Re: Fill column indicator functionality

[Ergus <spacibba@aol.com>]

Hi Eli:

On Fri, Apr 05, 2019 at 06:28:40PM +0300, Eli Zaretskii wrote:
>> Date: Fri, 05 Apr 2019 16:06:17 +0300
>> From: Eli Zaretskii <eliz@gnu.org>
>> Cc: emacs-devel@gnu.org
>>
>> Once you can reproduce the problem, I suggest to use the macro
>> FACE_FOR_CHAR to find the ID of a face suitable for displaying the
>> indicator character, instead of using DEFAULT_FACE_ID.  You can find
>> examples of using this macro elsewhere in xdisp.c.
>
>On second thought, I think using FACE_FOR_CHAR would produce
>unpleasant results.  If the font required to display U+2502 is not the
>default font, it could be higher than the default font, which will
>cause the line height become larger on those screen lines where the
>indicator is displayed.  The result will cause complaints.
>
I have a version already implemented with this solution and I don't
like it for the reasons you expose.

>So I think we should insist on being able to display the indicator
>character by the default face's font.  Whether this is possible could
>be tested by comparing the font returned by internal-char-font and the
>default face's font returned by face-font.  Like this:
>
>  (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.

Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
  aref(nil 0)
  eval((aref (car (internal-char-font nil 9474)) 0) nil)
  elisp--eval-last-sexp(nil)
  #f(compiled-function (eval-last-sexp-arg-internal) "Evaluate sexp before point; print value in the echo area.\nInteractively, with a non `-' prefix argument, print output into\ncurrent buffer.\n\nNormally, this function truncates long output according to the\nvalue of the variables `eval-expression-print-length' and\n`eval-expression-print-level'.  With a prefix argument of zero,\nhowever, there is no such truncation.\nInteger values are printed in several formats (decimal, octal,\nand hexadecimal).  When the prefix argument is -1 or the value\ndoesn't exceed `eval-expression-print-maximum-character', an\ninteger value is also printed as a character of that codepoint.\n\nIf `eval-expression-debug-on-error' is non-nil, which is the default,\nthis command arranges for all errors to enter t
 he debugger." (interactive "P") #<bytecode 0x1fdfdd428807>)(nil)
  apply(#f(compiled-function (eval-last-sexp-arg-internal) "Evaluate sexp before point; print value in the echo area.\nInteractively, with a non `-' prefix argument, print output into\ncurrent buffer.\n\nNormally, this function truncates long output according to the\nvalue of the variables `eval-expression-print-length' and\n`eval-expression-print-level'.  With a prefix argument of zero,\nhowever, there is no such truncation.\nInteger values are printed in several formats (decimal, octal,\nand hexadecimal).  When the prefix argument is -1 or the value\ndoesn't exceed `eval-expression-print-maximum-character', an\ninteger value is also printed as a character of that codepoint.\n\nIf `eval-expression-debug-on-error' is non-nil, which is the default,\nthis command arranges for all errors to e
 nter the debugger." (interactive "P") #<bytecode 0x1fdfdd428807>) nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

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)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

Could we use something else?