Pretty Org babel blocks

I show how to use prettify-symbols-mode to display Org Babel blocks as something like this:

where “☰” expands to the header arguments of the block as shown in the picture.

I have been using Org and Babel to create “interactive” slides. A slide is a narrowed subtree. To not confuse listeners I want to hide the details of the rather verbose source block syntax.

I tried to hack something together with font-lock-add-keywords, but even getting the compositing right I was still missing the “uncomposition” behavior of prettify-symbols-mode when prettify-symbols-unprettify-at-point is non-nil (I like right-edge). Unfortunately, prettify-symbols-alist only supports strings, not regexps as it uses regexp-opt behind the scene.

Thus, I decided to just hack something together with prettify-symbols-mode. rasmus/org-prettify-src–update goes through the buffer and add all occurrences of header arguments to prettify-symbols-alist and also removes old chun. This is probably not very efficient… Next, rasmus/org-prettify-src takes care of calling the update function whenever the cursor was in a #+begin_src before and now is not. Finally, rasmus/org-prettify-symbols is a hook that can be added to Org.

If you don’t want to use the burger icon you can change it by changing rasmus/ob-header-symbol.

This is clearly a hack! Some obvious flaws are:

Since prettify-symbols-mode doesn’t support regexp you can’t fully match the proper Org syntax. E.g. this is not valid syntax, but would still be prettified.
```
a #+begin_src
```
It might be nice to reveal the burger whenever the cursor is on the same line as the burger. I tried to get this working, but I couldn’t get it working property.
It would be nice to turn it into a proper minor-mode to easily toggle fortification of header arguments…

(with-eval-after-load 'org
  (defvar-local rasmus/org-at-src-begin -1
    "Variable that holds whether last position was a ")

  (defvar rasmus/ob-header-symbol ?☰
    "Symbol used for babel headers")

  (defun rasmus/org-prettify-src--update ()
    (let ((case-fold-search t)
          (re "^[ \t]*#\\+begin_src[ \t]+[^ \f\t\n\r\v]+[ \t]*")
          found)
      (save-excursion
        (goto-char (point-min))
        (while (re-search-forward re nil t)
          (goto-char (match-end 0))
          (let ((args (org-trim
                       (buffer-substring-no-properties (point)
                                                       (line-end-position)))))
            (when (org-string-nw-p args)
              (let ((new-cell (cons args rasmus/ob-header-symbol)))
                (cl-pushnew new-cell prettify-symbols-alist :test #'equal)
                (cl-pushnew new-cell found :test #'equal)))))
        (setq prettify-symbols-alist
              (cl-set-difference prettify-symbols-alist
                                 (cl-set-difference
                                  (cl-remove-if-not
                                   (lambda (elm)
                                     (eq (cdr elm) rasmus/ob-header-symbol))
                                   prettify-symbols-alist)
                                  found :test #'equal)))
        ;; Clean up old font-lock-keywords.
        (font-lock-remove-keywords nil prettify-symbols--keywords)
        (setq prettify-symbols--keywords (prettify-symbols--make-keywords))
        (font-lock-add-keywords nil prettify-symbols--keywords)
        (while (re-search-forward re nil t)
          (font-lock-flush (line-beginning-position) (line-end-position))))))

  (defun rasmus/org-prettify-src ()
    "Hide src options via `prettify-symbols-mode'.

  `prettify-symbols-mode' is used because it has uncollpasing. It's
  may not be efficient."
    (let* ((case-fold-search t)
           (at-src-block (save-excursion
                           (beginning-of-line)
                           (looking-at "^[ \t]*#\\+begin_src[ \t]+[^ \f\t\n\r\v]+[ \t]*"))))
      ;; Test if we moved out of a block.
      (when (or (and rasmus/org-at-src-begin
                     (not at-src-block))
                ;; File was just opened.
                (eq rasmus/org-at-src-begin -1))
        (rasmus/org-prettify-src--update))
      ;; Remove composition if at line; doesn't work properly.
      ;; (when at-src-block
      ;;   (with-silent-modifications
      ;;     (remove-text-properties (match-end 0)
      ;;                             (1+ (line-end-position))
      ;;                             '(composition))))
      (setq rasmus/org-at-src-begin at-src-block)))

  (defun rasmus/org-prettify-symbols ()
    (mapc (apply-partially 'add-to-list 'prettify-symbols-alist)
          (cl-reduce 'append
                     (mapcar (lambda (x) (list x (cons (upcase (car x)) (cdr x))))
                             `(("#+begin_src" . ?✎) ;; ➤ 🖝 ➟ ➤ ✎
                               ("#+end_src"   . ?□) ;; ⏹
                               ("#+header:" . ,rasmus/ob-header-symbol)
                               ("#+begin_quote" . ?»)
                               ("#+end_quote" . ?«)))))
    (turn-on-prettify-symbols-mode)
    (add-hook 'post-command-hook 'rasmus/org-prettify-src t t))
  (add-hook 'org-mode-hook #'rasmus/org-prettify-symbols))