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RE: address@hidden: RE: weird defadvicebugwithbyte-compilation]

From: Drew Adams
Subject: RE: address@hidden: RE: weird defadvicebugwithbyte-compilation]
Date: Sun, 11 Dec 2005 22:44:15 -0800

        > It does, as far as I can tell. I don't understand all of it,
        > however, I admit. I still don't understand, for instance:
        > 1. why byte-compiling the defadvice in bar.el would eval
        >    my-mode (my-mode does not even appear in bar.el)

        I think it doesn't eval it, it just sees that my-mode is not bound.
        The warning says "reference to free variable `my-mode'", see the
        backtrace.  This is a standard warning from the byte compiler, it is
        meant to help you detect typos in variable names.

    OK. I was confused because it was the byte-compilation that led
    ultimately to x-create-frame raising an unbound var error.

        > 2. why evaling the defadvice in foo.el would eval my-mode
        >    (the BODY of defadvice is not supposed to be quoted)

        Because defadvice byte-compiles the function it creates on
        the fly, I guess.

    But it is only the defadvice in bar.el, not the defadvice in
    foo.el, that has the keyword `compile'.

    And even if it does byte-compile foo.el on the fly (for
    whatever reason), why would it eval my-mode - the body of a
    defadvice is not supposed to need
    to be quoted (it is not evaled by defadvice).

    That is, I don't understand why a byte-compiler warning of a potentially
    unbound variable would lead to a *Backtrace* being created - a
    warning is not an error, and even byte-compiler errors (as opposed to
    warnings) do not result in a *Backtrace*.

        > 5. why selecting (require 'foo) and doing eval-region does
        >    not manifest the bug, but putting the cursor after
        >   (require 'foo) and doing `C-x C-e' does manifest the bug.

        Because, by default, eval-expression-debug-on-error is t, and it
        affects C-x C-e.  If I set eval-expression-debug-on-error to nil,
        Emacs behaves with C-x C-e like it does with eval-region: it doesn't
        pop up the *Backtrace* buffer in a separate frame, and the
        bug doesn't happen.

    Ah - thanks. That makes sense, given your explanation of the bug being
    manifested in x-create-frame.

    I don't understand, however, your statement that when the byte compiler
    tries to display special-buffer *Compile Log* in a separate
    frame "it hits
    the problem with the void variable again". What is the "problem with the
    void variable"? For the byte-compiler it was only a question of
    displaying a
    warning, there was no error to be raised. Why would display of
    *Compile Log*
    in a separate frame raise an unbound-variable error?

Nevermind; I understand now what happened (wrt the unbound var, not the
crash bug). I was confused about who was raising the unbound var error, but
I see now that x-create-frame was doing so because the menu-enable property
evaled it for a menu-bar menu when x-create-frame tried to create the
*Compile Log* frame (which has a menu-bar).

This makes me wonder now if byte-compiling in defadvice (i.e. on the fly)
should display a *Compile Log* buffer at all. I don't know. In any case,
that's clearly the cause of the problem here (but not the explanation of the
crash bug).

I still have a question, however, about how best to use define-minor-mode to
define the mode variable so that it can be used in a put 'menu-enable. Is it
necessary to do the define-minor-mode before doing the put? I guess so; but
in that case, I prefer the old method of defining a minor-mode function and
its variable (defcustom) separately. The problem is the same, but I always
place variable definitions at the top, and such a defcustom would be defined
before the  variable was used in the put 'menu-enable. In the new system, I
was calling the minor-mode function first thing (in .emacs), to set the
variable, but that function couldn't be called until its defining library
was loaded, and that meant that the variable wasn't defined when the
*Compile Log* was displayed when byte-compiling a defadvice in the file.

Moral (for me): Either don't use define-minor-mode at all or use it first
thing in a file, before you make any references (even quoted references,
like here) to its mode variable.

Thanks for helping me understand, and good luck with the crash bug. - Drew

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