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Re: Problem advising nreverse.

From: Sergei Organov
Subject: Re: Problem advising nreverse.
Date: Mon, 14 Dec 2009 18:05:23 +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:
>>>> 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.

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

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))
          (defadvice nreverse (after notice-nreverse activate)
            (setq last-nreverse-result ad-return-value))
      (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 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.

-- Sergei.

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