# 10.2.7 特殊表达式（DONE）

特殊表达式通常是基础函数，因此**其参数可能，也可能不会被求值**。大多数的特殊表达式有它们自己的控制结构，变量绑定，这是常规函数无法做到的。

每个特殊表达式都有它们自己的求值规则，甚至有些参数是否求值，取决于另一个参数的求值结果。

如果非空列表的第一个符号被解释为特殊表达式，那么这个列表会被按照该特殊表的规则进行求值；如果没有特殊表达式，Emacs 的行为将无法被良定义（即便不会出现数学上的冲突）。举个例子，((lambda (x) x . 3) 4) 包含了一个子表达式，这个子表达式以 lambda 开头，但并不是一个良定义的 lambda 表达式，因此 Emas 可能会抛出错误，也可能会返回 3， 或 4 或 nil，或者出现其他意想不到的行为。

Function: special-form-p object

这个谓语测试了参数object是否为特殊表达式，如果是则返回 t，否则返回 nil。

这里有一个依照字母顺序排列的表，列举了 Emcas Lisp 中所有的特殊表达式。

`and`

see [Combining Conditions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Combining-Conditions.html)

`catch`

see [Catch and Throw](https://www.gnu.org/software/emacs/manual/html_node/elisp/Catch-and-Throw.html)

`cond`

see [Conditionals](https://www.gnu.org/software/emacs/manual/html_node/elisp/Conditionals.html)

`condition-case`

see [Handling Errors](https://www.gnu.org/software/emacs/manual/html_node/elisp/Handling-Errors.html)

`defconst`

see [Defining Variables](https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Variables.html)

`defvar`

see [Defining Variables](https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Variables.html)

`function`

see [Anonymous Functions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Anonymous-Functions.html)

`if`

see [Conditionals](https://www.gnu.org/software/emacs/manual/html_node/elisp/Conditionals.html)

`interactive`

see [Interactive Call](https://www.gnu.org/software/emacs/manual/html_node/elisp/Interactive-Call.html)

`lambda`

see [Lambda Expressions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Lambda-Expressions.html)

`letlet*`

see [Local Variables](https://www.gnu.org/software/emacs/manual/html_node/elisp/Local-Variables.html)

`or`

see [Combining Conditions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Combining-Conditions.html)

`prog1prog2progn`

see [Sequencing](https://www.gnu.org/software/emacs/manual/html_node/elisp/Sequencing.html)

`quote`

see [Quoting](https://www.gnu.org/software/emacs/manual/html_node/elisp/Quoting.html)

`save-current-buffer`

see [Current Buffer](https://www.gnu.org/software/emacs/manual/html_node/elisp/Current-Buffer.html)

`save-excursion`

see [Excursions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Excursions.html)

`save-restriction`

see [Narrowing](https://www.gnu.org/software/emacs/manual/html_node/elisp/Narrowing.html)

`setq`

see [Setting Variables](https://www.gnu.org/software/emacs/manual/html_node/elisp/Setting-Variables.html)

`setq-default`

see [Creating Buffer-Local](https://www.gnu.org/software/emacs/manual/html_node/elisp/Creating-Buffer_002dLocal.html)

`unwind-protect`

see [Nonlocal Exits](https://www.gnu.org/software/emacs/manual/html_node/elisp/Nonlocal-Exits.html)

`while`

see [Iteration](https://www.gnu.org/software/emacs/manual/html_node/elisp/Iteration.html)

Common Lisp note：在 GNU Emcas Lisp 和 Common Lisp 中特殊表达式有一些不同。setq，if 和 catch 是两者中都有的特殊表达式。但 save-excursion 是 Emcas Lisp 中独有的特殊表达式；throw 是 Common Lisp 中特有的特殊表达式（因为 Common Lisp 允许抛出多个值，而Emcas Lisp 中没有这个概念）


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