The following describes writing commands that have a different
behavior when repeated the second time.  For a more general approach see
SequentialCommand.  See also RepeatKeyDifferentBehaviors.

----
This definition makes pressing  "End" to go to the end of the line and "End" again to go
to the end of the buffer.

  (defmacro make-double-command (name args doc-string interactive
                                      first-form second-form)
    (let ((int-form (if (not interactive)
                        '(interactive)
                      (list 'interactive interactive))))
      `(progn
       (defun ,name ,args ,doc-string
         ,int-form
         (if (eq last-command this-command)
             ,(if (and (listp second-form) (> (length second-form) 1))
                  (cons 'progn second-form)
                second-form)
           ,first-form)))))
  (put 'make-double-command 'lisp-indent-function 2)
  ;; See DefMacro for an explanation of this call to put
  
  (make-double-command my-home ()
    "Go to beginning of line, or beginning of buffer." 
    nil
    (beginning-of-line)
    (beginning-of-buffer))

-- MarioLang

----

Inserts the string arg if a key is pressed twice.  For example, insert HTML character entities: (by [EmacsWiki:Rick_Bielawski rgb])

{{{
 (defmacro rgb-insert-if-double (otherwise)
   "Insert OTHERWISE when the key mapped to this fcn is pressed twice.
  For example typing && can result in & appearing in place of &&.
  Use C-u <count> <key> to insert <key> more than once without replace."
   `(lambda (cnt raw)
      (interactive "p\nP")
      (if (and (equal (preceding-char) last-command-char)
               (not raw))
          (progn
              (backward-delete-char 1)
              (insert ,otherwise))
          (self-insert-command cnt))))
 
(add-hook 'html-mode-hook
 (lambda ()
   (define-key html-mode-map [(<)]  (rgb-insert-if-double "&lt;"))
   (define-key html-mode-map [(>)]  (rgb-insert-if-double "&gt;"))
   (define-key html-mode-map [(&)]  (rgb-insert-if-double "&amp;"))
   (define-key html-mode-map [(\")] (rgb-insert-if-double "&quot;"))))
}}}

----
CategoryKeys