bug#9090: 24.0.50; void-variable jka-compr-verbose

bug#9090: 24.0.50; void-variable jka-compr-verbose

[lee]

This bug report will be sent to the Bug-GNU-Emacs mailing list
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Please write in English if possible, as the Emacs maintainers
usually do not have translators for other languages.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':



When starting emacs, I'm getting a warning "Warning: defvar ignored
because jka-compr-verbose is let-bound".  This leads to errors when
dealing with compressed files and gives backtraces like:

,----
| Debugger entered--Lisp error: (void-variable jka-compr-verbose)
|   jka-compr-insert-file-contents("/home/lee/tmp/emacs-backtrace.txt.gz" t nil nil nil)
|   apply(jka-compr-insert-file-contents ("/home/lee/tmp/emacs-backtrace.txt.gz" t nil nil nil))
|   jka-compr-handler(insert-file-contents "/home/lee/tmp/emacs-backtrace.txt.gz" t nil nil nil)
|   insert-file-contents("~/tmp/emacs-backtrace.txt.gz" t)
|   byte-code([...] [inhibit-read-only filename t insert-file-contents] 3)
|   find-file-noselect-1(#<buffer emacs-backtrace.txt.gz> "~/tmp/emacs-backtrace.txt.gz" nil nil "~/tmp/emacs-backtrace.txt.gz" (76027656 2430))
|   find-file-noselect("~/tmp/emacs-backtrace.txt.gz" nil nil t)
|   find-file("~/tmp/emacs-backtrace.txt.gz" t)
|   call-interactively(find-file nil nil)
`----

As a work-around, I've put (setq jka-compr-verbose t) into my ~/.emacs.



If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/local/share/emacs/24.0.50/etc/DEBUG.


In GNU Emacs 24.0.50.1 (x86_64-unknown-linux-gnu, X toolkit)
 of 2011-07-15 on yun
configured using `configure  '--with-x-toolkit=lucid''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: de_DE.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Summary

Minor modes in effect:
  show-paren-mode: t
  desktop-save-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  size-indication-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-x b RET ESC x g n u s RET RET C-x b e m TAB i TAB 
RET ESC O A ESC O D ESC O D ESC O D ESC O D ESC O D 
ESC O D ESC O D ESC O D ESC O D ESC O D ESC O D ESC 
O D ESC O D ESC O D ESC O D ESC O D ESC O D ESC O D 
C-@ ESC [ 4 ~ ESC w C-x b RET ESC O A ESC O A ESC [ 
7 ^ ESC y g C-g M SPC ESC < C-s C-y C-s C-s C-s C-s 
C-s C-g c o m p r e s C-s C-s C-s C-g C-g C-g ESC x 
b u g TAB DEL DEL DEL DEL DEL DEL DEL DEL DEL DEL DEL 
DEL DEL DEL r e p o TAB r TAB RET

Recent messages:
Generating summary...done
Mark set
Couldn't fetch article <87y6mmc79m.fsf@kzsu.stanford.edu>
Buffer is read-only: #<buffer *Summary mail.lists.bug-gnu-emacs*>
bunzip2ing 822.bz2...done
Parsing BBDB... (frobnicating...done)
Quit
Mark set
Quit [2 times]
Making completion list...

Load-path shadows:
None found.

Features:
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bug#9090: 24.0.50; void-variable jka-compr-verbose

[Glenn Morris]

Version: 24.1

lee wrote:

> When starting emacs, I'm getting a warning "Warning: defvar ignored
> because jka-compr-verbose is let-bound".

Fixed.

Recipe:

emacs -Q -l info

(info-insert-file-contents "/usr/share/info/emacs.gz")
M-x find-file RET /usr/share/info/emacs.gz RET

bug#9090: 24.0.50; void-variable jka-compr-verbose

[Lars Magne Ingebrigtsen]

Glenn Morris <rgm@gnu.org> writes:

>> When starting emacs, I'm getting a warning "Warning: defvar ignored
>> because jka-compr-verbose is let-bound".
>
> Fixed.

Sorry; my fault.  The binding/later-loading thing is pretty annoying in
general, though.

Has changing the way `defvar' works in the presence of let-bindings been
discussed? 

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

bug#9090: 24.0.50; void-variable jka-compr-verbose

[Glenn Morris]

Lars Magne Ingebrigtsen wrote:

> The binding/later-loading thing is pretty annoying in general, though.

A general solution would be nice, yes. I guess if it were easy to do,
though, the code would do that rather than just emit a warning.

> Has changing the way `defvar' works in the presence of let-bindings been
> discussed? 

Can't remember.

defvar inside let-binding (was: bug#9090: 24.0.50; void-variable jka-compr-verbose)

[Stefan Monnier]

>>> When starting emacs, I'm getting a warning "Warning: defvar ignored
>>> because jka-compr-verbose is let-bound".
>> Fixed.
> Sorry; my fault.  The binding/later-loading thing is pretty annoying in
> general, though.

Yes.

> Has changing the way `defvar' works in the presence of let-bindings been
> discussed? 

No.  Personally I can't see how to make it work, because by the time we
get to defvar it's kind of too late.  Maybe we could try to change the
let-binding so that the "saved void state" (which will be restored at
the end of the outer let) gets changed as if the var had been defvarred
before the let-binding.  But it sounds terribly hackish.

Of course, another problem is that in the case of defcustom we don't
even detect the problem.


        Stefan

Re: defvar inside let-binding

[Lars Magne Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Has changing the way `defvar' works in the presence of let-bindings been
>> discussed? 
>
> No.  Personally I can't see how to make it work, because by the time we
> get to defvar it's kind of too late.  Maybe we could try to change the
> let-binding so that the "saved void state" (which will be restored at
> the end of the outer let) gets changed as if the var had been defvarred
> before the let-binding.  But it sounds terribly hackish.

That was exactly what I was thinking should happen.  :-)

I mean, if you say

(let ((foo-var nil))
  (foo-function))

and `foo-function' is autoloaded -- I can't think of a single use case
where you'd want `foo-var' to be unbound afterwards.

That `foo-function' is autoloaded or a real function is something that
should (in my opinion) have no effect on what you'd end up with after
calling it.

Leaving `foo-var' unbound is what is the hack, I think.  We should
strive to end up with the same result after calling `foo-function', no
matter whether it's autoloaded or not.

> Of course, another problem is that in the case of defcustom we don't
> even detect the problem.

I'm not sure I follow you there...

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

Re: defvar inside let-binding

[Stefan Monnier]

>> Of course, another problem is that in the case of defcustom we don't
>> even detect the problem.
> I'm not sure I follow you there...

defcustom works differently from defvar and doesn't use defvar
(e.g. because it may not even define a real variable: the "var" may
simply only be available through accessor/setter functions) and since
the current code that detects a "defvar within let" is written in C code
in Fdefvar, it doesn't apply to defcustom.


        Stefan

Re: defvar inside let-binding

[Lars Magne Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> defcustom works differently from defvar and doesn't use defvar
> (e.g. because it may not even define a real variable: the "var" may
> simply only be available through accessor/setter functions) and since
> the current code that detects a "defvar within let" is written in C code
> in Fdefvar, it doesn't apply to defcustom.

Right.

But does

(let ((foo nil))
  (foo-function))

leave `foo' unbound if `foo' is a defcustom?

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

Re: defvar inside let-binding

[Stefan Monnier]

> But does

> (let ((foo nil))
>   (foo-function))

> leave `foo' unbound if `foo' is a defcustom?

Yes, just like with defvar.  The difference is that defvar detects the
problem and outputs a warning.


        Stefan

Re: defvar inside let-binding

[Lars Magne Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Yes, just like with defvar.  The difference is that defvar detects the
> problem and outputs a warning.

Ah, ok.  So if we decide to change the way let-bindings affect defvar
when we load files, then we have to basically implement the same
detection for defcustom.

Which we probably should do anyway.  :-)

It's this code, right?

      if (NILP (tem))
	Fset_default (sym, eval_sub (Fcar (tail)));
      else
	{ /* Check if there is really a global binding rather than just a let
	     binding that shadows the global unboundness of the var.  */
	  volatile struct specbinding *pdl = specpdl_ptr;
	  while (--pdl >= specpdl)
	    {
	      if (EQ (pdl->symbol, sym) && !pdl->func
		  && EQ (pdl->old_value, Qunbound))
		{
		  message_with_string ("Warning: defvar ignored because %s is let-bound",
				       SYMBOL_NAME (sym), 1);
		  break;
		}
	    }
	}

Since `defcustom' seems to be purely Emacs Lisp, and I don't think this
data is available on the Emacs Lisp level (am I wrong?), perhaps just
creating a C-level function `symbol-let-bound-p' (or something) would
allow us to issue the warning, as a first step?
        
-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

Re: defvar inside let-binding

[Stefan Monnier]

> Since `defcustom' seems to be purely Emacs Lisp, and I don't think this
> data is available on the Emacs Lisp level (am I wrong?), perhaps just
> creating a C-level function `symbol-let-bound-p' (or something) would
> allow us to issue the warning, as a first step?
        
Yes.  Tho the "next" step you suggested (to cause defvar to change the
outer let-binding) will require another such function and will make
symbol-let-bound-p unneeded.


        Stefan

Re: defvar inside let-binding

[Lars Magne Ingebrigtsen]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Yes.  Tho the "next" step you suggested (to cause defvar to change the
> outer let-binding) will require another such function and will make
> symbol-let-bound-p unneeded.

Let's see...

  ;; Use defvar to set the docstring as well as the special-variable-p flag.
  ;; FIXME: We should reproduce more of `defvar's behavior, such as the warning
  ;; when the var is currently let-bound.
  (if (not (default-boundp symbol))
      ;; Don't use defvar to avoid setting a default-value when undesired.
      (when doc (put symbol 'variable-documentation doc))
    (eval `(defvar ,symbol nil ,@(when doc (list doc)))))

So what I would do here now would be to add

(when (symbol-let-bound-p symbol)
  (message "Warning: let-bound etc"))

now, and in Emacs 24.2, I'd change that to

(when (symbol-let-bound-p symbol)
  (change-outer-let-binding symbol default))

Or something along those lines?  

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/

Re: defvar inside let-binding

[Stefan Monnier]

>> Yes.  Tho the "next" step you suggested (to cause defvar to change the
>> outer let-binding) will require another such function and will make
>> symbol-let-bound-p unneeded.

> Let's see...

>   ;; Use defvar to set the docstring as well as the special-variable-p flag.
>   ;; FIXME: We should reproduce more of `defvar's behavior, such as the warning
>   ;; when the var is currently let-bound.
>   (if (not (default-boundp symbol))
>       ;; Don't use defvar to avoid setting a default-value when undesired.
>       (when doc (put symbol 'variable-documentation doc))
>     (eval `(defvar ,symbol nil ,@(when doc (list doc)))))

> So what I would do here now would be to add

> (when (symbol-let-bound-p symbol)
>   (message "Warning: let-bound etc"))

> now, and in Emacs 24.2, I'd change that to

> (when (symbol-let-bound-p symbol)
>   (change-outer-let-binding symbol default))

> Or something along those lines?  

Yes, although the above code is run after the var has been set,
i.e. too late.


        Stefan