类型参数[List [String]]不符合方法确保类型参数的界限[AA>：A]

Question

我有以下代码，不编译：

object CatsValidation {

  type FormData = Map[String, String]
  type FailFast[A] = Either[List[String], A]
  type NumFmtExn = NumberFormatException

  def getValue(name: String)(data: FormData)
  : FailFast[String] =
    data.get(name)
      .toRight(List(s"$name field not specified."))

  def readName(data: Map[String, String])
  : FailFast[String] =
    getValue("name")(data)
      .flatMap(nonBlank("name")(_))

  def readAge(data: Map[String, String])
  : FailFast[Int] =
    getValue("age")(data)
      .flatMap(parseInt)
      .flatMap(nonNegative("age")(_))

  def parseInt(number: String)
  : FailFast[Int] =
    Either.catchOnly[NumFmtExn](number.toInt)
      .leftMap(_ => List(s"$number must be an integer"))

  def nonBlank(name: String)(data: String): FailFast[String] =
    Right(data).
      ensure[List[String]](List(s"$name cannot be blank"))(_.nonEmpty)

  def nonNegative(name: String)(data: Int): FailFast[Int] =
    Right(data).
      ensure[List[String]](List(s"$name must be non-negative"))(_ >= 0)

  def main(args: Array[String]): Unit = {
    println(nonBlank("name")("Dade Murphy"))
    // res36: FailFast[String] = Right(Dade Murphy)
    println(nonBlank("name")(""))
    // res37: FailFast[String] = Left(List(name cannot be blank))
    println(nonNegative("age")(11))
    // res38: FailFast[Int] = Right(11)
    println(nonNegative("age")(-1))
    // res39: FailFast[Int] = Left(List(age must be non-negative))
  }

}  

错误消息是：

Error:(36, 13) type arguments [List[String]] do not conform to method ensure's type parameter bounds [AA >: A]
      ensure[List[String]](List(s"$name cannot be blank"))(_.nonEmpty)
Error:(40, 13) type arguments [List[String]] do not conform to method ensure's type parameter bounds [AA >: A]
      ensure[List[String]](List(s"$name must be non-negative"))(_ >= 0)  

为什么类型不正确？

Answer 1

你在Left输掉了Right(...)的类型

明确设置类型：

def nonBlank(name: String)(data: String): FailFast[String] =
  Right[List[String], String](data).
    ensure[List[String]](List(s"$name cannot be blank"))(_.nonEmpty)

def nonNegative(name: String)(data: Int): FailFast[Int] =
  Right[List[String], Int](data).
    ensure[List[String]](List(s"$name must be non-negative"))(_ >= 0)