Auto-insert " * " in block comments in Emacs

block-comments.el

(defun elmord-block-comment-auto-prefix ()
  (when (eq last-command-event ?\n)
    (let ((last-comment-start
           (save-excursion (search-backward "/*" nil t)))
          (last-comment-end
           (save-excursion (search-backward "*/" nil t))))
      (when (and last-comment-start
                 (or (null last-comment-end)
                     (> last-comment-start last-comment-end)))
        (insert "* ")
        (indent-for-tab-command)))))

(add-hook 'scala-mode-hook
  (defun elmord-scala-mode-hook ()
    (add-hook 'post-self-insert-hook
              'elmord-block-comment-auto-prefix
              nil t)))