Additional modifier keys in emacs?

Question

I have been modifying my Emacs setup quite alot recently but I reached a problem which is starting to annoy me. I would like to be able to introduce additional modifier like keys. What I am trying to do, to make things clearer, is when I am in dired-mode (which doesn't accept textual input so normal letters can be rebound) I would like it so that when I hold down the letter s and press j or l the cursor moves to the next and previous directory line respectively. Effectively making the s key act like a modifier.

I have looked into making the s apply a modifier such as super or hyper but those are all used for global things. Is this possible? if not then that's a shame.

Edit:

There seems to be some confusion with what I'm after. If I define a normal key sequence such as

(define-key map (kbd "s j") 'dired-next-dirline)

Then I have to keep pressing the s key every time before I press j to move to the next directory line. This is not what I am looking for (not to sound angry :P) I want s to act like a modifier where I can keep the s key held down and keep tapping j to move down the lines.

I hope I have made this more clear. Thanks.

Answer 1

I dug around in the code and came up with this. It binds s j to my-dired-next-dirline, but right after you've done that, just j is enough to do it again. Any other key resets the temporary binding.

Note that the function set-temporary-overlay-map was added in Emacs 24.2, which at the time of this writing hasn't been released yet, so you'll need to build Emacs from git.

(defun my-dired-next-dirline ()
  (interactive)
  (dired-next-dirline 1)
  (set-temporary-overlay-map
   (let ((map (make-sparse-keymap)))
     (define-key map [?j] 'my-dired-next-dirline)
     map)
   nil))

(eval-after-load "dired"
  '(progn
     (let ((prefix-map (make-sparse-keymap)))
       (define-key prefix-map "j" 'my-dired-next-dirline)  
       (define-key dired-mode-map "s" prefix-map))))

Answer 2

Key chords may be what you are looking for.