run()

Some of the other scope functions simply remix features of apply() and let() a bit differently. run(), for example, works a bit like apply(), in that whatever you call run() on becomes this in the scope of the lambda expression that you supply. However, run() also works a bit like let(), in that the result of run is whatever the last statement of the lambda expression returns.

run is useful when you want to call a bunch of functions on some object, but then the end result is not the object itself, but something else… such as the result of one of those functions:

data class IntPropertyBag(private val pieces: MutableMap<String, Int> = mutableMapOf()) {
  fun set(key: String, value: Int) {
    pieces[key] = value
  }
}

fun main() {
  val ultimateStuff = IntPropertyBag().run {
    set("ID", 330258648)
    set("YEAR", 1979)
    set("HOW_MANY_ROADS_MUST_A_MAN_WALK_DOWN", 42)

    toString()
  }

  println(ultimateStuff)
}

Here, we call run() on a IntPropertyBag instance, just as we did with apply() in the previous section. However, ultimateStuff is not the IntPropertyBag object, but instead the result of toString() called on the IntPropertyBag object. In the context of println(), there is no practical difference, as println() will call toString() to generate what to print. However, in other situations, the value that you generate from the run lambda expression might be more distinctly different.

In summary, run():

• Is called on some object
• Takes whatever you call it on and makes it be the “current” object — this — in the scope of the lambda expression
• Returns whatever the last statement of the lambda expression evaluates to