Type assertions

func detectCircle(g geometry) {
    if c, ok := g.(circle); ok {
        fmt.Println("circle with radius", c.radius)
    }
}

 whatAmI := func(i interface{}) {
        switch t := i.(type) {
        case bool:
            fmt.Println("I'm a bool")
        case int:
            fmt.Println("I'm an int")
        default:
            fmt.Printf("Don't know type %T\n", t)
        }
    }
    whatAmI(true)
    whatAmI(1)
    whatAmI("hey")