How to define a key in 'special-mode?

Question

I would like to have ; bound to other-window in special modes, unless the derived mode overrides the definition. It would be nice to also include modes not strictly deriving from special-mode (but defining 'mode-class), but that's not strictly necessary.

Ideas I've tried/considered:

• Use global-set-key: I thought the global keymap would be of lower precedence than major-mode keymaps, so ; would work in modes that defined it, but instead I just overwrote self-insert-command
• Add the keybinding in special-mode-hook: It doesn't seem like this hook is actually run, and this misses the 'mode-class special modes
• Add a keybinding that either calls other-window or defers to the major mode. Not sure how I would do this?

Any ideas?

Answer 1

I think you can use special-mode-map when possible. When it doesn't work, simply use the corresponding mode map such as Info-mode-map.

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The following (needs bind-key.el) might works (I tested in info-mode and help-mode), it uses bind-key.el's override-global-mode, which overrides any minor-mode binding of ;.

(defun override-global-mode-semicolon ()
  (interactive)
  (if (eq (get major-mode 'mode-class) 'special)
      (call-interactively 'other-window)
    (let (buf (current-buffer))
      (override-global-mode -1)
      (unwind-protect
          (call-interactively (key-binding ";"))
        (and (buffer-live-p buf)
             (with-current-buffer buf
               (override-global-mode 1)))))))

(bind-key* ";" 'override-global-mode-semicolon)

Answer 2

IIUC you're just looking for:

(define-key special-mode-map ";" #'other-window)

And for those other modes which don't (yet) derive from special-mode, you'll have to do something like

(add-hook '<foo>-mode-hook
          (lambda ()
            (local-set-key ";" #'other-window)))

tho I'd advise you to report those modes via M-x report-emacs-bug if you think they should derive from special-mode.