Typesafe builders

gistfile1.scala

package object typesafe_builder{

  import scala.language.higherKinds

  // Auxiliary type constructor. It allows us to represent 
  // that an argument has been set.
  type Id[T] = T

  // The target domain class
  case class Person(age: Int, name: String)

  // The builder, with a private constructor. It can only 
  // be initialised through the companion object.

  class PersonBuilder[AgeF[_], NameF[_]] private(
    val age: AgeF[Int],
    val name: NameF[String]
  ){
    def withAge(age: Int) =
      new PersonBuilder[Id,NameF](age, name)

    def withName(name: String) =
      new PersonBuilder[AgeF,Id](age, name)
  }

  object PersonBuilder{
    def apply(): PersonBuilder[Any, Any] = 
      new PersonBuilder(None, None)
  }

  // The builder is typesafe, i.e. the compiler won't let us build 
  // an object unless we specify *all* the arguments.
  implicit class ToPerson(builder: PersonBuilder[Id,Id]){
    def toPerson = Person(builder.age, builder.name) 
  }

  // Won't compile!
  // PersonBuilder().toPerson
  // PersonBuilder().withName("pepe").toPerson
  // PersonBuilder().withAge(10).toPerson
  
  // Compiles!
  assert( PersonBuilder().withName("pepe").withAge(10).toPerson == Person(10,"pepe") )

}