6.1.4.3 Examples of WHILE and UNTIL clauses

 (loop while (hungry-p) do (eat))
 
;; UNTIL NOT is equivalent to WHILE.
 (loop until (not (hungry-p)) do (eat))
 
;; Collect the length and the items of STACK.
 (let ((stack '(a b c d e f)))
   (loop for item = (length stack) then (pop stack)
         collect item
         while stack))
=>  (6 A B C D E F)
 
;; Use WHILE to terminate a loop that otherwise wouldn't terminate.
;; Note that WHILE occurs after the WHEN.
 (loop for i fixnum from 3
       when (oddp i) collect i
       while (< i 5))
=>  (3 5)