类型转换出错了 need type assertion

```go package main import ( "fmt" "container/ring" ) func main () { r := ring.New(1) r.Value = "A1" s := string(r.Value) fmt.Printf("%T", s[0]) } ``` 不知道怎么出现这个错误: # command-line-arguments main/main.go:14: cannot convert r.Value (type interface {}) to type string: need type assertion fmt.Printf("%T", r.Value) 结果是 string 啊?!