Restarts, interactive choices in the debugger

Restarts in Common Lisp

(define-condition odd-number-error (error) 
  ((message :initarg :message :reader message))
  (:report (lambda (condition stream)
	    (format stream "Ups! we have an error. ~%~a" (message condition))))
    (:documentation "Custom error when we encounter a odd number error."))

(defun filter-oddp (sequence)
  "Iterates over the elememets of the sequence and display
the odd number, and otherwise invoke a condition."
  (dolist (x sequence)
    (if (oddp x)
	(format t "Odd number: ~a ~%" x)
	(error 'odd-number-error :message (format nil "~a is not an odd number." x)))))

(defun filter-odd-numbers (sequence)
  "Display the number if is odd, and otherwise invoke restarts."
  ;; Restarts, interactive choices in the debugger
  (restart-case (filter-oddp sequence)
    ;; Creates a restart called "SKIP-FILTER-ODDP"
    (skip-filter-oddp ()
      nil)
    ;; Creates a restart called "RETURN-SEQUENCE"
    (return-sequence ()
      sequence)
    ;; Creates a restart called "SAY-GOOD-BYE"
    (say-good-by ()
      "Good bye.")))
      
;; call the function      
(filter-odd-numbers (list 1 3 5 8 9))