依赖类型的类型类实例

Question

假设我有一个Display[T] { apply(t: T): Unit } 类型类，用于显示实例（我有String、Int 等的实例）。而且我还有一个TCWithDependentType[T] { type DependentType ; def apply(t: T): DependentType }，它将T 转换为TCWithDependentType#DependentType。

我正在尝试显示DependentType 的实例，但它不起作用，因为编译器无法找到Display[TCWithDependentType#DependentType]（我假设是这个）实例（即使它应该是在编译时已知）。

这里是完整的例子（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)

你知道我为了让这件事发挥作用而缺少什么吗？

谢谢！ 阿德里安。

Answer 1

你丢失了类型优化

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干净，并且不会丢失类型信息。