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/software/emacs/manual/html_node/elisp/Lexical-Binding.html>:
the code in the body of a defun or defmacro cannot refer to surrounding lexical
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
misunderstand what the sentence really meant. So I decided to make some
experiments 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 warning
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
claims:
$ 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 design
decision of Emacs? Or is it temporary and removed in a (near) future release?
2. Why does the original source code behave differently from its compiled
code?
Best regards,
Deokhwan Kim