package main
import "fmt"
func do(i interface{}) {
    switch v := i.(type) {
        fmt.Printf("Twice %v is %v\n", v, v*2)
        fmt.Printf("%q is %v bytes long\n", v, len(v))
    default:
        fmt.Printf("I don't know about type %T!\n", v)
    }
func main() {
    do(21)
    do("hello")
    do(true)