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Re: Problem advising nreverse.

From: Pascal J. Bourguignon
Subject: Re: Problem advising nreverse.
Date: Mon, 14 Dec 2009 16:23:52 +0100
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/22.3 (darwin)

Sergei Organov <address@hidden> writes:

> address@hidden (Pascal J. Bourguignon) writes:
>> Sergei Organov <address@hidden> writes:
>>> address@hidden (Pascal J. Bourguignon) writes:
>>>> Sergei Organov <address@hidden> writes:
>>>>> Hello,
>>>>> It seems that an advice set for `nreverse' function fails to be called
>>>>> when `nreverse' is called from a byte compiled function:
>>> [...]
>>>>> Is it bug or feature? What's going on here? 
>>>> Indeed, it is a feature.
>>>> Advices are not available from several call points:
>>>> 1- when the advised function is called from C code.
>>>> 2- when the advised function is a an opcode of the virtual machine.
>>>>    You can observe the difference between the two primitives nreverse
>>>>    and buffer-name for example, with:     
>>>>      (disassemble (byte-compile (lambda (x) (nreverse x))))
>>>>    vs.:
>>>>      (disassemble (byte-compile (lambda (x) (buffer-name x))))
>>>>    In the former case, nreverse is a byte code, and therefore no
>>>>    advice applies.  In the later case, buffer-name is called with the
>>>>    call byte code, which will go thru the advice.
>>>> Notice that if you really want to advice such a low level primitive
>>>> function as nreverse, you can replace it with a lisp function (and
>>>> recompile all the code that uses it).
>>> Thanks a lot for the explanations and suggestion.
>>> Unfortunately this is not an option as intention was to advise
>>> `nreverse' during calls to a possibly buggy `ewoc-collect' function that
>>> for a long time errorneously called `nreverse' at the end (fixed since
>>> 2008).
>> Redefining nreverse and reloading the ewoc-collect function would help
>> as indicated...
> Yeah, but:
> 1. I don't really want `nreverse' to be always redefined.

Of course.  You do that only while "debugging" ewoc-collect.  Then you
can either put back the old nreverse, or reboot emacs.

(require 'cl)
(setf (symbol-function 'old-nreverse) (symbol-function 'nreverse))
(defun nreverse (&rest arguments) 
   (message "nreverse called with %S" arguments)
   (funcall old-nreverse arguments))
;; possibly add an advice, but you can as well put the code in the 
;; new definition of nreverse above.

;; When done:
(setf (symbol-function 'nreverse)  (symbol-function 'old-nreverse))

> 2. I don't know how to reload `ewoc-collect' (from elisp) provided it's
>    already byte-compiled.

C-h f ewoc-collect RET C-x o TAB RET

should bring you to the file where ewoc-collect is defined.  Then you
can load it, or just re-evaluate the ewoc-collect definition to have
it take into account the new nreverse.  If you want to debug it while
compiled, you can byte-compile it too.

> Here what I've actually originally tried (that didn't work):
> (defadvice ewoc-collect (around fix-ewoc-collect activate)
>   "Fix buggy `ewoc-collect' by reversing its result provided it
>    was altered by `nreverse'."
>   (let ((last-nreverse-result))
>     (unwind-protect
>         (progn
>           (defadvice nreverse (after notice-nreverse activate)
>             (setq last-nreverse-result ad-return-value))
>           ad-do-it)
>       (ad-unadvise 'nreverse))
>     (if (eq last-nreverse-result ad-return-value)
>         (setq ad-return-value (nreverse ad-return-value)))))
> This starts to work as soon as I re-evaluate the (defun ewoc-collect
> ...) in the ewoc.el file

Then you know where ewoc-collect is defined, you can load this ewoc.el

>  after the above defadvice is evaluated and
> continues to work even if I byte-compile the (defun ewoc-collect...)
> after that (though I still don't understand why it works after
> byte-compiling), but then it's both easier and cleaner to just re-define
> ewoc-collect to a correct version.


__Pascal Bourguignon__

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