @samp{variable, local}, defined
>  The @code{let} special form prevents confusion.  @code{let} creates a
>  name for a @dfn{local variable} that overshadows any use of the same
> -name outside the @code{let} expression.  This is like understanding
> -that whenever your host refers to ``the house'', he means his house, not
> -yours.  (Symbols used in argument lists work the same way.
> +name outside the @code{let} expression (in computer science jargon, we
> +call this ``binding'' the variable).  This is like understanding that
> +in your host's home, whenever he refers to ``the house'', he means his
> +house, not yours.  (Symbols used in argument lists work the same way.
>  @xref{defun, , The @code{defun} Macro}.)
>  
> -Local variables created by a @code{let} expression retain their value
> -@emph{only} within the @code{let} expression itself (and within
> -expressions called within the @code{let} expression); the local
> -variables have no effect outside the @code{let} expression.
> -
> -Another way to think about @code{let} is that it is like a @code{setq}
> -that is temporary and local.  The values set by @code{let} are
> -automatically undone when the @code{let} is finished.  The setting
> -only affects expressions that are inside the bounds of the @code{let}
> -expression.  In computer science jargon, we would say the binding of
> -a symbol is visible only in functions called in the @code{let} form;
> -in Emacs Lisp, the default scoping is dynamic, not lexical.  (The
> -non-default lexical binding is not discussed in this manual.)
> +@cindex lexical binding
> +@cindex binding, lexical
> +@cindex dynamic binding
> +@cindex binding, dynamic
> +Before we begin discussing @code{let} in detail, we must first mention
> +an important note.  For historical reasons, Emacs Lisp uses a form of
> +variable binding called ``dynamic binding''.  However, this manual
> +will discuss the preferred form of binding, called ``lexical binding''
> +(if you have programmed in other languages before, you're likely
> +already familiar with how lexical binding behaves).  In order to use
> +lexical binding, you should add something like this to the first line
> +of your Emacs Lisp file:
> +
> +@example
> +;;; -*- lexical-binding: t -*-
> +@end example
> +
> +For more information about this, @pxref{Selecting Lisp Dialect, , ,
> +elisp, The Emacs Lisp Reference Manual}.
> +
> +With that out of the way, we can get back to discussing @code{let}.
> +Another way to think about @code{let} is that it defines a place in
> +your code where the variables you named have their own local meaning.
> +Outside of the @code{let} body, they have another meaning (or they may
> +not be defined at all).
> +
> +This means that inside the @code{let} body, calling @code{setq}
> +for a variable named by the @code{let} expression will set the value
> +of the @emph{local} variable of that name.  This also means that
> +outside of the @code{let} body, calling @code{setq} for a variable
> +named by the @code{let} expression will @emph{not} affect that local
> +variable.
> +
> +For example, if you call a function inside of a @code{let}
> +body, that function's body would be unable to ``see'' (or modify) the
> +value of a local variable from the @code{let} expression:
> +
> +@example
> +(setq x 1)
> +
> +(defun getx ()
> +  x)
> +
> +(let ((x 2))
> +  (get-x))
> +     @result{} 1
> +@end example
>  
>  @code{let} can create more than one variable at once.  Also,
>  @code{let} gives each variable it creates an initial value, either a
> -- 
> 2.25.1
>