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Re: Help needed with defadvice


From: Alex Kost
Subject: Re: Help needed with defadvice
Date: Sat, 23 Nov 2013 01:56:53 +0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Perry Smith (2013-11-22 19:50 +0400) wrote:

> On Nov 22, 2013, at 1:09 AM, Alex Kost <address@hidden> wrote:
>
>> Perry Smith (2013-11-22 06:45 +0400) wrote:
>> 
>>> I have this defadvice:
>>> 
>>> (defadvice get-buffer-create (around inherit activate)
>>>  (let ((set-list (mapcar '(lambda ( v )
>>>                          (cons v (symbol-value v)))
>>>                       inherited-alist)))
>>>    (with-current-buffer ad-do-it 
>>>      (mapcar '(lambda ( c )
>>>              (message "Setting %s to %s inside %s"
>>>                       (car c) (cdr c) (buffer-name (current-buffer)))
>>>              (set (car c) (cdr c)))
>>>           set-list))))
>>> 
>>> inherited-alist is a list of symbols that I add to.  When a buffer is 
>>> created, I run through the list of variables and get their values as seen 
>>> from the current buffer.  I then call get-buffer-create (via ad-do-it) and 
>>> do a set on each of the variables.  The "message" is there just for 
>>> debugging.  I get the messages like I expect .... e.g. "Setting foo to dog 
>>> inside cat.c" or whatever.  All the symbols in inherited-alist are 
>>> buffer-local variables.
>
> To Eric: changing let to let* had not effect.
>
>> 
>> 1. With `set' you set a global value, you probably want
>> (set (make-local-variable (car c)) (cdr c)) instead of
>> (set (car c) (cdr c)).
>
> No change.  If this helps, when I do ^h-v cscope-out-buffer I get:
>
> cscope-out-buffer is a variable defined in `cscope.el'.
> Its value is "banana"
>
>   Automatically becomes buffer-local when set.
>
> Documentation:
> Buffer associated with the cscope process
>
> "banana" is the global I set with:
> (set-default 'cscope-out-buffer "banana") in *scratch*
>
> I'm coming to the conclusion that something after get-buffer-create
> is blasting the local variables back to their defaults.  (By the
> way, I have added another message after the set and it
> shows that it is being set.  I've also explicitly set one of these
> variables outside of the let and it is still not being set.

It looks like something interferes, because your initial advice works
for buffer-local variables.

Steps to reproduce:

1. emacs -q
2. Insert this into *scratch* buffer and eval it:

(defvar inherited-alist '(v1))
(defvar v1 nil)
(make-variable-buffer-local 'v1)
(set 'v1 123)

(defadvice get-buffer-create (around inherit activate)
  (let ((set-list (mapcar '(lambda ( v )
                             (cons v (symbol-value v)))
                          inherited-alist)))
    (with-current-buffer ad-do-it 
      (mapcar '(lambda ( c )
                 ;; (message "Setting %s to %s inside %s"
                 ;;          (car c) (cdr c) (buffer-name (current-buffer)))
                 (set (car c) (cdr c)))
              set-list))))

After evaluating you get `v1' variable with nil as global value and 123
as local value in *scratch* buffer.

3. Create new buffer with "C-x b" and the local value of v1 will be 123
there.

4. Switch to a buffer where v1 is nil (for example with "C-h e") and
create new buffer again.  And it will inherit nil for v1.




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