Cyclic reference immutable objects.scala

package org.example

abstract class Person(val name: String)

// cannot be case class, because case classes have all parameters as vals and it wouldn't make sense to lazily instantiate them
class Girl(val name2: String, _boyfriend: => Boy) extends Person(name2) {
    lazy val boyfriend = _boyfriend
}

class Boy(val name2: String, _girlfriend: => Girl) extends Person(name2) {
    lazy val girlfriend = _girlfriend
}

object Main extends App {
  // this will work because it is at object/class level
  val alice1: Girl = new Girl("alice", bob1)
  val bob1 = new Boy("Bob", alice1)
  println(alice1.boyfriend.girlfriend.name)

  // this will not work because it is at block/scope level
  // def method = {
  //   val alice: Girl = new Girl("alice", bob) <-- it will fail at this line with "forward reference extends over definition of value alice"
  //   val bob = new Boy("Bob", alice)
  //   println(alice.boyfriend.girlfriend.name)
  // }

  // this works if we make both of them lazy and they are both instantiated at println line, when the first one is needed
  def method = {
    lazy val alice: Girl = new Girl("alice", bob)
    lazy val bob = new Boy("Bob", alice)
    println(alice.boyfriend.girlfriend.name)
  }

  method
}