1. 18.5 接口
（1）如何检测一个值v是否实现了接口Stringer：
if v, ok := v.(Stringer); ok {
fmt.Printf("implements String(): %s\n", v.String())
}
（2）如何使用接口实现一个类型分类函数：
func classifier(items ...interface{}) {
for i, x := range items {
switch x.(type) {
case bool:
fmt.Printf("param #%d is a bool\n", i)
case float64:
fmt.Printf("param #%d is a float64\n", i)
case int, int64:
fmt.Printf("param #%d is an int\n", i)
case nil:
fmt.Printf("param #%d is nil\n", i)
case string:
fmt.Printf("param #%d is a string\n", i)
default:
fmt.Printf("param #%d’s type is unknown\n", i)
}
}
}