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Re: How can I undo a change from a modification-hook?

From: Stefan Monnier
Subject: Re: How can I undo a change from a modification-hook?
Date: Wed, 26 Apr 2017 08:26:13 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

> (by raising an error), and to allow changes when in a comment.  I can't
> find a good way to allow whitespace edits at the end of a line, though.
> I can detect such changes from the second call to the modification hook
> (during which it's easy to know whether the change is only adding or
> removing space at the end of a line), but I can't easily undo the changes
> and error out if they involve more than whitespace, can I?

Technically you can, but it's a bit messy and will likely not be
100% reliable.  Personally, in your case, I wouldn't bother doing it,
but here's how I'd do it otherwise:

- in the before-hook stash the value of buffer-undo-list somewhere.
  [ While you're at it, stash the beg/end there as well.  ]
- in the after-hook check the change (using beg/end/len) [ and while
  you're there, check that the beg/end/len is within the announces
  beg/end stashed earlier, and if it's not just don't do anything
  since it means the stash buffer-undo-list is likely unrelated.  ]
  and if it doesn't fit your criteria, call undo until you reach the
  previously stashed value of buffer-undo-list, and then signal the error.

-- Stefan

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