Main.scala

package demo

import java.util.concurrent.atomic.AtomicInteger

class IdGenerator {
  private val seq = new AtomicInteger(0)
  def next(): Int = seq.incrementAndGet()
}

final case class Visitor(id: Int)

object Conference {
  def apply(title: String, size: Int)(implicit generator: IdGenerator): Conference = {
    Conference(title, (1 to size).map(_ => Visitor(generator.next())).toList)
  }
}

final case class Conference(title: String, visitors: List[Visitor])

object Main extends App {
  implicit val generator = new IdGenerator()
  val conference = Conference("Scala Russia 2019", 10)
  println(s"Welcome to ${conference.title}, ${conference.visitors}!")
}