List<out T>

List<in T>

sealed class Result {
  data class Success<out T, out V>(val t: T, val v: V) : Result()
  data class Failure(val msg: String) : Result()
}

class MyView : View() {
  var size = 10.0
  ...
  override val root = borderpane {
    center = stackpane { /* use size */ }
    top = hbox {
      textfield(size.toString()) {
        action {
          size = text.toDouble()
          text = ""
        }
      }
    }
  }
}