如何使用Scala猫的Kleisli

Question

我正在尝试使用Kleisli来编写函数，返回一个monad。它适用于一种选择：

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)
}

但我不能和一个.：

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)
}

我想两者都有两个类型参数，Kleisle包装器需要一个输入和一个输出类型参数。我不知道我怎么能把左边的类型藏起来.

Answer 1

正如您正确地指出的，问题在于Either接受两个类型参数，而Kleisli则期望一个只需要一个类型的类型构造函数。我建议你看看放映机插件，因为它可以解决你的问题。

你可以从几个方面解决这个问题：

如果Either中的错误类型总是相同的，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，则可以让Result类型接受3种类型的参数，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。