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From: Deokhwan Kim <deokhwan...@gmail.com>
Newsgroups: gnu.emacs.help
Subject: Surrounding Lexical Variable Reference in the Body of defun
Date: Mon, 20 Aug 2012 23:07:00 -0700 (PDT)
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Hi there,

I'm having trouble understanding lexical binding in Emacs 24. I came across=
 the following sentence in the Emacs Lisp manual <http://www.gnu.org/softwa=
re/emacs/manual/html_node/elisp/Lexical-Binding.html>:

> the code in the body of a defun or defmacro cannot refer to surrounding l=
exical variables.

It was a great shock to me because it sounded quite awkward and Common Lisp=
 does not have such restriction AFAIK. Rather, I suspected that I might mis=
understand what the sentence really meant. So I decided to make some experi=
ments with the following code stored in foo.el:

  ;;; -*- lexical-binding: t -*-
  (let ((x 0))
    (defun counter ()
      (setq x (1+ x))))

  (message "%d" (counter))
  (message "%d" (counter))

Surprisingly, when I ran it in the form of source code, it worked:

  $ emacs -Q -batch -l foo.el
  1
  2

On the other hand, when I tried to byte-compile it, I got the following war=
ning messages:

  $ emacs -Q -batch -f batch-byte-compile foo.el
  In toplevel form:
  foo.el:2:1:Warning: Function counter will ignore its context (x)
  foo.el:2:1:Warning: Unused lexical variable `x'
  foo.el:4:11:Warning: reference to free variable `x'
  foo.el:4:17:Warning: assignment to free variable `x'

  In end of data:
  foo.el:8:1:Warning: the function `counter' is not known to be defined.
  Wrote foo.elc

When I ran the resulting byte-compiled code, I got an error as the manual c=
laims:

  $ emacs -Q -batch -l foo.elc
  Symbol's value as variable is void: x

Now I'm so confused. Here are my two questions:

  1. Why does this restriction exists? Is it inevitable because of some des=
ign decision of Emacs? Or is it temporary and removed in a (near) future re=
lease?
  2. Why does the original source code behave differently from its compiled=
 code?

Best regards,
Deokhwan Kim