Understanding Contravariance and Covariance via Kotlin

variance.kt

class Producer<out T: Any>(val e:T) {
  fun read(): T = e
}

class Consumer<in T: Any>() {
  private lateinit var e: T
  fun write(v: T): Unit { e = v }
}

fun main() {
  var p1: Producer<Number> = Producer<Double>(0.4)
  p1.read()
  var p2: Producer<Double> = Producer<Number>(3)  // Disallowed
  p2.read()
  var c1: Consumer<Number> = Consumer<Double>()   // Disallowed
  c1.write(3)
  var c2: Consumer<Double> = Consumer<Number>()
  c2.write(0.4)
}