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

From: Michael Albinus <[email-masked]>
Date: Sun, 19 Nov 2023 09:38:54 +0100

Jim Porter <[email-masked]> writes:

Hi Jim,

> +In some cases, both lexical and dynamic binding behave identically.
> +However, in other cases, they can change the meaning of your program.
> +For example, under lexical binding, 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
> +;;; -*- lexical-binding: t -*-
> +
> +(setq x 1)
> +
> +(defun getx ()
> +  x)
> +
> +(let ((x 2))
> +  (getx))
> +     @result{} 1
> +@end example
> +
> +@noindent
> +If we use dynamic binding instead, the behavior is different:
> +
> +@example
> +;;; -*- lexical-binding: nil -*-
> +
> +(setq x 1)
> +
> +(defun getx ()
> +  x)
> +
> +(let ((x 2))
> +  (getx))
> +     @result{} 2
> +@end example
> +
> +Now, the result of @samp{(getx)} is @samp{2}!  That's because under
> +dynamic binding, when @code{getx} looks for the value of @code{x}, it
> +sees the value we set in our @code{let} expression.  In other words,
> +the call to @code{getx} happens during the @emph{time} when our
> +@code{let} expression is active.  Under lexical binding, @code{getx}
> +doesn't see the value from our @code{let} expression.  That's because
> +it happens in a different @emph{place} than the @code{let} body.

Would it be worth to emphasize, that a declaration of x changes this?
That is, when a variable is declared, both lexical and dynamic binding
behave identically.

@example
;;; -*- lexical-binding: t -*-

(devfar x 1)

(defun getx ()
  x)

(let ((x 2))
  (getx))
     @result{} 2
@end example

Best regards, Michael.