scala - 如何将 Scala Cats 的 Kleisli 与 Either 一起使用

Question

我正在尝试使用 Kleisli 来组合返回 monad 的函数。它适用于一个选项:

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

object KleisliOptionEx extends App {
  case class Failure(msg: String)
  sealed trait Context
  case class Initial(age: Int)                                   extends Context
  case class AgeCategory(cagetory: String, t: Int)                    extends Context
  case class AgeSquared(s: String, t: Int, u: Int)             extends Context

  type Result[A, B] = Kleisli[Option, A, B]
  val ageCategory: Result[Initial,AgeCategory] =
    Kleisli {
      case Initial(age) if age < 18 => {
        Some(AgeCategory("Teen", age))
      }
    }

  val ageSquared: Result[AgeCategory, AgeSquared] = Kleisli {
      case AgeCategory(category, age) =>  Some(AgeSquared(category, age, age * age))
    }

  val ageTotal = ageCategory andThen ageSquared
  val x = ageTotal.run(Initial(5))
  println(x)
}

但我无法让它与 Either 一起工作...:

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

object KleisliEx extends App {
  case class Failure(msg: String)

  sealed trait Context
  case class Initial(age: Int)                                   extends Context
  case class AgeCategory(cagetory: String, t: Int)                    extends Context
  case class AgeSquared(s: String, t: Int, u: Int)             extends Context

  type Result[A, B] = Kleisli[Either, A, B]

  val ageCategory: Result[Initial,AgeCategory] =
    Kleisli {
      case Initial(age) if age < 18 => Either.right(AgeCategory("Teen", age))
    }

  val ageSquared : Result[AgeCategory,AgeSquared] = Kleisli {
      case AgeCategory(category, age) =>  Either.right(AgeSquared(category, age, age * age))
    }

  val ageTotal = ageCategory andThen ageSquared
  val x = ageTotal.run(Initial(5))

  println(x)
}

我猜 Either 有两个类型参数，而 Kleisle 包装器需要一个输入和一个输出类型参数。我不知道如何从 Either 中隐藏左侧类型...

Answer 1

正如您正确陈述的那样，问题是 Either 采用两个类型参数，而 Kleisli 期望类型构造函数只采用一个。我建议你看看kind-projector插件，因为它可以解决您的问题。

您可以通过多种方式解决这个问题:

如果 Either 中的错误类型始终相同，您可以这样做:

    sealed trait MyError
    type PartiallyAppliedEither[A] = Either[MyError, A]
    type Result[A, B] = Kleisli[PartiallyAppliedEither, A, B]
    // you could use kind projector and change Result to
    // type Result[A, B] = Kleisli[Either[MyError, ?], A, B]

如果需要更改错误类型，您可以让您的 Result 类型改为采用 3 个类型参数，然后遵循相同的方法

type Result[E, A, B] = Kleisli[Either[E, ?], A, B]

注意 ? 来自 kind-projector。