Re: Integration of undo-tree in Emacs

[Toby Cubitt <tsc25@cantab.net>]

On Wed, May 28, 2014 at 10:08:08PM -0400, Stefan Monnier wrote:
> > Toby, are there other reasons undo-tree needs to transfer undo
> > elements from the buffer-undo-list to its own data model?
> 
> Toby's position is that the undo data should be kept as a tree, not as
> a list.  That makes a lot of sense: For every branch in a tree, the
> undo-list keeps 2 bundles (one going forward and the other going back),
> but one of the two is always redundant, so the representation
> is inefficient.  Of course, this inefficiency only applies to the
> *branches*, i.e. only for those elements generated by `undo', so this is
> irrelevant as long as most of the changes are not undos.

Just to be clear, I'm not strongly advocating changing Emacs' undo data
structures. I'm just pointing out that *if* you're going to make
substantial changes to the undo system for other reasons, you might as
well consider whether changing the data structures would be useful too.

I can see a number of arguments against changing Emacs' undo model, not
least that `buffer-undo-list' is documented in the Elisp manual so is
part of the Elisp API that packages may rely on. (I very occasionally get
reports that undo-tree is incompatible with some package, for this
reason.)

Toby
-- 
Dr T. S. Cubitt
Royal Society University Research Fellow
Fellow of Churchill College, Cambridge
Centre for Quantum Information
DAMTP, University of Cambridge

email: tsc25@cantab.net
web:   www.dr-qubit.org