如何使用带有猫的嵌套 Reader monad 返回值？

Question

有没有办法避免调用“run”方法两次而只在 main 方法中执行一次，或者这是在嵌套 Readers 中执行此操作的正确方法？

  case class Dependencies(showService: ShowService, sumService: SumService)

  class ShowService {
    def show(s: String): IO[Unit] = IO {println(s)}
  }

  class SumService() {
    def sum(a: Int, b: Int): Reader[Dependencies, IO[Int]] = Reader {_ => IO {a + b} }
  }

  object ModuleA {
    def sumAndShow: ReaderT[IO, Dependencies, Unit] = for {
      x <- ReaderT[IO, Dependencies, Int] (deps => deps.sumService.sum(10, 10).run(deps))
      r <- ReaderT[IO, Dependencies, Unit] (deps => deps.showService.show(x.toString))
    } yield r
  }
  
  override def run(args: List[String]): IO[ExitCode] = {
    val dependencies = Dependencies(new ShowService, new SumService)
    ModuleA.sumAndShow.run(dependencies) *> IO(ExitCode.Success)
  }

Answer 1

我不确定您为什么选择以这种方式定义服务，但如果您的问题是避免在 sumAndShow 中调用 run，您可以使用 @ 将您的 Reader/IO 提升为 ReaderTs 987654327@/liftF 并用ReaderT.ask 组合它们：

def sumAndShow: ReaderT[IO, Dependencies, Unit] =
  for {
    deps <- ReaderT.ask[IO, Dependencies]
    x <- deps.sumService.sum(10, 10).lift[IO].flatMap(ReaderT.liftF)
    r <- ReaderT.liftF(deps.showService.show(x.toString))
  } yield r

具有 Cats/Cats Effect 的现有类型类约束的替代通用实现，以及如何将 newtype 用于替代类型类实例：

import cats.implicits._
import cats.effect._
import cats.effect.std.Console
import cats.{Semigroup, Show}
import io.estatico.newtype.macros.newtype


object Main extends IOApp {
  @newtype case class MInt(value: Int)
  object MInt {
    implicit val multiplicativeSemigroup: Semigroup[MInt] =
      deriving(Semigroup.instance(_ * _))
    implicit val show: Show[MInt] = deriving
  }

  def sumAndShow[F[_]: Console, A: Show: Semigroup](a: A, b: A): F[Unit] =
    Console[F].println(a |+| b)

  override def run(args: List[String]): IO[ExitCode] =
    for {
      _ <- sumAndShow[IO, Int](10, 10) //20

      //you can also "inject" default/custom typeclass "dependencies" explicitly
      _ <- sumAndShow[IO, Int](10, 10)(Console[IO], Show[Int], Semigroup.instance(_ * _)) //100

      //custom typeclass instance with newtype
      _ <- sumAndShow[IO, MInt](MInt(10), MInt(10)) //100

      _ <- sumAndShow[IO, String]("Hello", "World") //HelloWorld
    } yield ExitCode.Success
}