Re: disabling undo boundaries

[Phillip Lord]

Stefan Monnier <address@hidden> writes:

>> I think that the current undo-boundary behaviour wrt a single buffer
>> makes sense. It's the fact that inserting content into *that* buffer
>> forces an undo-boundary into *this* buffer.  I don't know why Emacs
>> does this.
>
> I think the reason is not very deep: for reasons of efficiency (back in
> the days when 8MB was a lot of memory), Emacs keeps track of a few
> undo-related pieces of information in global variables, so it can only
> handle a single buffer at a time.  If a command touches some other
> buffer, the C code has to choose between "just switch" and "push
> a boundary and then switch", and the choice it makes is to err on the
> "safe" side of adding a potentially unnecessary undo-boundary rather
> than risking to let the undo-log grow without intervening boundaries.


I meant "deep" in the sense of "deep in version control history" -- or
in this case, before the version control system. Still, I didn't know
this was the reason.

Arguing against this are my experiments which suggest that
undo-boundaries are, in practice, rarely inserted in this way (about 1%
of times these functions are called, although, of course, it will depend
on how you are using Emacs).


> In your cases, this is complicated by the fact that you're concerned
> about self-insert-command which is very special in that it was the only
> command that doesn't push a boundary every time, but only once every
> N repetitions.


I am slightly concerned about this, but not overly concerned. The main
thing that I am worried about is here:

https://github.com/phillord/lentic/issues/20

where, for example, a single "fill-paragraph" command takes many undos
to undo because of multiple changes in *another* buffer.

Anyway, the patch is there. Any chance? If not, anything (further) that
I can add (or remove!) to the branch to make it happen?

Failing that, I will have to think about how to remove the undo-boundary
post-hoc, or give up and accept that lentic will interfere with undo.
Haven't decided which yet.

BTW, I did a demonstration at a conference last, which pitted Emacs
(lagging badly on a badly overloaded netbook) with CIDER, and lentic
against several slick websites. We won the best demo prize. You have to
laugh...

Phil