（1）如何检测一个值 v 是否实现了接口 Stringer：

    1. if v, ok := v.(Stringer); ok {
    2.     fmt.Printf("implements String(): %s\n", v.String())
    3. }

    （2）如何使用接口实现一个类型分类函数：

    1. func classifier(items ...interface{}) {
    2.     for i, x := range items {
    3.         switch x.(type) {
    4.         case bool:
    5.             fmt.Printf("param #%d is a bool\n", i)
    6.         case float64:
    7.             fmt.Printf("param #%d is a float64\n", i)
    8.         case int, int64:
    9.             fmt.Printf("param #%d is an int\n", i)
    10.         case nil:
    11.             fmt.Printf("param #%d is nil\n", i)
    12.         case string:
    13.             fmt.Printf("param #%d is a string\n", i)
    14.         default:
    15.             fmt.Printf("param #%d’s type is unknown\n", i)
    16.         }
    17.     }
    18. }