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我正在尝试使用 Kleisli 来编写返回 monad 的函数。它适用于选项：

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)
}

但我不能让它与 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。我不知道我怎么能从 Either 中隐藏左类型...

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1 回答 1

5

正如您正确陈述的那样，问题在于Either接受两个类型参数的事实，而 Kleisli 期望一个类型构造函数只接受一个。我建议您看一下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此问题：

如果错误类型Either始终相同，您可以执行以下操作：

    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类型取 3 个类型参数，然后采用相同的方法

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

注意?来自kind-projector.

于 2018-04-20T12:15:05.127 回答