bug#66706: [PATCH] Automatic elisp dialect insertion

[Jim Porter <jporterbugs@gmail.com>]

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On 10/25/2023 6:19 PM, Jim Porter wrote:
> I'll start with a patch here then. I think this is also a prime spot to 
> add an example or two that would actually show lexical binding in action 
> (i.e. a sample where the code would do something different under dynamic 
> binding).

Here's a first attempt. I'm not sure I'm entirely happy with it (the 
digression into setting 'lexical-binding' to 't' is a bit disruptive), 
but hopefully it's an improvement. Of course, we can keep adjusting this 
further as needed.

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From 6bc9bbbb98105f700bb8d5b04e8de5e261efa777 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): Rename to...
(Why Use let?): ... this, and rework the explanation to discuss
lexical binding (including how to enable it).
---
 doc/lispintro/emacs-lisp-intro.texi | 81 +++++++++++++++++++++--------
 1 file changed, 58 insertions(+), 23 deletions(-)

diff --git a/doc/lispintro/emacs-lisp-intro.texi b/doc/lispintro/emacs-lisp-intro.texi
index fce7583fe91..ebbcc08b9ff 100644
--- a/doc/lispintro/emacs-lisp-intro.texi
+++ b/doc/lispintro/emacs-lisp-intro.texi
@@ -3587,39 +3587,74 @@ let
 @code{let} special form prevents this kind of confusion.
 
 @menu
-* Prevent confusion::
+* Why Use let?::
 * Parts of let Expression::
 * Sample let Expression::
 * Uninitialized let Variables::
 @end menu
 
 @ifnottex
-@node Prevent confusion
-@unnumberedsubsec @code{let} Prevents Confusion
+@node Why Use let?
+@unnumberedsubsec Why Use @code{let}?
 @end ifnottex
 
 @cindex @samp{local variable} defined
 @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.
-@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.)
+The @code{let} special form provides a way to confine your variables
+to a particular section of your code (in computer science jargon, a
+``scope'').  @code{let} creates a name for a @dfn{local variable} that
+overshadows any use of the same name outside the @code{let} expression
+(we call this ``binding'' the variable).  This prevents any accidental
+usage of these variables outside of 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.  @xref{defun, , The @code{defun} Macro}.)
+
+@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 return to discussing @code{let}.
+Local variables created by a @code{let} expression hold their value
+@emph{only} within the body of the @code{let} expression itself; the
+local variables have no effect outside of the @code{let} expression.
+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