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如何將Scala Cat's Kleisli與任何一種一起使用

[英]How to use Scala Cats' Kleisli with Either

我正在嘗試使用Kleisli編寫返回monad的函數。 它適用於Option:

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)
}

但是我無法使其與Either ...一起使用:

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)
}

我猜每個都有兩個類型參數,一個Kleisle包裝器需要一個輸入和一個輸出類型參數。 我不怎么向Either隱藏左類型...

正如您正確指出的那樣,問題是這樣一個事實: Either接受兩個類型參數,而Kleisli期望一個僅接受一個類型構造函數。 我建議您看看可以解決您問題的kind-projector插件。

您可以通過幾種方式解決此問題:

如果錯誤類型Either是永遠不變的,你可以這樣做:

    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]

如果錯誤類型需要更改,則可以使Result類型改為使用3個類型參數,然后采用相同的方法

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

注意? 來自kind-projector

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