Help Understanding syntax-propertize-function

[Reza Nikoopour]

Hello All,

I'm trying to implement here doc syntax highlighting.  I've reviewed the implementations in shell-script-mode and hcl-mode but I don't really understand what is happening. I've read the documentation for syntax-propertize-function (https://www.gnu.org/software/emacs/manual/html_node/elisp/Syntax-Properties.html) but that didn't help me understand what's going on.

Could someone help explain the following code:

(defun hcl--syntax-propertize-heredoc (end)

  (let ((ppss (syntax-ppss)))

    (when (eq t (nth 3 ppss))

      (let ((key (get-text-property (nth 8 ppss) 'hcl-here-doc-marker))

            (case-fold-search nil))

        (when (re-search-forward

               (concat "^\\(?:[ \t]*\\)" (regexp-quote key) "\\(\n\\)")

               end 'move)

          (let ((eol (match-beginning 1)))

            (put-text-property eol (1+ eol)

                               'syntax-table (string-to-syntax "|"))))))))

(defun hcl--font-lock-open-heredoc (start string eol)

  (unless (or (memq (char-before start) '(?< ?>))

     (save-excursion

                (goto-char start)

                (hcl--in-string-or-comment-p)))

    (let ((str (replace-regexp-in-string "['\"]" "" string)))

      (put-text-property eol (1+ eol) 'hcl-here-doc-marker str)

      (prog1 (string-to-syntax "|")

        (goto-char (+ 2 start))))))

(defun hcl--syntax-propertize-function (start end)

  (goto-char start)

  (hcl--syntax-propertize-heredoc end)

  (funcall

   (syntax-propertize-rules

    (hcl--here-doc-beg-re

     (2 (hcl--font-lock-open-heredoc

         (match-beginning 0) (match-string 1) (match-beginning 2))))

    ("\\s|" (0 (prog1 nil (hcl--syntax-propertize-heredoc end)))))

   (point) end))

Cheers,
Reza