小编use*_*362的帖子

我正在读这本书成功的lisp,有一个例子:

(defstruct (ship
              (:print-function
               (lambda (struct stream depth)
                 (declare (ignore depth))
                 (format stream "[ship ~A of ~A at (~D, ~D) moving (~D, ~D)]"
                         (ship-name struct)
                         (ship-player struct)
                         (ship-x-pos struct)
                         (ship-y-pos struct)
                         (ship-x-vel struct)
                         (ship-y-vel struct)))))
    (name "unnamed")
    player
    (x-pos 0.0)
    (y-pos 0.0)
    (x-vel 0.0)
    (y-vel 0.0))

我怎么能理解这部分:

(lambda (struct stream depth)
                     (declare (ignore depth))

为什么要宣布忽略深度？我觉得很困惑,为什么不把lambda写成

(lambda (struct stream)
         .....)

谢谢