这是字面弊端的特定于实现的行为吗? [英] Is this an implementation-specific behavior for literal cons?

问题描述

我正在测试这个有趣的答案中的代码.

CL-USER> (defun literal-cons ()
        (let ((cons '(1 . 2)))
          (incf (cdr cons))
          cons))
; in: DEFUN LITERAL-CONS
;     (INCF (CDR CONS))
; --> LET* 
; ==>
;   (SB-KERNEL:%RPLACD #:CONS1 #:NEW0)
; 
; caught WARNING:
;   Destructive function SB-KERNEL:%RPLACD called on constant data.
;   See also:
;     The ANSI Standard, Special Operator QUOTE
;     The ANSI Standard, Section 3.2.2.3
; 
; compilation unit finished
;   caught 1 WARNING condition
LITERAL-CONS
CL-USER> (literal-cons)
(1 . 3)
CL-USER> (literal-cons)
(1 . 3)
CL-USER> (literal-cons)
(1 . 3)

由于行为不同，我想知道SBCL是否已使用上述警告将行为更改为用户认为更可能期望的行为?预期:

As the behavior is not the same, I am wondering if SBCL has used the mentioned warning to change the behavior to something it thinks is more likely expected from the user? Expected:

TEST> (defun literal-cons ()
        (let ((cons '(1 . 2)))
          (incf (cdr cons))
          cons))
LITERAL-CONS
TEST> (literal-cons)
(1 . 3)
TEST> (literal-cons)
(1 . 4)
TEST> (literal-cons)
(1 . 5)

推荐答案

简短的回答是，是的，这是实现特定的行为.如数据的意外持久性，

The short answer is that yes, this is implementation specific behavior. As discussed in Unexpected persistence of data,

quote :

如果文字对象(包括引号)的后果是不确定的 对象)进行破坏性的修改.

The consequences are undefined if literal objects (including quoted objects) are destructively modified.

这意味着您从此类功能中看到的任何行为都是特定于实现的(即使某些行为在实现中比其他行为更常见).

This means that any behavior you see from such a function is implementation specific (even if certain behaviors are more common among implementations than others).

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