Scala類型類找不到隱式實例

Question

我試圖在Scala中創建一個類型類，並使用它來創建一個簡單的多態案例類。 本示例未編譯，並給出了“找不到參數writer的隱式值：A $ A228.this.ValueWriter [T]”。 我真的不知道可能出什么問題或從哪里開始。

trait Keeper

case class StringKeeper(measure: String) extends Keeper

case class StringLengthKeeper(measure: Int) extends Keeper


trait ValueWriter[A] {
  def write(value: String): A
}

object DefaultValueWriters {
  implicit val stringWriter = new ValueWriter[StringKeeper] {
    def write(value: String) = StringKeeper(value)
  }
  implicit val stringLengthWriter = new ValueWriter[StringLengthKeeper] {
    def write(value: String) = StringLengthKeeper(value.length)
  }
}

object Write {
  def toWrite[A](value: String)(implicit writer: ValueWriter[A]) = {
    writer.write(value)
  }
}


case class WriterOfKeepers[T <: Keeper](value: String) {

  def run: T = {
    Write.toWrite[T](value)
  }

}

import DefaultValueWriters._

val writerLengthKeeper = WriterOfKeepers[StringLengthKeeper]("TestString")

writerLengthKeeper.run

Answer 1

調用Write.toWrite時，作用域中沒有隱式ValueWriter 。

您需要在構造函數中包含隱式參數

case class WriterOfKeepers[T <: Keeper : ValueWriter](value: String) {

  def run: T = {
    Write.toWrite[T](value)
  }

}

或在方法中

case class WriterOfKeepers[T <: Keeper](value: String) {

  def run(implicit writer: ValueWriter[T]): T = {
    Write.toWrite(value)
  }

}

或找到其他符合您要求的方式（我不知道這些方式）。