Using path dependent types to ensure invariants in Scala

Example.scala

object StandardUnit {
  type PersonId = Int

  final class Person(id: PersonId, name: String)

  object Person {
    def lookupName(id: PersonId): Option[String] = ???

    def lookupBlacklistStatus(id: PersonId): Option[Unit] = ???

    def generatePerson(id: PersonId)(associatedName: String): Person = {
      new Person(id, associatedName)
    }
  }

  val exampleId: PersonId = 5

  val myPerson: Option[Person] = for {
    _ <- Person.lookupBlacklistStatus(exampleId)
    person <- Person.generatePerson(exampleId)
  } yield person
}

object UsingPathDependentTypes {
  type PersonId = Int

  final case class IdRelations(underlyingId: PersonId) {
    final class NameAssociatedWithId private (val name: String)

    final class IsNotBlackListed()
  }

  // You might actually want to store IdRelations, IsNotBlackListed, on Person
  // as well to allow downstream consumers to make use of these facts
  final class Person private (id: PersonId, name: String)

  object Person {
    def lookupName(id: IdRelations): Option[id.NameAssociatedWithId] = ???

    def lookupBlacklistStatus(id: IdRelations): Option[id.IsNotBlackListed] = ???

    def generatePerson(id: IdRelations)(associatedName: id.NameAssociatedWithId, proofNotBlacklisted: id.IsNotBlackListed): Person = {
      new Person(id.underlyingId, associatedName.name)
    }
  }

  val exampleId: PersonId = 5

  val relationsForExampleId: IdRelations = IdRelations(exampleId)

  val anotherId: PersonId = 6

  val relationsForAnotherId: IdRelations = IdRelations(anotherId)

  val myPerson: Option[Person] = for {
    notBlacklisted <- Person.lookupBlacklistStatus(relationsForExampleId)
    name <- Person.lookupName(relationsForExampleId)
  } yield Person.generatePerson(relationsForExampleId)(name, notBlacklisted)

  // Does not compile
  val myIncorrectPerson: Option[Person] = for {
    // I tried to look up the black list status of an incorrect Person ID
    proofNotBlacklisted <- Person.lookupBlacklistStatus(relationsForAnotherId)
    name <- Person.lookupName(relationsForExampleId)
  } yield Person.generatePerson(relationsForExampleId)(name, proofNotBlacklisted)
}