> For the complete documentation index, see [llms.txt](https://emacs-lisp.ivory.cafe/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://emacs-lisp.ivory.cafe/qiu-zhi/biao-da-shi/10.2.2-fu-hao-biao-da-shi-done.md).

# 10.2.2 符号表达式（DONE）

对符号求值时，该符号会被当成变量。如果这个符号的变量部分有值，那么其求值的结果便是该变量部分的值。如果这个符号没有对应的变量值，那么 Lisp 解释器会抛出一个错误。查阅变量章节以获取更多信息。

在下面这个例子中，我们使用 setq 给符号设置一个值。然后，我们对该符号求值，获得其 用 setq 储存的值。

```
(setq a 123)
     ⇒ 123
(eval 'a)
     ⇒ 123
a
     ⇒ 123
```

符号 nil 和 t 是两个特例，nil 的值永远是 nil，t 的值永远是 t；你不能将它们和其他的值绑定。因此，这两个符号看起来很像 自求值表达式，甚至 eval 都将它们单独考虑。此外，以":"开头的符号同样为自求值；类似的，它们的值通常也不能改变。查阅 常数变量。


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