RETURN 子句与 RETURN 表单

在 LOOP 中，你可以在任何表达式中使用 Common Lisp (return) 表单，这将使 LOOP 表单立即计算为 return 的值。

LOOP 也有一个 return 子句几乎完全相同，唯一的区别是你不用括号括起来。该子句在 LOOP 的 DSL 中使用，而表单在表达式中使用。

(loop for x in list
      do (if (listp x) ;; Non-barewords after DO are expressions
             (return :x-has-a-list)))

;; Here, both the IF and the RETURN are clauses
(loop for x in list
     if (listp x) return :x-has-a-list)

;; Evaluate the RETURN expression and assign it to X...
;; except RETURN jumps out of the loop before the assignment
;; happens.
(loop for x = (return :nothing-else-happens)
      do (print :this-doesnt-print))

finally 之后的东西必须是一个表达式，所以必须使用 (return) 形式而不是 return 子句：

 (loop for n from 1 to 100
       when (evenp n) collect n into evens
       else collect n into odds
      finally return (values evens odds)) ;; ERROR!

 (loop for n from 1 to 100
       when (evenp n) collect n into evens
       else collect n into odds
      finally (return (values evens odds))) ;; Correct usage.