* 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
* Re: Surrounding Lexical Variable Reference in the Body of defun
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
1 sibling, 0 replies; 5+ messages in thread
From: Andreas Röhler @ 2012-08-22 6:42 UTC (permalink / raw
To: help-gnu-emacs
Am 21.08.2012 08:07, schrieb Deokhwan Kim:
> 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
>
as this question isn't raised first time, maybe Emacs developers consider to revert that change?
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Surrounding Lexical Variable Reference in the Body of defun
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
1 sibling, 2 replies; 5+ messages in thread
From: Stefan Monnier @ 2012-08-22 15:11 UTC (permalink / raw
To: help-gnu-emacs
> 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?
It is because of a technical limitation in the byte-compiler's handling
of dynamically loaded docstrings.
You can work around this by replacing your defun
with (defalias counter (lambda () (setq x (1+ x)))).
I believe this is fixed in the Emacs trunk (where defun is just a macro
that expands to a defalias+lambda and where the docstrings handling was
improved accordingly).
Stefan
^ 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
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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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