Case Class Option 参数的不明确的隐式值 [英] Ambiguous Implicit Values for Case Class Option Parameter

问题描述

我正在做一些我在案例类和类型类上发明的练习.我遇到的问题之一如下:

I am going through some exercises I have invented on case classes and typeclasses. One of the problems I have faced is the following:

object Example extends App {

  sealed trait Serializer[T] {
    def serialize(seq: List[T]): String
  }

  implicit object StringSerializer extends Serializer[String] {
    def serialize(seq: List[String]): String = seq.toString()
  }

  implicit object IntSerializer extends Serializer[Int] {
    def serialize(seq: List[Int]): String = seq.toString()
  }

  case class Marker[T: Serializer](lst: Option[List[T]] = None)
  
  Marker() // ambiguous implicit values: here...
}

现在这给出了关于不明确的隐含值的错误.我认为这与我之前问过的一个问题有关(尽管是不同的错误消息):

Now this gives an error about ambiguous implicit values. I think this is related to a question I have asked before (albeit a different error message):

​​在嵌套列表中键入擦除给定的上下文绑定

我是否正确，即使错误消息不同，这里的工作过程也是相同的?

Am I correct in that it is the same process at work here, even though the error message is different?

推荐答案

编译器无法推断 T.尝试明确指定 T

Compiler can't infer T. Try to specify T explicitly

Marker[String]() // compiles
Marker[Int]() // compiles

当您提供 lst 时，它可以推断 T 本身

When you provide lst it can infer T itself

Marker(Some(List(1, 2)))
Marker(Some(List("a", "b")))

出于同样的原因

Marker(Option.empty[List[Int]])
Marker(Option.empty[List[String]])
Marker[Int](None)
Marker[String](None)
Marker(None: Option[List[Int]])
Marker(None: Option[List[String]])

编译而 Marker(None) 不编译.

或者你可以优先考虑你的隐含

Alternatively you can prioritize your implicits

trait LowPrioritySerializer {
  implicit object StringSerializer extends Serializer[String] {
    def serialize(seq: List[String]): String = seq.toString()
  }
}

object Serializer extends LowPrioritySerializer {
  implicit object IntSerializer extends Serializer[Int] {
    def serialize(seq: List[Int]): String = seq.toString()
  }
}

然后 IntSerializer 将首先尝试，如果 IntSerializer 不起作用(如果类型不同)，将第二次尝试 StringSerializer.

Then IntSerializer will be tried firstly and StringSerializer will be tried secondly if IntSerializer didn't work (if type is different).

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