Scala：“含糊的隐式值”，但找不到正确的值

问题描述

我正在编写一个小型Scala程序，该程序应该：

从本地FS逐行读取文件
从每一行解析三个双精度值
根据这三个值创建案例类的实例
将这些实例传递到二进制堆

为了能够将String和Double都解析为CoordinatePoint，我想出了以下特征：

trait Parseable[T] {
  def parse(input: String): Either[String,T]
}

并且我为后者提供了许多类型对象实现：

object Parseable {
  implicit val parseDouble: Parseable[Double] = new Parseable[Double] {
    override def parse(input: String): Either[String,Double] = {
      val simplifiedInput = input.replaceAll("[ \\n]","").toLowerCase
      try Right(simplifiedInput.toDouble) catch {
        case _: NumberFormatException =>
          Left(input)
      }
    }
  }

  implicit val parseInt: Parseable[Int] = new Parseable[Int] {
    override def parse(input: String): Either[String,Int] = {
      val simplifiedInput = input.replaceAll("[ \\n]","").toLowerCase
      try Right(simplifiedInput.toInt) catch {
        case _: NumberFormatException =>
          Left(input)
      }
    }
  }

  implicit val parseCoordinatePoint: Parseable[CoordinatePoint] = new Parseable[CoordinatePoint] {
    override def parse(input: String): Either[String,CoordinatePoint] = {
      val simplifiedInput = input.replaceAll("[ \\n]","").toLowerCase
      val unparsedPoints: List[String] = simplifiedInput.split(",").toList
      val eithers: List[Either[String,Double]] = unparsedPoints.map(parseDouble.parse)
      val sequence: Either[String,List[Double]] = eithers.sequence
      sequence match {
        case Left(value) => Left(value)
        case Right(doublePoints) => Right(CoordinatePoint(doublePoints.head,doublePoints(1),doublePoints(2)))
      }
    }
  }
}

我有一个公共对象，该对象将调用委派给相应的隐式Parseable（在同一文件中）：

object InputParser {
  def parse[T](input: String)(implicit p: Parseable[T]): Either[String,T] = p.parse(input)
}

仅供参考-这是CoordinatePoint案例类：

case class CoordinatePoint(x: Double,y: Double,z: Double)

在我的主程序中（确认文件在那里，并且不为空，等等。）我想将每一行转换为CoordinatePoint的实例，如下所示：

  import Parseable._
  import CoordinatePoint._

  ...
  private val bufferedReader = new BufferedReader(new FileReader(fileName))

  private val streamOfMaybeCoordinatePoints: Stream[Either[String,CoordinatePoint]] = Stream
    .continually(bufferedReader.readLine())
    .takeWhile(_ != null)
    .map(InputParser.parse(_))

我得到的错误是：

[error] /home/vgorcinschi/data/eclipseProjects/Algorithms/Chapter 2 Sorting/algorithms2_1/src/main/scala/ca/vgorcinschi/algorithms2_4/selectionfilter/SelectionFilter.scala:42:27: ambiguous implicit values:
[error]  both value parseDouble in object Parseable of type => ca.vgorcinschi.algorithms2_4.selectionfilter.Parseable[Double]
[error]  and value parseInt in object Parseable of type => ca.vgorcinschi.algorithms2_4.selectionfilter.Parseable[Int]
[error]  match expected type ca.vgorcinschi.algorithms2_4.selectionfilter.Parseable[T]
[error]     .map(InputParser.parse(_))
[error]                           ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 1 s,completed Sep 1,2020 10:38:18 PM

我不理解，也不知道在哪里寻找编译器为何找到Parseable[Int]和Parseable[Double]而不是唯一的权利-Parseable[CoordinatePoint]。

所以我想，好的，让我先指定一下转换函数来帮助编译器：

  private val bufferedReader = new BufferedReader(new FileReader(fileName))

  val stringTransformer: String => Either[String,CoordinatePoint] = s => InputParser.parse(s)

  private val streamOfMaybeCoordinatePoints: Stream[Either[String,CoordinatePoint]] = Stream
    .continually(bufferedReader.readLine())
    .takeWhile(_ != null)
    .map(stringTransformer)

可惜这只是在函数声明中的代码上方产生了相同的错误。

我很想知道是什么导致了这种行为。既要纠正代码又要获取个人知识。在这一点上，我很好奇。

解决方法

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either implicit scala typeclass