使用参数化验证结果验证的 Scala Cats

Question

我正在按照 https://typelevel.org/cats/datatypes/validated.html 中的示例进行操作，但有一点不同：NEL 中的元素类型不是 DomainValidation 而是 DomainValidation[A]，我不确定如何执行此操作。以下是我的尝试，但在使用类型时遇到了问题。这可能既是一个 Scala 问题，也是一个经过 Cats 验证的问题。

import cats.data._
import cats.implicits._

sealed trait Widget
case object WidgetA extends Widget
case object WidgetB extends Widget

object Widget{
  val values = List(WidgetA, WidgetB)
  def fromString(s: String): Option[Widget] = values.filter(_.toString == s).headOption
}

case class Order( quantity: Float, widget: Widget )

trait MyValidation[A] {
  def errorMessage: String
  def is: A
}

type ValidationResult[A,B] = ValidatedNel[MyValidation[A], B]

case class WidgetQuantityIsNegative(is: Float) extends MyValidation[Float] {
  def errorMessage = s"$is widget quantity is negative"
}

case class WidgetTypeIsInvalid(is: String) extends MyValidation[String] {
  def errorMessage = s"$is is not a valid widget type"
}

def validateQuantity(q: Float): ValidationResult[Float, Float] =
  if(q >= 0) q.validNel else WidgetQuantityIsNegative(q).invalidNel

/*
Here I would like to validate the string.
If it represents a valid Widget, return the valid Widget
else return the invalid */
def validateWidgetType(s: String) =
  Widget.fromString(s) map {widget => widget.validNel } getOrElse WidgetTypeIsInvalid.invalidNel

def validateOrder( quantity: Float, s: String ): ValidationResult[MyValidation[Any], Order] = // NOT SURE IF ANY IS THE RIGHT TYPE
  (
    (validateQuantity(quantity)),
    (validateWidgetType(s))
  ).mapN[Order](Order.apply) // THIS DOES NOT COMPILE

Answer 1

我稍微调整了你的代码并编译：

import cats.data._
import cats.implicits._

sealed trait Widget
case object WidgetA extends Widget
case object WidgetB extends Widget

object Global {

  object Widget {
    val values = List(WidgetA, WidgetB)

    def fromString(s: String): Option[Widget] = values.find(_.toString == s)
  }

  case class Order(quantity: Float, widget: Widget)

  trait MyValidation[+A] {
    def errorMessage: String

    def is: A
  }

  type ValidationResult[A, B] = ValidatedNel[MyValidation[A], B]

  case class WidgetQuantityIsNegative(is: Float) extends MyValidation[Float] {
    def errorMessage = s"$is widget quantity is negative"
  }

  case class WidgetTypeIsInvalid(is: String) extends MyValidation[String] {
    def errorMessage = s"$is is not a valid widget type"
  }

  def validateQuantity(q: Float): ValidationResult[Float, Float] =
    if (q >= 0) q.validNel else WidgetQuantityIsNegative(q).invalidNel

  /*
Here I would like to validate the string.
If it represents a valid Widget, return the valid Widget
else return the invalid string */
  def validateWidgetType(s: String): ValidationResult[String, Widget] =
    Widget.fromString(s) map { widget => widget.validNel } getOrElse WidgetTypeIsInvalid(s).invalidNel

  def validateOrder(quantity: Float, s: String): ValidationResult[Any, Order] = 
    (
      validateQuantity(quantity),
      validateWidgetType(s)
    ).mapN[Order](Order(_, _)) // Order.apply without _ works too but would add one more red line in intellij

}

基本上我所做的就是标记 MyValidation 是协变的并将 validateWidgetType 和 validateQuantity 都固定为 return 相同的类型

关于协方差的注意事项：

当我们删除协方差注释时，会出现以下编译器消息：

Error:(50, 18) type mismatch;
 found   : cats.data.Validated[cats.data.NonEmptyList[cats.Global.MyValidation[_ >: String with Float]],cats.Global.Order]
 required: cats.Global.ValidationResult[Any,cats.Global.Order]
    (which expands to)  cats.data.Validated[cats.data.NonEmptyList[cats.Global.MyValidation[Any]],cats.Global.Order]
Note: cats.data.NonEmptyList[cats.Global.MyValidation[_ >: String with Float]] <: Any, but class Validated is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
    ).mapN[Order](Order.apply)

这或多或少表明既是 Float 又是 String 的子类型的具体类型不完全是 Any 而是 Any 的子类型

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