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Re: Misery with undo. Help, please!

From: Alan Mackenzie
Subject: Re: Misery with undo. Help, please!
Date: Tue, 21 Oct 2008 20:34:45 +0000
User-agent: Mutt/1.5.9i

Hi, Andreas!

On Mon, Oct 20, 2008 at 11:49:23PM +0200, Andreas Schwab wrote:
> Alan Mackenzie <address@hidden> writes:

> > [*] Actually, it inserts an extra, spurious, nil too, or I've badly
> > misunderstood something.  This seems to be a bug in the macro `push'.

> No, it isn't.  The command loop automatically inserts an undo boundary.
> You should use ielm for your experiments.

Ah, thanks!  How come I didn't know about ielm?

I understand what's been happening, now.  While primitive-undo is busily
pulling stuff off pending-undo-list, it's piling stuff back onto
buffer-undo-list.  That's not obvious from looking at the source code.

The problem, with a buffer-undo-list of (nil 6 nil ......) is that
primitive-undo piles nothing onto it, merely moving point to 6.
buffer-undo-list thus remains unaltered after C-_.  This is a bug.

There seem two alternative ways to fix this:
(i) In the Elisp manual declare a buffer-undo-list of this form as
  malformed, and not to be passed to `undo'.
(ii) In `undo', check for (not (car buffer-undo-list)), and strip off
  the initial nil from b-u-l.

I think (ii) is better.  What do others think?

> Andreas.

Alan Mackenzie (Nuremberg, Germany).

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