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bug#18059: 24.3.92; defvar and special variables

From: Michael Heerdegen
Subject: bug#18059: 24.3.92; defvar and special variables
Date: Sun, 11 Feb 2018 00:59:10 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Hello Noam,

thanks for working on this.

> +Note that specifying a value, even @code{nil}, marks the variable as
> +special permanently.  Whereas if @var{value} is omitted then the
> +variable is only marked special locally (i.e. within the current file
> +or let-block).  This can be useful for suppressing byte compilation
> +warnings, see @ref{Compiler Errors}.

After reading this and thinking about it, I'm confused now what the
extent of `defvar' without a specified value is.

In the case of a file, does it mean that the variable is considered
special for the rest of the file, or for the whole file?

And for locally special variables, when you eval

#+begin_src emacs-lisp
;; -*- lexical-binding: t -*-

(defvar testfun nil)

(let ((x 1) f g)
  (defvar x)
  (setq testfun (lambda () x)))

(funcall testfun)

you get 1, but OTOH

#+begin_src emacs-lisp
;; -*- lexical-binding: t -*-

(defvar testfun nil)

  (defvar x)
  (let ((x 1) f g)
    (setq testfun (lambda () x))))

(funcall testfun)

gives you an error.  I don't really see the connection to a `let' block:
in the first example, the defvar is in a let block, but the variable is
not treated as special in the block.  In the second example, it's
outside, but the variable is treated as special (though the extent is
surely not limited to any `let' block).


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