使用 Scala 抽象类型的奇怪的重复模板模式约束

Question

我有一个 Scala 编译器错误难题，我想不通。由于此处未给出的原因，我需要使用 curiously recurring template pattern 声明我的类。我的一些类需要包含其他作为参数。例如，我的代码的精简版本如下所示：

trait Container[C <: Container[C]] {
  def stuff: C
}

trait Aggregation {
  def fill(x: Double): Unit
}

class Counted extends Container[Counted] {
  def stuff = this
}

class Counting extends Container[Counting] with Aggregation {
  def stuff = this
  def fill(x: Double) { }
}

class Binned[V <: Container[V]](v: V) extends Container[Binned[V]] {
  def stuff = this
  def substuff = v
}

class Binning[V <: Container[V] with Aggregation](v: V) extends Container[Binning[V]] with Aggregation {
  def stuff = this
  def substuff = v
  def fill(x: Double) { }
}

以上工作正常。现在我想从每个 -ing 添加一个抽象类型到每个 -ed:

trait Container[C <: Container[C]] {
  type Ed <: Container[Ed]
  def stuff: C
}

trait Aggregation {
  def fill(x: Double): Unit
}

class Counted extends Container[Counted] {
  type Ed = Counted
  def stuff = this
}

class Counting extends Container[Counting] with Aggregation {
  type Ed = Counted
  def stuff = this
  def fill(x: Double) { }
}

class Binned[V <: Container[V]](v: V) extends Container[Binned[V]] {
  type Ed = Binned[V]
  def stuff = this
  def substuff = v
}

class Binning[V <: Container[V] with Aggregation](v: V) extends Container[Binning[V]] with Aggregation {
  type Ed = Binned[V#Ed]
  def stuff = this
  def substuff = v
  def fill(x: Double) { }
}

而且编译器还好意思告诉我

<console>:34: error: type arguments [V#Ed] do not conform to class Binned's type parameter bounds [V <: Container[V]]
             type Ed = Binned[V#Ed]

当 Ed 对所有 Containers 来说显然是 <: Container[Ed]。

严格来说，只有 Containers with Aggregation 才需要输入 Ed，如果我把它移到那里，我会得到同样的错误。

有人知道我如何向编译器说明我的意图吗？

Answer 1

class Binning[V <: Container[V] with Aggregation](val v: V) extends Container[Binning[V]] with Aggregation {
  type Ed = Binned[v.Ed]
  def stuff = this
  def substuff = v
  def fill(x: Double) { }
}

有效。但在我看来你的版本也应该编译。

使用 Scala 抽象类型的奇怪的重复模板模式约束

Curiously recurring template pattern constraints with Scala abstract types

scala

crtp

abstract-type