在Lisp中打印defstruct

Question

我有一个非常简单的数据结构,我在Lisp中定义:

;;Data structure for a person

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

然后我继续实例化一些人物:

(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,我将兄弟姐妹联系起来(假设他们都是彼此的兄弟姐妹):

(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,我如何打印有关我已实例化和修改的对象的信息？我已经研究了关于print-object和print-function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.

Answer 1

常见的lisp有一个控制递归结构打印的变量:*print-circle*.在你的Lisp中nil,默认情况下可能是false()(因为它在SBCL和clisp中 - 我不熟悉ACL),这可能导致无限循环.如果将其设置为t,则应打印结构:

(setf *print-circle* t)

我用以下文件测试了这个:

(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)

以下是运行该代码的脚本:

> 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,那么我得到一个堆栈溢出错误(SB-KERNEL::CONTROL-STACK-EXHAUSTED在sbcl中).

HTH,

凯尔