约束案例类和特征的类型参数

Question

我有点困惑如何约束类型参数，以便它们只接受已实现特定类型类的类型。这是一些相当做作的示例代码：

// I want to tag my favourite types of number with this trait
trait MyFavouriteTypesOfNumber[A]
// implicits necessary because I cannot directly extend Int or Double
implicit def iLikeInts(i: Int): MyFavouriteTypesOfNumber[Int] = new MyFavouriteTypesOfNumber[Int] {}
implicit def iAlsoLikeFloats(f: Float): MyFavouriteTypesOfNumber[Float] = new MyFavouriteTypesOfNumber[Float] {}

// Is the type constraint here correct? 
// I am trying to say 'only accept types that implement MyFavouriteTypesOfNumber'
trait PresentMyFavourite[Num <: MyFavouriteTypesOfNumber[Num]] {
  val originalNum: Num
  def packageAsString: String = s"***... ${originalNum} ...***"
  def printPackaged: Unit = println(packageAsString)
}

// again, the case class should only accept types that have implemented MyFavouriteTypesOfNumber
case class ILikeThisNumberAsWellAsItsType[Num: MyFavouriteTypesOfNumber] (
  val originalNum: Num
) extends PresentMyFavourite[Num]

// Would expect these next two lines to work
val three = ILikeThisNumberAsWellAsItsType[Int](3: Int)
three.printPackaged
// But not this one, because I don't like Doubles
val four = ILikeThisNumberAsWellAsItsType[Double](3.0: Double)

我写这篇文章是希望它在最后一行 val four = ... 上引发错误，但实际上它给了我一个新错误（超出了我的预期） - could not find implicit parameter for evidence value of type MyFavouriteTypesOfNumber[Int]。

如果有人可以 a) 让我知道我的原始代码对于我想要实现的目标是否正确，或者 b) 对这个意外错误消息有所了解，那么我将不胜感激。

Answer 1

将val 用于类型类实现。

trait MyFavouriteTypesOfNumber[A]
implicit val iLikeInts: MyFavouriteTypesOfNumber[Int] =
  new MyFavouriteTypesOfNumber[Int] {}
implicit val iAlsoLikeFloats: MyFavouriteTypesOfNumber[Float] =
  new MyFavouriteTypesOfNumber[Float] {}

将PresentMyFavourite 更改为abstract class，以便我们可以使用上下文边界。

abstract class PresentMyFavourite[Num : MyFavouriteTypesOfNumber] {
  val originalNum: Num
  def packageAsString: String = s"***... ${originalNum} ...***"
  def printPackaged(): Unit = println(packageAsString)
}

现在除了最后一行 val four ... 之外的其余部分应该可以工作了，正如预期的那样，它不会编译。

应该注意的是，您将必须为每个“FavoriteTypesOfNumber”实现您想要使用的每个数学运算。此时编译器知道该类型被限制为Int 或Float，但是，因为它可能是任何一个，所以除非您提供代码，否则它不知道如何将它们中的两个相加。