6.1.6.1 Examples of WHEN clause

;; Signal an exceptional condition.
 (loop for item in '(1 2 3 a 4 5)
       when (not (numberp item))
        return (cerror "enter new value" "non-numeric value: ~s" item))
Error: non-numeric value: A
 
;; The previous example is equivalent to the following one.
 (loop for item in '(1 2 3 a 4 5)
       when (not (numberp item))
        do (return 
            (cerror "Enter new value" "non-numeric value: ~s" item)))
Error: non-numeric value: A

;; This example parses a simple printed string representation from 
;; BUFFER (which is itself a string) and returns the index of the
;; closing double-quote character.
 (let ((buffer "\"a\" \"b\""))
   (loop initially (unless (char= (char buffer 0) #\")
                     (loop-finish))
         for i of-type fixnum from 1 below (length (the string buffer))
         when (char= (char buffer i) #\")
          return i))
=>  2
 
;; The collected value is returned.
 (loop for i from 1 to 10
       when (> i 5)
         collect i
       finally (prin1 'got-here))
>>  GOT-HERE
=>  (6 7 8 9 10) 

;; Return both the count of collected numbers and the numbers.
 (loop for i from 1 to 10
       when (> i 5)
         collect i into number-list
         and count i into number-count
       finally (return (values number-count number-list)))
=>  5, (6 7 8 9 10)