Question

我正在阅读Programming in Emacs Lisp

Here is another list, this time with a list inside of it:

     '(this list has (a list inside of it))

我对嵌套列表感到困惑，为什么它没有前缀引用为

 '(this list has  '(a list inside of it))

如果没有前缀`，为什么不将a解析为函数？

Answer 1

's-expression是(quote s-expression)的缩写： s表达式中的任何内容都被视为基准，并且不进行评估。

所以

'(this list has (a list inside of it))

的缩写：

(quote (this list has (a list inside of it)))

包含以下列表：

(this list has (a list inside of it))

这是整个quote表单的值，因为未评估。

通过编写以下代码很容易验证这一点：

'(this list has '(a list inside of it))

如果进行了评估，它将产生以下列表作为值：

(this list has (quote (a list inside of it)))

Answer 2

这是Lisp的轻微困难之一：列表是数据，也可以是程序。如果希望列表成为Lisp程序中的数据，则需要用引号引起来。

这样的列表：人们可以使用READ读取它们

(this list has (a list inside of it))
(this list has no list inside of it)
(+ 1 2)
(1 2 +)
(1 + 2)
(quote (this list has (a list inside of it)))
(quote (this list has (quote (a quote list inside of it))))
(quote quote)

有效的Lisp表单：可以使用EVAL对其进行评估

(+ 1 2)
Evaluates to -> 3

(quote (+ 1 2))
Evaluates to -> (+ 1 2)

(quote (this list has (a list inside of it)))
Evaluates to -> (this list has (a list inside of it))

(quote quote)
Evaluates to -> quote

这也是有效的Lisp表单：

(quote (this list has (quote (a quoted list inside of it))))

其评估结果为：

(this list has (quote (a quoted list inside of it)))

Lisp教程列表中的列表

2 个答案: