Type safe scala builder, with required arguments, without a build()!

builder.scala

object BuilderTest {

  class NAME_WAS_SET

  case class Person(name: String, age: Option[Int])

  case class PersonBuilder[NAME_NOT_SET](name: Option[String], age: Option[Int]) {
    def withName(name: String) = new PersonBuilder[NAME_WAS_SET](Some(name), age)
    def withAge(age: Int)      = new PersonBuilder[NAME_NOT_SET](name, Some(age))
  }

  object PersonBuilder {
    def apply() = new PersonBuilder(None, None)
    implicit def builder2Instance(builder: PersonBuilder[NAME_WAS_SET]) = new Person(builder.name.get, builder.age)
  }

  def main(args: Array[String]) {
    val p1: Person = PersonBuilder() withName("Torgeir") withAge(29)
    val p2: Person = PersonBuilder() withAge(29) withName("Torgeir")
    val p3: Person = PersonBuilder() withName("Torgeir")
    // val p4: Person = PersonBuilder() withAge(29) // does not compile
    
    println(p1, p2, p3) // (Person(Torgeir,Some(29)),Person(Torgeir,Some(29)),Person(Torgeir,None))
  }
}

Yess!