Scala 奇怪的代码无法理解这一点：访问者 [String] ⇒

我对 Scala 很陌生，所以请不要投票给我。表达this: Visitor[Double] ⇒是什么意思？对我来说，它看起来像不完整的代码。我试图在谷歌上找到这个概念，但由于我不知道这个概念名称，我无法找到它。

import scala.language.higherKinds

trait Expressions {

  trait TVisitor[T] {
    def visitConstant(v: Double): T
  }
  type Visitor[T] <: TVisitor[T]

  trait Expression {
    def accept[T](visitor: Visitor[T]): T
  }

  class Constant(val v: Double) extends Expression {
    def accept[T](visitor: Visitor[T]): T = visitor.visitConstant(v)
  }

  trait EvalVisitor extends TVisitor[Double] {
    def visitConstant(v: Double): Double = v
  }
}

trait AddExpressions extends Expressions {
  class Add(val l: Expression, val r: Expression) extends Expression {
    def accept[T](visitor: Visitor[T]): T = visitor.visitAdd(l, r)
  }

  trait TVisitor[T] extends super.TVisitor[T] {
    def visitAdd(l: Expression, r: Expression): T
  }
  type Visitor[T] <: TVisitor[T]

  trait EvalVisitor extends super.EvalVisitor with TVisitor[Double] {
    this: Visitor[Double] ⇒
    def visitAdd(l: Expression, r: Expression): Double =
      l.accept(this) + r.accept(this)
  }
}

trait ExtendedExpressions extends AddExpressions with MultExpressions {
  type Visitor[T] = TVisitor[T]
  trait TVisitor[T]
      extends super[AddExpressions].TVisitor[T]
      with super[MultExpressions].TVisitor[T]
  object EvalVisitor
      extends super[AddExpressions].EvalVisitor
      with super[MultExpressions].EvalVisitor
      with TVisitor[Double] {
    this: Visitor[Double] ⇒
  }
}

trait PrefixNotationForExpressions extends ExtendedExpressions {
  object PrefixNotationVisitor extends super.TVisitor[String] {
    this: Visitor[String] ⇒
    def visitConstant(v: Double): String = v.toString + " "
    def visitAdd(l: Expression, r: Expression): String =
      "+ " + l.accept(this) + r.accept(this)
    def visitMult(l: Expression, r: Expression): String =
      "* " + l.accept(this) + r.accept(this)
  }
}