Scala变量和常量的声明

标示符的命名规则

1. 字母或下划线开头

2. 以操作符开头，且只包含操作符（+ - * / # !等）

3. 用反引号`....`包括的任意字符串，即使是 Scala 关键字（39 个）也可以
? package, import, class, object, trait, extends, with, type, for
? private, protected, abstract, sealed, final, implicit, lazy, override
? try, catch, finally, throw
? if, else, match, case, do, while, for, return, yield
? def, val, var
? this, super

? new
? true, false, null