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Re: Problem advising nreverse.


From: Sergei Organov
Subject: Re: Problem advising nreverse.
Date: Mon, 14 Dec 2009 19:01:14 +0300
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.2 (gnu/linux)

address@hidden (Pascal J. Bourguignon) writes:

> 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.
> (do-stuff-with-new-nreverse)
>
> ;; When done:
> (setf (symbol-function 'nreverse)  (symbol-function 'old-nreverse))

I'm not now sure, but I think I've tried replacing defadvice with
re-defining of the function, but it worked (didn't work) exactly the
same way as defadvice.

>> 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.
>

You see, I wanted automatic elisp-only solution to "fixing"
ewoc-collect, but only if it indeed happens to be buggy. Manual
reloading is not an option then. Alternate approach would be to execute
simple test-case to determine if ewoc-collect is indeed buggy and then
re-define it to the correct version if so. That's what I'm going to
implement now, provided there doesn't seem to be a way to do what I want
by means of advising ewoc-collect and nreverse.

>
>> 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
> file!

I do know, but I wanted lisp-only, entirely automated solution.

-- Sergei.





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