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Re: bug#3688: Also need (auto-fill-mode nil) to trigger this bug

From: Alan Mackenzie
Subject: Re: bug#3688: Also need (auto-fill-mode nil) to trigger this bug
Date: Mon, 29 Jun 2009 21:37:11 +0000
User-agent: Mutt/1.5.9i

Hi, Kees!

On Mon, Jun 29, 2009 at 11:18:10AM +0200, Kees Bakker wrote:
> On Monday 29 June 2009, Stefan Monnier wrote:

> > >   (auto-fill-mode nil)

> > All minor modes are turned on with a positive arg and turned off with
> > a negative arg.  A nil argument may toggle it, but that might change
> > in the future.  A `toggle' argument toggles most (but not all) minor
> > modes.

> OK. I didn't know that (and I'm using emacs for almost two decades
> now). Still I want to express my dislike about the way a "nil" arg is
> handled.

OK, but look on the bright side - your confusion led directly to a bug
being fixed.  Your discomfort for everybody's gain.  Think of yourself as
a hero, not a victim.  ;-)

> First, is 0 a positive arg? Some people might think so (I do).

I don't think so, and I'm a graduate mathematician.  But I still remember
long unresolved discussions amongst maths people about whether zero is
positive, and sometimes terms like "strictly positive" and "non-negative"
were used.  Trouble is, "strictly positive" is a bit of a mouthful, and
"non-negative" is a bit, well, negative - do we really want to describe
something by what it isn't rather than what it is?

> It is a bit confusing.  (auto-fill-mode 0) will turn the mode off, so
> in that sense a zero is not a positive arg. To summarize

>    (auto-fill-mode)         ;;; toggles
>    (auto-fill-mode 0)       ;;; turn off
>    (auto-fill-mode nil)     ;;; toggles
>    (auto-fill-mode -1)      ;;; turn off
>    (auto-fill-mode 1)       ;;; turn on
>    (auto-fill-mode t)       ;;; turn on

(auto-fill-mode 0) shouldn't be troublesome to a C hacker.  :-)

> Toggling for a "nil" argument does not make sense to me.

`auto-fill-mode' is also an interactive command, and as such it has an
&optional argument.  The Right Thing to do with "M-x auto-fill-mode" is
to toggle.  The omitted argument becomes nil, hence the the above is
(auto-fill-mode nil) in lisp.  It does make sense, sort of.  But I admit,
I sometimes find it confusing too.

Actually, looking at the doc string for this command:

    Toggle Auto Fill mode.
    With ARG, turn Auto Fill mode on if and only if ARG is positive.
    In Auto Fill mode, inserting a space at a column beyond
    `current-fill-column' automatically breaks the line at a previous

, it is incomplete in that it doesn't specify what happens when ARG isn't
positive.  Maybe this would be better:

    Turn AutoFill mode on if ARG is positive, turn it off if ARG is zero
    or negative, toggle it if ARG is nil (or absent).

> Kees

Alan Mackenzie (Nuremberg, Germany).

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