使用参数化验证结果验证了斯卡拉猫

Question

我遵循 https://typelevel.org/cats/datatypes/validated.html 中的示例，但有一个转折：NEL中元素的类型不是DomainValidation，而是DomainValidation[A]，我不确定如何做到这一点。 以下是我的尝试，但类型有问题。 这可能既是 Scala 的问题，也是 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

我稍微调整了一下你的代码，这编译了：

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
} 

基本上我所做的只是标记MyValidation是协变的，并修复了validateWidgetType和validateQuantity以返回相同的类型

---

关于协方差的说明：

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)

这或多或少地说，既是Float子类型又是String子类型的混凝土类型不完全是Any而是Any的子类型