MonadTrans.scala

import cats.data.OptionT
import cats.implicits._
import java.util.Date

object MonadTrans {
  val person1 = Person(None, new Date())
  var person2 = Person(Some(person1), new Date())
  var person3 = Person(None, new Date())
  var person4 = Person(Some(person3), new Date())

  val persons: List[Person] = List(person1, person2, person3, person4)

  def main(args: Array[String]): Unit = {
    println(getBirthdays(persons))
    println(getBirthdaysMonadTransformer(persons))
  }

  def getBirthdays(persons: List[Person]): List[Option[Date]] = {
    val spouses: List[Option[Person]] = persons.map(_.marriedTo)
    spouses.map(_.map(_.birthDay))
  }

  def getBirthdaysMonadTransformer(persons: List[Person]): List[Option[Date]] = {
    val spouses: List[Option[Person]] = persons.map(_.marriedTo)
    val spousesT: OptionT[List, Person] = OptionT(spouses)
    spousesT.map(_.birthDay).value
    
  }

  case class Person(marriedTo: Option[Person], birthDay: Date)

}