Challenge 1: A Lambda-with-Receiver Warm-Up — Solution

fun buildGreeting(init: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.init()
    return sb.toString()
}

fun main() {
    val greeting = buildGreeting {
        append("Hello, ")
        append("Philip")
        append("!")
    }
    println(greeting)
}

/*
Output:
Hello, Philip!

Notes:
- init: StringBuilder.() -> Unit declares init as a lambda WITH RECEIVER
  — StringBuilder is the receiver type, so inside the lambda passed to
  buildGreeting, "this" is the StringBuilder, and its methods (append)
  are callable directly without a prefix.
- sb.init() runs that lambda with sb as the receiver — equivalent to
  temporarily treating the lambda's code as if it were written as
  methods on sb itself.
- Each append(...) call inside the lambda operates on the same sb
  instance, building up the final string before buildGreeting returns
  sb.toString().
*/