Re: "Buttons" in a message buffer that evaluate to plain text?

Michael Heerdegen wrote:

>> With Emacs 30.2, what is the best practice to achieve that
>> plain text ↔ "buttons" conversion?  Is there an existing
>> ELPA package that does something similar?

> The simplest way to convert a region of buffer text into buttons is
> probably `buttonize-region' - see

>   (info "(elisp) Making Buttons")

> I don't know if there are more high-level tools that handle your use
> case are available.  Not so far in the past I needed something similar
> (a button toggling between different string values that are/get part of
> the resulting text) and wrote it by myself.

It was indeed rather simple:

| (while (re-search-forward "#\\([1-9a]\\) +\\(\\[ +\\]\\)" (point-max) t)
|   (let
|       ((choices (if (string= (match-string 1) "a")
|                     '("JA" "NEIN" "ANNULLIERUNG")
|                   '("JA" "NEIN" "ENTHALTUNG"))))
|     (make-button
|      (match-beginning 2)
|      (match-end 2)
|      'action (lambda (button)
|                (save-mark-and-excursion
|                  (let
|                      ((current-label (button-label button)))
|                    (if (string-match "\\`\\[ *\\([^ ]*\\) *\\]\\'" current-label)
|                        (replace-region-contents
|                         (+ (button-start button) 1)
|                         (- (button-end button) 1)
|                         (lambda
|                           nil
|                           (let*
|                               ((current-choice (match-string 1 current-label))
|                                (next-choice (if (string-empty-p current-choice)
|                                                 (car choices)
|                                               (or (cadr (member current-choice choices)) "")))
|                                (original-field-length (- (point-max) (point-min))))
|                             (concat
|                              (make-string (max (/ (- original-field-length (length next-choice)) 2) 0) ?\s)
|                              actual-choice
|                              (make-string (max (/ (- (+ original-field-length 1) (length next-choice)) 2) 0) ?\s))))))))))))

message-mode will just ignore overlays, so the sent message
contains the text only.  Very nice, thanks for the pointer.

One minor flaw: Replacing the button's "label" (i. e., the
text "[ ENTHALTUNG ]", etc.) will move point to either the
beginning or the end of the replaced text.  Aside from
replace-region-contents I also tried delete-char/insert with
overwrite-mode set to 'overwrite-mode-textual, but the
latter does not seem to have any effect to turn insert into
a "replace".

While I probably could do some arithmetics with point or
mark myself, any (in my case hypothetical) marker would
still be moved.

How can I actually replace the character/text at point
without deleting and inserting?

Tim