Scala compilation error

Question

Can't figure out what's wrong with StrangeIntQueue extending Queue, why there is an error "Not enough arguments for constructor Queue: (leading: Int)list.Lister.Queue[Int]. Unspecified value parameter leading". How can I specify it?

  class Queue[+T](
    private val leading: T
  ) {
    def enqueue[U >: T](x: U) =
      new Queue[U](leading: U) // ...
  }

  class StrangeIntQueue(private val leading: Int) extends Queue[Int] {
    override def enqueue(x: Int) = {
      println(math.sqrt(x))
      super.enqueue(x)
    }
  }

Answer 1

extends Queue[Int](leading)

You have to pass on the arguments even if it seems "obvious" what to do.

Note also that since you have declared leading private, you'll actually get two copies: one for StrangeIntQueue and one for Queue. (Otherwise you could have just StrangeIntQueue(leading0: Int) extends Queue[Int](leading0) and use the inherited copy of leading inside.)

Answer 2

The primary constructor of class Queue, which StrangeIntQueue extends, takes a parameter, but you're not passing it anything for the parameter. Try this:

class StrangeIntQueue(leading: Int) extends Queue[Int](leading) {
  // ...
}