I've installed j-mode and it works.

But in an org-babel code block I have a strange issue.

Spelling like begin_src J (capital J) allows me to evaluate the block, but has no syntax highlighting:

#+begin_src J
(+/%#) 0.2 + i.24  NB. avg

: 11.7

Also, I can't use C-c ' to edit the code in a popup; instead I get an error message: No such language mode: J-mode.

Here's the strangest thing: when I change the spelling to begin_src j (lowercase j), then I get syntax highlighting and can use C-c ' to edit the code in a popup as expected.

However, now org-babel won't evaluate the source block, and when I try it complains with ob-j.elc failed to provide feature ‘ob-j’.

Is there anything I can do to get both sets of features at the same time?

  • ob-j.elc failed to provide feature ‘ob-j’ sounds strange - maybe ob-j.elc is corrupt or needs to be recompiled?
    – Tyler
    Oct 7, 2021 at 20:13
  • This looks like a bug to me. I think org-babel uses ob-J (uppercase) but org's own syntax highlighting uses j-mode (lowercase).
    – Evan Davis
    Oct 7, 2021 at 20:46
  • I have ob-J.elc but no ob-j.elc. Will try a recompile. Oct 7, 2021 at 20:53
  • ob-J.el ends with (provide 'ob-J). Should I edit it so it says (provide 'ob-j) instead? Oct 7, 2021 at 20:56
  • I wonder how ob-C.el and c-mode coexist. If there is a trick there, it might be applied to ob-J.el. You might want to bring it up on the Org mode mailing list.
    – NickD
    Oct 7, 2021 at 21:56

2 Answers 2


I believe you are looking for this in your init file:

(add-to-list 'org-src-lang-modes '("J" . j))

that should allow you to edit and give you syntax highlighting


Found a hacky workaround. A the bottom of .emacs.d/.local/straight/repos/org/lisp/ob-J.el, I added a line:

(provide 'ob-J)
(provide 'ob-j) ;; this line added by me

Now everything works as expected. Seems dumb that this is necessary though.

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