Implementation of the Bonding comic featured in xkcd blog https://xkcd.com/1188

bonding.kt

class Ball: Throwable()
class P(var target: P?) {
  fun aim(ball: Ball) {
    try {
      throw ball
    } catch (b: Ball) {
      target?.aim(b)
    }
  }
}

fun main() {
  val parent = P(null)
  val child = P(parent)
  parent.target = child
  parent.aim(Ball())
}