组合子类型(Scala)

object Example {
  abstract class Person(val name: String) {
    def occupation: Occupation
    implicit val self = this
    abstract class Occupation(implicit val practitioner: Person) {
       def title: String
       def advanceCareer: Person
    }
    class Programmer extends Occupation {
      def title = "Code Monkey"
      def advanceCareer = practitioner
    }
    class Student extends Occupation {
      def title = "Undecided"
      def advanceCareer = new Person(practitioner.name) {
        def occupation = new Programmer
      }
    }
  }

  def main(args: Array[String]) {
    val p = new Person("John Doe") { def occupation = new Student }
    val q = p.occupation.advanceCareer
    val r = q.occupation.advanceCareer
    println(p.name + " is a " + p.occupation.title)
    println(q.name + " is a " + q.occupation.title)
    println(r.name + " is a " + r.occupation.title)
    println("I am myself: " + (r eq r.occupation.practitioner))
  }
}

让我们试试吧

scala> Example.main(Array())
John Doe is a Undecided
John Doe is a Code Monkey
John Doe is a Code Monkey
I am myself: true

所以这样做有点有用.

这里的诀窍是,每当一个职业(这是一个内在类)决定改变事情时,你创建你的人的匿名子类.它的工作是创造一个新角色,新角色完整无缺;这是通过隐含的val self = this和对职业的隐式构造函数来帮助的,这有助于自动加载人的正确实例.

您可能需要一个职业列表,因此可能会需要帮助方法来重新生成专业列表.就像是

object Example {
  abstract class Person(val name: String) {
    def occupations: List[Occupation]
    implicit val self = this
    def withOccupations(others: List[Person#Occupation]) = new Person(self.name) {
      def occupations = others.collect {
        case p: Person#Programmer => new Programmer
        case s: Person#Pirate => new Pirate
      }
    }
    abstract class Occupation(implicit val practitioner: Person) {
       def title: String
       def addCareer: Person
       override def toString = title
    }
    class Programmer extends Occupation {
      def title = "Code Monkey"
      def addCareer: Person = withOccupations( this :: self.occupations )
    }
    class Pirate extends Occupation {
      def title = "Sea Monkey"
      def addCareer: Person = withOccupations( this :: self.occupations )
    }
  }

  def main(args: Array[String]) {
    val p = new Person("John Doe") { def occupations = Nil }
    val q = (new p.Programmer).addCareer
    val r = (new q.Pirate).addCareer
    println(p.name + " has jobs " + p.occupations)
    println(q.name + " has jobs " + q.occupations)
    println(r.name + " has jobs " + r.occupations)
    println("I am myself: " + (r eq r.occupations.head.practitioner))
  }
}