Re: Editing change groups

[Joost Kremers <joostkremers@fastmail.fm>]

On Fri, Nov 09, 2018 at 08:52:16AM +0100, Michael Heerdegen wrote:
> Hello,
> 
> I have the following requirement for a user interface: if the user hits
> a key, something in the buffer is replaced with something else.  Doing
> this potentially involves several editing operations.  If the user hits
> the same key again, the old contents are restored.
> 
> My question is: are change groups, as described in (info "(elisp) Atomic
> Changes"), a suitable tool to do this?

From the description in the manual, it would seem they're not: you can only finish the change group once, so once the change has been accepted (and becomes visible for the user), they cannot be undone as a group.

The purpose of atomic change groups seems to be to make changes that can be undone as a group when an error occurs *while the changes are being made*. You want to be able for a group of editing operations to be undone as a group.

>  AFAIK it works, but I must admit
> I don't understand the implementation of change groups.  I tried to use
> an example like
> 
> (defun my-test-change-groups ()
>   (interactive)
>   (insert "0\n")
>   (let ((g (prepare-change-group)))
>     (insert "a\n")
>     (activate-change-group g)
>     (insert "b\n")
>     (insert "c\n")
>     (cancel-change-group g)))
> 
> to see how things work but that doesn't work as I expected: calling the
> command leaves all insertions intact as if the change group G hadn't
> been canceled.

That's something I cannot make sense of. Looks like a bug, though might very well not be.

>  OTOH, if I remove the first insertion (insert "0\n")
> from the example, calling the function causes nothing to be inserted at
> all.

Because the change group is being undone. ;-) Try adding an insert after the cancel-change-group:

```
(defun my-test-change-groups ()
  (interactive)
  (let ((g (prepare-change-group)))
    (insert "a\n")
    (activate-change-group g)
    (insert "b\n")
    (insert "c\n")
    (cancel-change-group g)
    (insert "d\n")))
```

You'll see "d" inserted in the buffer.

-- 
Joost Kremers
Life has its moments