Can I get whitespace-mode to display a sequence of two leading spaces , specially?

Question

Like this:

I have been able to get whitespace-mode to display dots for leading spaces.

But I want to specially display each sequence of 2 leading spaces, with maybe a vertical bar.

Possible?

Maybe not with whitespace-mode?

As a separate question, how do I get whitespace-mode to display only leading spaces as middle-dot?

I have

    whitespace-space-regexp "\\(^ +\\)" 

...but that seems only to apply to the FACE used, not to the character substitution.

I've been fiddling with this for 20 minutes. I'm amazed that I still have not cracked this. Doesn't everyone want something like this?

Answer 1

Relying on font-lock and compose-region, I can sort of get emacs to do what I want. This stuff:

(defun yaml-pretty-search-leading-ws (limit)
  "Function to search for leading whitespace, and invoke
`compose-region' on ranges within it, to display the whitespace
differently: display a dot and a vertical bar for the sequence.

This fn is intended to be invoked indirectly via font-lock as a
search/matcher. It doesn't return a positive match to font-lock.

At the finish, all of the leading whitespace up to LIMIT should
be composed and fontified."

  (let ((start (point)))
    (save-match-data ;; we don't want font-lock to fontify.
      ;; 2 consecutive spaces
      (while (re-search-forward "  " limit t)
        (let ((start (match-beginning 0))
              (stop (match-end 0)))
          (put-text-property start stop 'face 'yaml-pretty-leading-ws-face)
          (compose-region start (1- stop) ?·)
          (compose-region (1- stop) stop ?|)
          )))))

(defvar yaml-pretty-flock-keywords
  '((yaml-pretty-search-leading-ws . yaml-pretty-leading-ws-face)))

(font-lock-add-keywords nil yaml-pretty-flock-keywords)

Results in :

For static text display, this is exactly what I wanted.

Now I need to get it to do the inverse - "un-compose" the region, and remove the face property - as I modify the leading whitespace.