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* Surrounding Lexical Variable Reference in the Body of defun
@ 2012-08-21  6:07 Deokhwan Kim
  2012-08-22  6:42 ` Andreas Röhler
  2012-08-22 15:11 ` Stefan Monnier
  0 siblings, 2 replies; 5+ messages in thread
From: Deokhwan Kim @ 2012-08-21  6:07 UTC (permalink / raw)
  To: help-gnu-emacs

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


^ permalink raw reply	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2012-08-23 12:52 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2012-08-21  6:07 Surrounding Lexical Variable Reference in the Body of defun Deokhwan Kim
2012-08-22  6:42 ` Andreas Röhler
2012-08-22 15:11 ` Stefan Monnier
2012-08-23  2:56   ` justinhj
2012-08-23 12:52   ` Deokhwan Kim

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