[英]How to use Scala Cats' Kleisli with Either
我正在尝试使用Kleisli编写返回monad的函数。 它适用于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)
}
但是我无法使其与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)
}
我猜每个都有两个类型参数,一个Kleisle包装器需要一个输入和一个输出类型参数。 我不怎么向Either隐藏左类型...
正如您正确指出的那样,问题是这样一个事实: Either
接受两个类型参数,而Kleisli期望一个仅接受一个类型构造函数。 我建议您看看可以解决您问题的kind-projector插件。
您可以通过几种方式解决此问题:
如果错误类型Either
是永远不变的,你可以这样做:
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]
如果错误类型需要更改,则可以使Result
类型改为使用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
。
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