bug#15925: 24.3.50; error when customizing whitespace-display-mappings

[Claudio Bley <claudio.bley@googlemail.com>]

At Tue, 19 Nov 2013 03:24:02 -0500,
Glenn Morris wrote:
> 
> Claudio Bley wrote:
> 
> > I think the regexp should be changed to "\\`\(.\|\n\)\\'" to allow a
> > single newline also.
> 
> We already tried that once; it caused more problems than it solved:
> http://debbugs.gnu.org/2689

I see. So lets get this solved.

At first, changing the regexp is just the right thing to do and
in itself, IMO, does not break anything.

Alas, it does not solve the problem at hand because
`widget-specify-field' behaves /badly/ when the field ends with a
newline.

So, actually, this hunk of your patch trying to work around
this behaviour broke other things.

How about using a special :value-create function for character fields?

Furthermore, the presentation of the character values is bad,
usability-wise, because for nonprintable chars you cannot see what
value the field actually has.

How about displaying those chars in the buffer in another format?
E.g. use escape sequences like \n \r et cetera?

I (rather hackish) way to fix this would be to always insert a ?\n
as part of the value of a character field:

(defun widget-field-char-create (widget)
  "Create an editable character field."
  (let ((value (widget-get widget :value))
	(from (point))
	;; This is changed to a real overlay in `widget-setup'.  We
	;; need the end points to behave differently until
	;; `widget-setup' is called.
	(overlay (cons (make-marker) (make-marker))))
    (widget-put widget :field-overlay overlay)
    (insert value)
    (insert ?\n) ;; EEEK
    (unless (memq widget widget-field-list)
      (setq widget-field-new (cons widget widget-field-new)))
    (move-marker (cdr overlay) (point))
    (set-marker-insertion-type (cdr overlay) nil)
    (insert ?\n)
    (move-marker (car overlay) from)
    (set-marker-insertion-type (car overlay) t)))

What do you think?
-- 
Claudio