Re: Fill column indicator functionality

[Eli Zaretskii]

> Date: Fri, 5 Apr 2019 20:11:33 +0200
> From: Ergus <address@hidden>
> Cc: address@hidden
> 
> >  (if (eq (aref (car (internal-char-font nil ?│)) 0)
> >          w(face-font 'default))
> >       ;; can use the default font for U+2502
> >
> >(But we should use the actual value of
> >display-fill-column-indicator-char instead of the literal character.)
> >
> >WDYT?
> 
> Actually this is what we have now:
> 
> (define-minor-mode display-fill-column-indicator-mode
>   "Toggle display fill column indicator.
> This uses `display-fill-column-indicator' internally.
> 
> To change the position of the column displayed by default,
> customize `display-fill-column-indicator-column' you can change the
> character for the indicator setting 
> `display-fill-column-indicator-character'."
>   :lighter nil
>   (if display-fill-column-indicator-mode
>       (progn
>         (setq display-fill-column-indicator t)
>         (unless display-fill-column-indicator-character
>           (if (char-displayable-p ?\u2502)
>               (setq display-fill-column-indicator-character ?\u2502)
>             (setq display-fill-column-indicator-character ?|))))
>     (setq display-fill-column-indicator nil)))
> 
> 
> I think we only need to extend the inner if with your condition, but I
> got an error because (internal-char-font nil ?│) always return nil in a
> terminal and aref has issues with that.

For terminal frames, char-displayable-p is okay and should be left
alone as the only test there.  The test I proposed is only needed for
GUI frames.

> In a gui I get this error:
> 
> Debugger entered--Lisp error: (wrong-type-argument arrayp #<font-object 
> "-1ASC-Liberation Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1">)
>   aref(#<font-object "-1ASC-Liberation 
> Mono-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1"> 0)
>   eval((aref (car (internal-char-font nil 9474)) 0) nil)

Oops, sorry, I copied the wrong expression.  This is the correct one:

  (aref (query-font (car (internal-char-font nil ?│))) 0)

Btw, there could be a problem when the mode is turned on on a GUI
frame, where '│' can be displayed OK, and then a TTY frame is created
in the same session, where that character cannot be displayed.  I
wonder how to deal with this issue.