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Re: Support for undo-amalgamate in a version of the atomic-change-group

From: Campbell Barton
Subject: Re: Support for undo-amalgamate in a version of the atomic-change-group macro (with patch)
Date: Tue, 2 Nov 2021 12:04:33 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.1

On 11/2/21 04:43, Stefan Monnier wrote:
Hi, there have been a handful of times I've needed to treat multiple
actions as a single action,
others too (see stackoverflow question [0]).

While this is supported using change groups, the resulting macro ends
up being nearly identical to the existing `atomic-change-group' macro.
The only difference is a call to undo-amalgamate-change-group.

The other difference would be the call to `cancel-change-group` (you
may want to not undo the changes in case of errors).

Good point (updated my answer)

Attached a patch to add `atomic-change-group-undo-amalgamate' so
packages don't need to use a modified copy of this macro.

I suspect that we'd be better off with a separate macro which only does
the amalgamation without the cancel-change-group-on-error.

Agree, if the cancel group is removed it's no longer atomic and can be a separate macro.

But I wonder how often such a macro would be useful: undo boundaries are
normally added only by the command-loop (i.e. the loop which repeatedly
reads a key and calls the corresponding command), so unless your macro's
body calls `recursive-edit` it's very unlikely that you'll get any undo
entries that you would then want to remove.

I'm not sure why most other people are running into this (the original question relates specifically voice commands), nevertheless the stats on the stackexchange question lead me to think it's not so unusual to spawn multiple commands that happen to be treated as separate undo steps.

(Re)reading your stackexchange question, it seems you want to amalgamate
undo entries from different commands.  So do you run those commands
within a `recursive-edit`?  If not, how do you manage to run those
commands from within your (new) macro?

As far as I know I'm not using recursive edit (unless indirectly).

In these situations collapsing undo data was necessary.

- Calling kmacro-exec-ring-item (based on this answer [0])
- Calling some evil functions for e.g.

  (defun evil-paste-and-indent-after ()
      (evil-paste-after 1)
      (evil-indent (evil-get-marker ?\[)
                   (evil-get-marker ?\]))))

[0]: https://emacs.stackexchange.com/questions/70


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