当 class 及其参数从同一特征扩展时，方法签名会发生冲突

Question

抱歉标题有点模糊，很难将我的问题概括为 single-liner。希望以下代码片段能更好地解释我的意图。

让我们首先为所有 Reduceable:

的事物引入一个特征

trait Reduceable[A] {
  def reduceWith(other: A): A
}

此类 Reduceable 事物的示例可以是这样的计数器：

case class GenderCounter(male: Int, female: Int)
    extends Reduceable[GenderCounter] {
  override def reduceWith(other: GenderCounter): GenderCounter =
    GenderCounter(male + other.male, female + other.female)
}

case class FruitCounter(apples: Int, oranges: Int)
    extends Reduceable[FruitCounter] {
  override def reduceWith(other: FruitCounter): FruitCounter =
    FruitCounter(apples + other.apples, oranges + other.oranges)
}

GenderCounter(5, 10) reduceWith GenderCounter(4, 11)
// res: GenderCounter = GenderCounter(9,21)

一个class以上面的计数器为参数，本身也可以是一个Reduceable

case class CompoundCounter(c1: GenderCounter, c2: FruitCounter)
    extends Reduceable[CompoundCounter] {
  override def reduceWith(
    other: CompoundCounter)
    : CompoundCounter = CompoundCounter(
    c1 reduceWith other.c1,
    c2 reduceWith other.c2
  )
}

CompoundCounter(GenderCounter(5, 10), FruitCounter(11, 2)) reduceWith CompoundCounter(GenderCounter(5, 10), FruitCounter(11, 2))
// res: CompoundCounter = CompoundCounter(GenderCounter(10,20),FruitCounter(22,4))

但是，当我尝试引入一个泛型 Reduceable class 时，问题出现了，它的参数也是泛型的 Reduceables

case class CollectionReduceable[A, B](r1: Reduceable[A], r2: Reduceable[B])
    extends Reduceable[CollectionReduceable[A, B]] {
  override def reduceWith(
    other: CollectionReduceable[A, B])
    : CollectionReduceable[A, B] = CollectionReduceable(
    r1 reduceWith other.r1, 
    r2 reduceWith other.r2
  )
}

// error: type mismatch
// found   : other.r1.type (with underlying type Reduceable[A])
// required: A
//           r1 reduceWith other.r1,

// Desired outcome: same as CompoundCounter

我知道错误消息的来源 - 这是因为特征的 reduceWith 签名需要 other: A，但如果我更改它，则 GenderCounter 和 FruitCounter 会断。什么可以改变我以实现我的预期结果？谢谢！！

Answer 1

尝试

case class CollectionReduceable[A <: Reduceable[A], B <: Reduceable[B]](r1: A, r2: B) extends Reduceable[CollectionReduceable[A, B]] {
  override def reduceWith(other: CollectionReduceable[A, B]): CollectionReduceable[A, B] =
    CollectionReduceable(
      r1.reduceWith(other.r1),
      r2.reduceWith(other.r2)
    )
  }

CollectionReduceable(GenderCounter(5, 10), FruitCounter(11, 2)) reduceWith CollectionReduceable(GenderCounter(5, 10), FruitCounter(11, 2))
// CollectionReduceable(GenderCounter(10,20),FruitCounter(22,4))

当 class 及其参数从同一特征扩展时，方法签名会发生冲突

Method signature clashes when class and its parameters extends from the same trait

generics

scala

implicit

hierarchy