这是sbcl中的错误吗? [英] This is a bug in sbcl?

问题描述

为什么在sbcl中发生这种情况?也许是个错误?

Why happen this in sbcl? Maybe a bug?

(defclass myclass ()
  ((s1
    :initform '((a . 1) (b . 2))) 
   (s2
    :initform '((a . 1) (b . 2)))))

(defparameter ins (make-instance 'myclass))

(setf (cdr (assoc 'a (slot-value ins 's1))) 43) ;; change only slot s1

;; here my problem

(slot-value ins 's1)  ;; => ((a . 44) (b . 2)))
(slot-value ins 's2)  ;; => ((a . 44) (b . 2)))

但是如果将:initform更改为:

But if change :initform to :

(defclass myclass ()
  ((s1
    :initform '((a . 1) (b . 2))) 
   (s2
    :initform '((a . 1) (b . 3)))))

问题消失了

我在 sbcl 1.4.3 和 1.4.11 中对此进行了测试.似乎没有出现问题.

I test this in sbcl 1.4.3 and 1.4.11. In clisp it seems that the problem does not arise.

推荐答案

否.您正在修改文字数据，这会带来不确定的后果.

No. You are modifying literal data, which has undefined consequences.

在源代码中引用列表时，这意味着您要使用的东西正是阅读器从源代码生成的列表，这是文字列表.读者可能会记住这些事情，因此不会重复两个相同的列表.

When you quote a list in source code, it means that the thing you want to work with is exactly the list that the reader produced from your source code—this is a literal list. Such things may be remembered by the reader so that two identical lists are not duplicated.

解决此问题的一种方法是在运行时使用list和cons创建列表:

One way to fix this is to create the lists at runtime, using list and cons:

(defclass myclass ()
  ((s1
    :initform (list (cons a 1) (cons b 2))) 
   (s2
    :initform (list (cons a 1) (cons b 2)))))

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