print-method 函数行为异常

Question

我在尝试自定义 CLOS 对象的 print-object 函数时遇到了一些奇怪的行为。该函数确实生成了预期的字符串，但它出现在对象定义之外（即 <OBJECT-NAME > 后跟 > 字符后我想要的字符串）。下面的框架代码重现了这个问题：

(defclass object ()                            ;;; Define some object...
  ((element-count
    :initarg :element-count
    :accessor n)
   (first-element
    :initform nil
    :accessor 1st)
   (last-element
    :initform nil
    :accessor end)))

(defmethod print-object ((obj object) stream)  ;;; ...and then its print-object method
  (print-unreadable-object (obj stream :type t))
  (with-accessors ((first 1st)
                   (last end)
                   (n n))
      obj
    (format stream " ~[empty~:;:elements ~:*~d :content ~:*(~[~;~a~;~a ~a~:;~a ... ~a~])~]"
            n first last)))

输出字符串如下：

(defvar o1 (make-instance 'object :element-count 0)) => #<OBJECT > empty

(defvar o2 (make-instance 'object :element-count 1)) => #<OBJECT > :elements 1 :content (NIL)

(defvar o3 (make-instance 'object :element-count 2)) => #<OBJECT > :elements 2 :content (NIL NIL)

(defvar o4 (make-instance 'object :element-count 10)) => #<OBJECT > :elements 10 :content (NIL ... NIL)

我不明白这个输出。根据文档，在 space 之外打印不应该发生。还是应该？好吧，交给你了！顺便说一句，我在 Windows.

的 Portacle 上使用 SBCL

Answer 1

你放错了括号 - print-unreadable-object creates an environment which should encompass the whole print-object 正文：

(defclass airplane () ((tail-number :initarg tail :accessor plane-tail)))
(defmethod print-object ((plane airplane) stream)
  (print-unreadable-object (plane stream :type t)
    (princ (plane-tail plane) stream)))
(make-instance 'airplane 'tail 1234)
==> #<AIRPLANE 1234>

print-method 函数行为异常

print-method function behaves weirdly

common-lisp

clos