org-mode code block parentheses mismatch

Question

Here's some emacs-lisp code in a babel block:

#+begin_src emacs-lisp
(defun factor (number)
  (labels
      ((*factor (divisor number)
        (if (> (* divisor divisor) number)
            (list number)
          (if (= (mod number divisor) 0)
              (cons divisor (*factor divisor (/ number divisor)))
            (*factor (+ divisor 1) number)))))
    (*factor 2 number)))
#+end_src

So in the org buffer I get a parentheses matching problem, i.e., that very last ) at the bottom is flagged as one too many. It flashes red and check-parens flags it as well. Trouble is, this is not an error, i.e., the same code in scratch is perfectly paren-matched, likewise when I bring it up in the Org Source C-c ' window. BTW, this code compiles and runs giving good output. Any ideas what org-mode is doing here? I've got (show-paren-mode 1). When I turn it off, obviously, no problem because no highlighting matching parens.

Answer 1

The reason for the mismatch is the code contains an angled bracket (">") which is treated as a matching paren.

I don't know the solution for how to specify that orgmode emacs-elisp should ignore angled brackets when matching parens (but I would like to know a solution).

Note: I posted this as an "answer" because I don't have enough reputation points to post it as a comment in the OP.

Answer 2

Here is a potential solution:

(defun org-mode-<>-syntax-fix (start end)
  (when (eq major-mode 'org-mode)
    (save-excursion
      (goto-char start)
      (while (re-search-forward "<\\|>" end t)
    (when (get-text-property (point) 'src-block)
      ;; This is a < or > in an org-src block
      (put-text-property (point) (1- (point))
                 'syntax-table (string-to-syntax "_")))))))

(add-hook 'org-mode-hook
      (lambda ()
        (setq syntax-propertize-function 'org-mode-<>-syntax-fix)
        (syntax-propertize (point-max))))

This basically just changes the syntax table for <> inside src blocks I think. It seems to solve the problem.

---

The code above depends on source blocks already propertized with src-block.

The src-block property is put on source blocks by org-fontify-meta-lines-and-blocks during keyword fontification through font-lock. But, syntax fontification comes before keyword fontification. So maybe the following function is better. For easier copy-paste the full code is given. The main change is the search for source blocks by re-search-backward.

(defun org-mode-<>-syntax-fix (start end)
  "Change syntax of characters ?< and ?> to symbol within source code blocks."
  (let ((case-fold-search t))
    (when (eq major-mode 'org-mode)
      (save-excursion
        (goto-char start)
        (while (re-search-forward "<\\|>" end t)
          (when (save-excursion
                  (and
                   (re-search-backward "[[:space:]]*#\\+\\(begin\\|end\\)_src\\_>" nil t)
                   (string-equal (downcase (match-string 1)) "begin")))
            ;; This is a < or > in an org-src block
            (put-text-property (point) (1- (point))
                               'syntax-table (string-to-syntax "_"))))))))

(defun org-setup-<>-syntax-fix ()
  "Setup for characters ?< and ?> in source code blocks.
Add this function to `org-mode-hook'."
  (setq syntax-propertize-function 'org-mode-<>-syntax-fix)
  (syntax-propertize (point-max)))

(add-hook 'org-mode-hook #'org-setup-<>-syntax-fix)

Answer 3

Since I never use <> as brackets in org mode documents, I have these two lines in my org mode hook function:

(modify-syntax-entry ?< ".")
(modify-syntax-entry ?> ".")

No need to play around with properties etc. Of course, if you use XML or HTML, you may wish to avoid this.