Question

抱歉，标题有点含糊，很难将我的问题总结成一个单一的类。希望以下代码片段能更好地说明我的意图。

让我们开始为Reduceable的所有事物引入特征：

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

Reduceable这样的例子可以作为计数器：

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)

以上述计数器作为参数的类本身也可以是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，当我尝试引入泛型Reduceable类其参数也是泛型Reduceable s

时，就会出现问题

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

我得到了错误消息的来源-其原因是特征的reduceWith签名要求other: A，但是如果我更改它，则GenderCounter和FruitCounter将打破。我可以怎样改变才能达到预期的结果？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))

当类及其参数从同一特征扩展时，方法签名发生冲突

1 个答案: