猫用mapN验证

Question

我是Cats的初学者.验证猫有错误.我使用这样的列表累加器:

case class Type(
 name: String,
 pattern: String,
 primitiveType: PrimitiveType = PrimitiveType.string,
 sample: Option[String] = None,
 comment: Option[String] = None,
 stat: Option[Stat] = None
) {
 type ValidationResult[A] = Validated[List[String], A]

 def checkValidity(): ValidationResult[Boolean] = {
  val errorList: mutable.MutableList[String] = mutable.MutableList.empty

  val patternIsValid = Try {
   primitiveType match {
    case PrimitiveType.struct =>
    case PrimitiveType.date =>
      new SimpleDateFormat(pattern)
    case PrimitiveType.timestamp =>
      pattern match {
        case "epoch_second" | "epoch_milli" =>
        case _ if PrimitiveType.formatters.keys.toList.contains(pattern) =>
        case _ =>
          DateTimeFormatter.ofPattern(pattern)
      }
    case _ =>
      Pattern.compile(pattern)
   }
 }
 if (patternIsValid.isFailure)
  errorList += s"Invalid Pattern $pattern in type $name"
 val ok = sample.forall(this.matches)
 if (!ok)
  errorList += s"Sample $sample does not match pattern $pattern in type $name"
 if (errorList.nonEmpty)
  Invalid(errorList.toList)
 else
  Valid(true)
}
}

当我在我的case类类型中使用此函数时:

case class Types(types: List[Type]) {

 type ValidationResult[A] = Validated[List[String], A]

 def checkValidity(): ValidationResult[Boolean] = {
   val typeNames = types.map(_.name)
   val dup: ValidationResult[Boolean] =
   duplicates(typeNames, s"%s is defined %d times. A type can only be defined once.")
  (dup,types.map(_.checkValidity()).sequence).mapN((_,_) => true)
 }
}

我有这个错误

Error:(29, 39) Cannot prove that cats.data.Validated[List[String],Boolean] <:< G[A].
(dup,types.map(_.checkValidity()).sequence: _*).mapN((_,_) => true)

你能帮我解决一下这个错误吗？

谢谢你的帮助.

Answer 1

很多年前,我写了一篇关于你在这里遇到的潜在问题的博文,如果你对历史或一些旧的解决方法感兴趣,但幸运的是现在解决方案要容易得多(假设你使用的是Scala 2.11或者2.12):只需添加-Ypartial-unification到Scala编译器选项中.例如,如果您使用的是sbt,则可能如下所示:

scalacOptions += "-Ypartial-unification"

而且你已经完成了.

如果由于某种原因您无法添加编译器选项,则必须显式提供某些类型参数.这是一个快速简化版本:

import cats.data.Validated, cats.implicits._


case class Foo(i: Int) {
  type ValidationResult[A] = Validated[List[String], A]

  def check: ValidationResult[Boolean] =
    if (i < 0) Validated.invalid(List("bad")) else Validated.valid(true)
}

case class Foos(values: List[Foo]) {
  type ValidationResult[A] = Validated[List[String], A]

  def dup: ValidationResult[Unit] = Validated.valid(())
  def check: ValidationResult[Boolean] =
    (dup, values.map(_.check).sequence).mapN((_, _) => true)
}

这将因您看到的错误而失败(假设您尚未添加-Ypartial-unification):

<console>:22: error: Cannot prove that cats.data.Validated[List[String],Boolean] <:< G[A].
           (dup, values.map(_.check).sequence).mapN((_, _) => true)
                                     ^

要修复它,您可以编写以下内容:

case class Foos(values: List[Foo]) {
  type ValidationResult[A] = Validated[List[String], A]

  def dup: ValidationResult[Unit] = Validated.valid(())
  def check: ValidationResult[Boolean] =
    (dup, values.map(_.check).sequence[ValidationResult, Boolean]).mapN((_, _) => true)
}

我想你也可能只是将类型别名移动到包级别,但我不是百分之百确定,并且没有动力去检查,抱歉.

一个脚注:任何时候你有map,然后sequence,你可以通过traverse改为使事情变得更快:

  def check: ValidationResult[Boolean] =
    (dup, values.traverse[ValidationResult, Boolean](_.check)).mapN((_, _) => true)

您可以再次删除类型参数,如果已-Ypartial-unification启用,则允许类型推断计算出来.