An example of using the async package to implement sleep-sort.

sleep-sort-with-async

(setq lexical-binding t)
(defun sleep-sort (numbers)
  (let ((sorted-list))
    (dolist (n numbers)  
      (async-start (lambda ()
                     (sleep-for n))
                   (lambda (val)
                     (add-to-list 'sorted-list n)
                     (when (= (length numbers)
                              (length sorted-list))
                       (message "Sorted list: %s"
                                (nreverse sorted-list))))))))