为什么此函数每次都返回不同的值? [英] Why does this function return a different value every time?

问题描述

有人可以解释以下行为吗?具体来说，为什么函数每次都返回不同的列表?为什么每次调用该函数时some-list都没有初始化为'(0 0 0)?

Can someone explain the following behavior? Specifically, why does the function return a different list every time? Why isn't some-list initialized to '(0 0 0) every time the function is called?

(defun foo ()
  (let ((some-list '(0 0 0)))
    (incf (car some-list))
    some-list))

输出:

> (foo)
(1 0 0)
> (foo)
(2 0 0)
> (foo)
(3 0 0)
> (foo)
(4 0 0)

谢谢！

此外，假设我希望函数每次输出'(1 0 0)，建议的实现此功能的方法是什么?

Also, what is the recommended way of implementing this function, assuming I want the function to output '(1 0 0) every time?

推荐答案

'(0 0 0)是一个文字对象，假定为常量(尽管不受修改).因此，您每次都在有效地修改相同的对象.要在每个函数调用中创建不同的对象，请使用(list 0 0 0).

'(0 0 0) is a literal object, which is assumed to be a constant (albeit not protected from modification). So you're effectively modifying the same object every time. To create different objects at each function call use (list 0 0 0).

因此，除非您知道自己在做什么，否则应始终仅将文字列表(如'(0 0 0))用作常量.

So unless you know, what you're doing, you should always use literal lists (like '(0 0 0)) only as constants.

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