有沒有辦法避免兩次呼叫“run”方法并從主方法中只呼叫一次，或者這是在嵌套閱讀器中執行此操作的正確方法？
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)
}
uj5u.com熱心網友回復：
我不確定您為什么選擇以這種方式定義您的服務，但是如果您的問題是為了避免呼叫run，sumAndShow您可以將Reader/提升IO到ReaderT帶有lift/ 的sliftF并將它們與 組合起來ReaderT.ask：
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 的現有型別類約束的替代泛型實作，以及如何newtype用于替代型別類實體：
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
}
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