DogBuilder.scala

// Compile time validated builder pattern using type level programming

sealed trait Fur

case object Short extends Fur
case object Long extends Fur
case object Curly extends Fur

case class Dog(name: String, age: Int, fur: Fur)


sealed trait Bool
trait True extends Bool
trait False extends Bool

class DogBuilder[HasName <: Bool, HasAge <: Bool, HasFur <: Bool](
  val name: Option[String], 
  val age: Option[Int], 
  val fur: Option[Fur])

object DogBuilder {
  def apply() = new DogBuilder[False, False, False](None, None, None)

  implicit def withNameBuilder[HasAge <: Bool, HasFur <: Bool](db: DogBuilder[False, HasAge, HasFur]) = new {
    def withName(name: String) = new DogBuilder[True, HasAge, HasFur](Some(name), db.age, db.fur)
  }

  implicit def withAgeBuilder[HasName <: Bool, HasFur <: Bool](db: DogBuilder[HasName, False, HasFur]) = new {
    def withAge(age: Int) = new DogBuilder[HasName, True, HasFur](db.name, Some(age), db.fur)
  }

  implicit def withFurBuilder[HasName <: Bool, HasAge <: Bool](db: DogBuilder[HasName, HasAge, False]) = new {
    def withFur(fur: Fur) = new DogBuilder[HasName, HasAge, True](db.name, db.age, Some(fur))
  }

  implicit def realizeBuilder(db: DogBuilder[True, True, True]) = new {
    def toDog: Dog = Dog(db.name.get, db.age.get, db.fur.get)
  }
}


/* Example User */
val fidoBuilder = DogBuilder()
val fido = fidoBuilder.withAge(3).withFur(Short).withName("fido").toDog

val namelessBuilder = DogBuilder()
val nameless = namelessBuilder.withAge(7).withFur(Curly).toDog // this won't compile because name is missing