lisp - 在 Lisp 中打印 defstruct

Question

我在 Lisp 中定义了一个非常简单的数据结构:

;;Data structure for a person

(defstruct person
  (name nil)
  (age 0)
  (siblings nil :type list))   ;; Siblings is a list of person objects

然后我开始实例化一些人物对象:

(setf person-a (make-person :name 'Tim :age 23))
(setf person-b (make-person :name 'Sally :age 21))
(setf person-c (make-person :name 'Louis :age 24))

然后我将 sibling 联系起来(假设他们都是彼此的 sibling ):

(setf (person-siblings person-a) (list person-b person-c))
(setf (person-siblings person-b) (list person-a person-c))
(setf (person-siblings person-c) (list person-b person-a))

然后我怎样才能打印关于我已经实例化和修改的对象的信息？我已经研究了关于打印对象和打印功能的 defstruct 选项，但我无法弄清楚如何正确打印我的对象。使用类似的东西:

(print person-a)

将我的 ACL 解释器发送到无限循环中。

Answer 1

Common lisp 有一个控制递归结构打印的变量:*print-circle* .在您的 Lisp 中，默认情况下它可能为 false (nil)(因为它在 SBCL 和 clisp 中 - 我不熟悉 ACL)，这可能导致无限循环。如果你将它设置为 t，你的结构应该打印:

(setf *print-circle* t)

我使用以下文件对此进行了测试:

(defstruct person
  (name nil)
  (age 0)
  (siblings nil :type list))

(setf person-a (make-person :name 'Tim :age 23))
(setf person-b (make-person :name 'Sally :age 21))
(setf person-c (make-person :name 'Louis :age 24))

(setf (person-siblings person-a) (list person-b person-c))
(setf (person-siblings person-b) (list person-a person-c))
(setf (person-siblings person-c) (list person-a person-b))

(setf *print-circle* t)
(format t "~a~&" person-a)
(format t "~a~&" person-b)
(format t "~a~&" person-c)

(print person-a)
(print person-b)
(print person-c)

这是运行该代码的记录:

> sbcl
This is SBCL 1.0.55, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (load "defstruct.lisp")

#1=#S(PERSON
        :NAME TIM
        :AGE 23
        :SIBLINGS (#2=#S(PERSON
                         :NAME SALLY
                         :AGE 21
                         :SIBLINGS (#1#
                                    #3=#S(PERSON
                                          :NAME LOUIS
                                          :AGE 24
                                          :SIBLINGS (#1# #2#))))
                   #3#))
#1=#S(PERSON
      :NAME SALLY
      :AGE 21
      :SIBLINGS (#2=#S(PERSON
                       :NAME TIM
                       :AGE 23
                       :SIBLINGS (#1#
                                  #3=#S(PERSON
                                        :NAME LOUIS
                                        :AGE 24
                                        :SIBLINGS (#2# #1#))))
                 #3#))
#1=#S(PERSON
      :NAME LOUIS
      :AGE 24
      :SIBLINGS (#2=#S(PERSON
                       :NAME TIM
                       :AGE 23
                       :SIBLINGS (#3=#S(PERSON
                                        :NAME SALLY
                                        :AGE 21
                                        :SIBLINGS (#2# #1#))
                                  #1#))
                 #3#))

#1=#S(PERSON
      :NAME TIM
      :AGE 23
      :SIBLINGS (#2=#S(PERSON
                       :NAME SALLY
                       :AGE 21
                       :SIBLINGS (#1#
                                  #3=#S(PERSON
                                        :NAME LOUIS
                                        :AGE 24
                                        :SIBLINGS (#1# #2#))))
                 #3#)) 
#1=#S(PERSON
      :NAME SALLY
      :AGE 21
      :SIBLINGS (#2=#S(PERSON
                       :NAME TIM
                       :AGE 23
                       :SIBLINGS (#1#
                                  #3=#S(PERSON
                                        :NAME LOUIS
                                        :AGE 24
                                        :SIBLINGS (#2# #1#))))
                 #3#)) 
#1=#S(PERSON
      :NAME LOUIS
      :AGE 24
      :SIBLINGS (#2=#S(PERSON
                       :NAME TIM
                       :AGE 23
                       :SIBLINGS (#3=#S(PERSON
                                        :NAME SALLY
                                        :AGE 21
                                        :SIBLINGS (#2# #1#))
                                  #1#))
                 #3#)) 
T
* (sb-ext:quit)

如果我离开 *print-circle* nil，那么我会得到一个堆栈溢出错误(SB-KERNEL::CONTROL-STACK-EXHAUSTED 在 sbcl 中)。

HTH,

凯尔