;;; This function sets up a toggleable swank listener in a Lisp
;;; program. This enables a REPL directly in the running process,
;;; providing unparalled introspection, debugging, and speed of
;;; development. It performs some initial setup, sets up a closure
;;; over a status variable, and then swaps out the setup code for the
;;; production code.
(defun start-swank (&key (port 4006))
  "Setup a swank server in the running process."
  ;;; The `swank-running-p` variable is used to keep track of the
  ;;; function's state: is swank running?
  (let ((swank-runningp nil))
    ;;; We create a new, locally-scoped function.
    (labels ((fn ()
                 (progn
                   (if swank-runningp
                       (progn
                         ;;; if swank is running, we print a message to the
                         ;;; console alerting the programmer that swank is
                         ;;; shutting down.
                         (format t "Stopping swank server...~%")
                         ;;; Then, stop swank.
                         (swank:stop-server port)
                         ;;; Finally, update the swank status
                         ;;; variable.
                         (setf swank-runningp nil))
                       (progn
                         ;;; If swank is not running, let the
                         ;;; programmer know that it is being started.
                         (format t "Starting swank server on port ~A.~%" port)
                         ;;; Start the server, and make sure that it
                         ;;; doesn't close when the function exits.
                         (swank:create-server :port port :dont-close t)
                         ;;; Finally, update our status.
                         (setf swank-runningp t)))
                   ;;; After taking the appropriate action, we'll return the
                   ;;; current swank state so that it can be programmatically
                   ;;; acted on.
                   swank-runningp)))
      ;;; Swap out the setup code for our locally defined function.
      (setf (symbol-function 'start-swank) #'fn)
      ;;; The function is called `start-swank`, so we start swank.
      (start-swank))))