# 10.3 反引用（DONE）

反引用允许你引用一个列表，并可以控制列表的哪些部分求值，哪些不被求值。在某些情况下，反引用和特殊表达式 quote 完全等同。举个例子，下面两个表达式会生成完全等价的结果：

```
`(a list of (+ 2 3) elements)
     ⇒ (a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
     ⇒ (a list of (+ 2 3) elements)
```

特殊记号','，用在反引用中，用于标记哪些位置需要求值，而不是作为常量对待。Emacs Lisp 求值器会对有逗号标记的参数求值，并将求值结果放在原先的列表结构中：

```
`(a list of ,(+ 2 3) elements)
     ⇒ (a list of 5 elements)
```

','同样支持深层嵌套列表结构，例如：

```
`(1 2 (3 ,(+ 4 5)))
     ⇒ (1 2 (3 9))
```

你还可以将求值的结果拼接到列表中，并作为结果返回，此时你需要使用特殊符号',@'，这个符号会将列表内的元素作为外层列表的元素，拼接到同等层级。而等价的，不使用反引号的写法往往不具备可读性。这里有一些例子：

```
(setq some-list '(2 3))
     ⇒ (2 3)
(cons 1 (append some-list '(4) some-list))
     ⇒ (1 2 3 4 2 3)
`(1 ,@some-list 4 ,@some-list)
     ⇒ (1 2 3 4 2 3)

(setq list '(hack foo bar))
     ⇒ (hack foo bar)
(cons 'use
  (cons 'the
    (cons 'words (append (cdr list) '(as elements)))))
     ⇒ (use the words foo bar as elements)
`(use the words ,@(cdr list) as elements)
     ⇒ (use the words foo bar as elements)

```

如果反引用结果中没有替换和拼接，那么它表现起来和 quote 很像，同样生成点对，向量，字符串这些不可变对象。


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