Examples of Transfer of Control during a Destructive Operation (ANSI and GNU Common Lisp Document)

3.7.2.1 Examples of Transfer of Control during a Destructive Operation

The following examples illustrate some of the many ways in which the implementation-dependent nature of the modification can manifest itself.

 (let ((a (list 2 1 4 3 7 6 'five)))
   (ignore-errors (sort a #'<))
   a)
⇒  (1 2 3 4 6 7 FIVE)
OR⇒ (2 1 4 3 7 6 FIVE)
OR⇒ (2)

 (prog foo ((a (list 1 2 3 4 5 6 7 8 9 10)))
   (sort a #'(lambda (x y) (if (zerop (random 5)) (return-from foo a) (> x y)))))
⇒  (1 2 3 4 5 6 7 8 9 10)
OR⇒ (3 4 5 6 2 7 8 9 10 1)
OR⇒ (1 2 4 3)