let

let就跟*.run一样。必须有对象，并且这对象会带进匿名函数里，名字叫it

fun main(args : Array<String>){
    var v1 = "fd"
    var v2:String?
    
    v2 = v1?.let{
        it + " hi"
    }
    
    println(v2)
}