# 10.2.5 函数表达式求值（DONE）

若非空列表的第一个元素被求值为Lisp 函数，或字节码函数，或基础函数，那我们称这个列表为 函数调用。举个例子，这里展示了如何调用函数 +：

```
(+ 1 x)
```

函数调用的**第一步是对其参数依次从左向右求值，求值的结果才真正的是参数的值。**第二步为对参数列表，使用 apply 调用该函数(详情查阅 函数调用)。如果函数是使用 Lisp 编写的，那么其参数求值结果会绑定到函数的参数变量(详情查阅 Lambda 表达式)；第三步，函数主体会依次运算，并将主体中最后运算值作为函数调用的值。


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