【Go Flow Control】
1、for没有()，必须有{}。
2、for的前后表达式可以为空。
3、没有while，for即是while。
4、无穷循环。
5、if没有()，必须有{}。
6、if临时变量。
Like for, the if statement can start with a short statement to execute before the condition.
Variables declared by the statement are only in scope until the end of the if.
7、if临时变量可以在else中使用。
8、switch会自动break。
9、case中表达式可被计算。
10、Switch without a condition is the same as switch true.
11、延迟执行。
A defer statement defers the execution of a function until the surrounding function returns.
The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.
12、Deferred function calls are pushed onto a stack.
输出是 9 8 7 6 ...