类的构造函数引用回该类的类型成员

Question

I am trying to implement a case class which is meant to describe an automaton. I am currently doing following:

case class Automaton(alphabet: Set[String], initial: String, finish: Set[String],
  transitions: Map[String, self.Transition] { self =>

  trait Transition

  // concrete things omitted
}

Note that how I want transitions to be defined. I hope that makes sense to you why I want to implement in such way. However, when I tried to compile, compiler reported following error:

Error:(21, 47) not found: value self
                     transitions: Map[String, self.Transition]) {

I understand the error. But how then can I force the transition is the exact transition belongs to the automaton I am describing? Any idea how to get around this?

Answer 1

I got the problem myself. Sure this cannot type check. Even if it did, I won't be able to instantiate any instance of this type. To get away with this, I need to define dependent function.

trait DepFunc {
  def apply(automaton: Automaton): Map[String, automaton.Transition]
}

Then inside of the body of the case class, I then can construct the transitions :

val transitions: Map[String, self.Transition] = transitionsGen(this)

One extra observation is, it turns out that Scala's type system is not powerful enough to describe a dependent function. For example, the trait DepFunc above cannot be described by Function1 , that is because at type level, scala disallows to refer to a variable name. This gives a concrete feeling of scala's incapability of handling dependent types.