依赖类型的Typeclass实例

Question

假设我有一个用来显示实例的Display[T] { apply(t: T): Unit }类型类(我有String、Int等的实例)。我还有一个将T转换成TCWithDependentType#DependentType的TCWithDependentType[T] { type DependentType ; def apply(t: T): DependentType }。

我正在尝试显示DependentType的实例，但它无法工作，因为编译器找不到Display[TCWithDependentType#DependentType] (我假设它就是这个)实例(即使它在编译时应该是已知的)。

下面是完整的示例(相关的Scastie在这里：https://scastie.scala-lang.org/RXmC776DQeywLOxj2xbHQg)：

trait TCWithDependentType[T]:
  type DependentType

  def apply(t: T): DependentType

def dependentTypeOf[T](t: T)(using
    tcWithDependentType: TCWithDependentType[T]
): tcWithDependentType.DependentType =
  tcWithDependentType(t)

given TCWithDependentType[Int] =
  new TCWithDependentType[Int]:
    type DependentType = String

    def apply(i: Int): String =
      s"${i}"

given TCWithDependentType[String] =
  new TCWithDependentType[String]:
    type DependentType = String

    def apply(i: String): String =
      s"${i}"

trait Display[T]:
  def apply(t: T): Unit

def display[T](t: T)(using displayForT: Display[T]): Unit =
  displayForT(t)

/* given [T]: Display[T] =
    new Display[T]:
        def apply(t: T): Unit =
            println(s"I display \"${t}\" which is unknown") */

given Display[String] =
  new Display[String]:
    def apply(t: String): Unit =
      println(s"I display \"${t}\" which is a String")

given Display[Int] =
  new Display[Int]:
    def apply(t: Int): Unit =
      println(s"I display \"${t}\" which is a Int")

val dv = dependentTypeOf(1)
display(dv)

你知道为了让这玩意儿工作我错过了什么吗？

谢谢!艾德里安。

Answer 1

您丢失了类型优化

given (TCWithDependentType[Int] {type DependentType = String}) =
  new TCWithDependentType[Int]:
    type DependentType = String
    def apply(i: Int): String = s"${i}"

given (TCWithDependentType[String] {type DependentType = String}) =
  new TCWithDependentType[String]:
    type DependentType = String
    def apply(i: String): String = s"${i}"

Answer 2

您应该使用标准的given ... with语法：

given TCWithDependentType[Int] with
  type DependentType = String

  def apply(i: Int): String =
    s"${i}"

这样，您的代码看起来更整洁，类型信息也不会丢失。