在Scala中构建`this.type`的实例

Question

我正在研究一个称为Graphlike的图特征，其中我对顶点使用从属/关联类型。在solving my polymorphism issues之后，我的实现如下所示：

trait Graphlike {
  type Vertex

  def subgraph(selectedVertices: Set[Vertex]): this.type
}

我也有各种抽象的原子态自动机，它们的行为当然类似于图形：

trait Automaton extends Graphlike {
  type State
  type Vertex = State

  def states: Iterable[State]
  def initialState: State
  def getBuilder: AutomatonBuilder[this.type]


  def subgraph(selectedVertices: Set[Vertex]) = {
    val builder = getBuilder
    // Some logic to actually do something to the builder here
    builder.getAutomaton
  }
}

trait AutomatonBuilder[A <: Automaton] {
  def getAutomaton: A
}

但是，当我尝试实际实现一个具体的自动机时，我遇到了麻烦：

class ConcreteAutomaton extends Automaton {
  type State = Int

  def states = List(1, 2, 3)
  def initialState = 1
  def getBuilder = new ConcreteAutomatonBuilder

}

class ConcreteAutomatonBuilder extends AutomatonBuilder[ConcreteAutomaton] {
  def getAutomaton = new ConcreteAutomaton
}

class UsesAutomataAsGraphs {
  val aut = new ConcreteAutomaton
  aut.subgraph(Set(aut.initialState)).subgraph(Set(aut.initialState))
}

给予：

[info] Compiling 1 Scala source to /Users/albin/Downloads/example/target/scala-2.12/classes ...
[error] /Users/albin/Downloads/example/src/main/scala/example/Example.scala:33:20: type mismatch;
[error]  found   : ConcreteAutomatonBuilder
[error]  required: AutomatonBuilder[ConcreteAutomaton.this.type]
[error] Note: ConcreteAutomaton >: ConcreteAutomaton.this.type (and ConcreteAutomatonBuilder <: AutomatonBuilder[ConcreteAutomaton]), but trait AutomatonBuilder is invariant in type A.
[error] You may wish to define A as -A instead. (SLS 4.5)
[error]   def getBuilder = new ConcreteAutomatonBuilder
[error]                    ^

遵循建议并设定A的对立会给我带来其他问题。这也不是我真正想要的。我想要我的建造者生产完全相同类型的自动机。

Answer 1

“ this.type的构建实例”听起来很奇怪。无论如何：

trait Graphlike {
  type Vertex

  def subgraph(selectedVertices: Set[Vertex]): this.type
}

trait Automaton extends Graphlike {
  type State
  type Vertex = State

  def states: Iterable[State]
  def initialState: State
  def getBuilder: AutomatonBuilder[this.type]

  def subgraph(selectedVertices: Set[Vertex]): this.type = {
    val builder = getBuilder
    // Some logic to actually do something to the builder here
    builder.getAutomaton
  }
}

trait AutomatonBuilder[A <: Automaton] {
  def getAutomaton: A
}

class ConcreteAutomaton extends Automaton {
  type State = Int

  def states = List(1, 2, 3)
  def initialState = 1
  def getBuilder = new ConcreteAutomatonBuilder[this.type](this)   
}

class ConcreteAutomatonBuilder[A <: Automaton with Singleton](a: A) extends AutomatonBuilder[A] {
  def getAutomaton = a
}