我试图使用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)
}
但是我不能让它与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)
}
我猜要么有两个类型参数,而Kleisle包装器需要一个输入和一个输出类型参数。我不知道如何隐藏Either中的左侧类型......
答案 0 :(得分:5)
正确地说明问题是Either
接受两个类型参数,而Kleisli期望一个类型构造函数只接受一个。
我建议您查看kind-projector插件,因为它会处理您的问题。
您可以通过多种方式解决这个问题:
如果Either
中的错误类型始终与您相同:
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,您可以改为使用Result
类型取3类型参数,然后按照相同的方法
type Result[E, A, B] = Kleisli[Either[E, ?], A, B]
请注意?
来自kind-projector
。