restart-case example

(define-condition error-503 (error)
  ((url :initarg :url)))

(defclass webpage ()
  ((url :initarg :url)))

(defun http-get (url)
  (format t "Retrieving: ~S~%" url)
  (when (= (random 10) 0)
    (error 'error-503 :url url))
  (make-instance 'webpage :url url))

(defun crawl (urls)
  (handler-bind ((error-503 #'(lambda (e)
                                (format t "Info: ~A~%" e)
                                (invoke-restart 'retry-a-bit-later))))
    (let ((web-pages '()))
      (dolist (url urls)
        :again
        (restart-case
            (push (http-get url) web-pages)
          (retry-a-bit-later ()
            (format t "Let's retry in 10 seconds again...~%")
            (sleep 10)
            (go :again))))
      web-pages)))

;;; run e.g. with
;;;   (crawl (loop for i from 0 below 10 collect (format nil "http://foo.com/~D.html" i)))