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
", 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
", i)
        case float64:
            fmt.Printf("param #%d is a float64
", i)
        case int, int64:
            fmt.Printf("param #%d is an int
", i)
        case nil:
            fmt.Printf("param #%d is nil
", i)
        case string:
            fmt.Printf("param #%d is a string
", i)
        default:
            fmt.Printf("param #%d’s type is unknown
", i)
        }
    }
}