Lambda with receiver

Feature in Kotlin: https://kotlinlang.org/docs/lambdas.html#function-types

Maybe not actually that groundbreaking but allows for some pretty cool meta-programming / DSL construction, such as Kotlin’s builders. For example:

val map = HashMap().apply {
  put(0, "0")
  for (i in 1..10) {
      put(i, "$i")
  }
}

put is an instance method of the HashMap but we can call it implicitly without prefixing this allowing for a fairly terse syntax.