免费的monad翻译器，只有副作用

Question

我需要编写免费的monad解释器，它只执行有效的操作，但不返回任何结果。对于免费的应用程序，我使用了带有MyAction ~> Const[Unit, ?]签名的解释器，并使用foldMap函数调用它。但是，Const没有monad实例。Id不适合，因为它需要返回实际值(评估嵌入式语言(。我应该编写自己的monad实例，类似于Id但没有底层值，还是有一种更方便的方式来创建不产生任何值的解释器？

Answer 1

"不产生任何价值"是什么意思？这是Freemonad 示例，其中解释器什么都不返回，但会做一些事情。

import cats.{Id, ~>}
import cats.effect.{ExitCode, IO, IOApp}
import cats.free.Free
import cats.free.Free.liftF

object Main extends IOApp {
// define algebra
sealed trait LaunchMissilesA[A]
case class LaunchMissileType1(arg: Int) extends LaunchMissilesA[Unit]
case class LaunchMissileType2(arg: String) extends LaunchMissilesA[Unit]
// define Free type
type LaunchMissiles[A] = Free[LaunchMissilesA, A]
// smart constructors
def launchMissileType1(arg: Int): LaunchMissiles[Unit] = liftF[LaunchMissilesA, Unit](LaunchMissileType1(arg))
def launchMissileType2(arg: String): LaunchMissiles[Unit] = liftF[LaunchMissilesA, Unit](LaunchMissileType2(arg))
// final program
def launchVariousMissiles: LaunchMissiles[Unit] =
for {
_ <- launchMissileType1(1)
_ <- launchMissileType2("str2")
} yield ()
// interpreter #1: IO
def ioMissileLauncher: LaunchMissilesA ~> IO = new (LaunchMissilesA ~> IO) {
override def apply[A](fa: LaunchMissilesA[A]): IO[A] = fa match {
case LaunchMissileType1(arg) =>
IO(println(s"[IO] launching Int missile of type 1: $arg").asInstanceOf[A])
case LaunchMissileType2(arg) =>
IO(println(s"[IO] launching String missile of type 2: $arg").asInstanceOf[A])
}
}
// interpreter #2: Id
def idMissileLauncher: LaunchMissilesA ~> Id = new (LaunchMissilesA ~> Id) {
override def apply[A](fa: LaunchMissilesA[A]): Id[A] = fa match {
case LaunchMissileType1(arg) =>
println(s"[Id] launching Int missile of type 1: $arg").asInstanceOf[A]
case LaunchMissileType2(arg) =>
println(s"[Id] launching String missile of type 2: $arg").asInstanceOf[A]
}
}
// end of the world
override def run(args: List[String]): IO[ExitCode] = for {
_ <- launchVariousMissiles.foldMap(ioMissileLauncher)
_ <- IO(launchVariousMissiles.foldMap(idMissileLauncher))
} yield ExitCode(0)
}