Scala (scalaz) State monad - 映射 Option 状态类型

Question

如何申请实现以下功能？

def wrapIntoOption(state: State[S, A]): State[Option[S], Option[A]]

大图是这样的：

case class Engine(cylinders: Int)
case class Car(engineOpt: Option[Engine])

val engineOptLens = Lens.lensu((c, e) => c.copy(engineOpt = e), _.engineOpt)

def setEngineCylinders(count: Int): State[Engine, Int] = State { engine =>
  (engine.copy(cylinders = count), engine.cylinders)
}

def setEngineOptCylinders(count: Int): State[Option[Engine], Option[Int]] = {
  ??? // how to reuse setEngineCylinders?
}

def setCarCylinders(count: Int): State[Car, Option[Int]] = {
  engineOptLens.lifts(setEngineOptCylinders)
}

有没有更好的方法来处理选项属性？

Answer 1

创建 wrapIntoOption 函数的一种方法是在传递的 State[S, A] 上调用 run 并将生成的 (S, A) 元组转换为 (Option[S], Option[A])。

import scalaz.State
import scalaz.std.option._
import scalaz.syntax.std.option._

def wrapIntoOption[S, A](state: State[S, A]): State[Option[S], Option[A]] =
  State(_.fold((none[S], none[A])){ oldState =>
    val (newState, result) = state.run(oldState)
    (newState.some, result.some)
  })

然后您可以将 setEngineOptCylinders 定义为：

def setEngineOptCylinders(count: Int): State[Option[Engine], Option[Int]] = 
  wrapIntoOption(setEngineCylinders(count))

您可以将其用作：

scala> val (newEngine, oldCylinders) = setEngineOptCylinders(8).run(Engine(6).some)
newEngine: Option[Engine] = Some(Engine(8))
oldCylinders: Option[Int] = Some(6)

Scala (scalaz) State monad - 映射 Option 状态类型

Scala (scalaz) State monad - map over Option state type

monads

state

scala

scalaz

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