ReaderT + EitherT stack

mtl-example.scala

import cats.*
import cats.syntax.all.*
import cats.data.*
import cats.effect.*

trait AppError(val msg: String)
case object InvalidConfig extends AppError("ERROR: invalid config :(")

final case class Config(port: Int)
type Result = String

val validConfig: Function1[Config, Boolean] = (config) => config.port % 3 == 2
val serviceCall: Function1[Config, IO[String]] = (config) =>
  s"Running service on port ${config.port}".pure

type EitherApp[A] = EitherT[IO, AppError, A]
// defined type alias EitherApp

type Stack[A] = ReaderT[EitherApp, Config, A]
// defined type alias Stack

def program: Stack[Result] = for {
  config <- ReaderT.ask[EitherApp, Config]
  _ <- ReaderT
    .liftF(
      EitherT.liftF[IO, AppError, Unit](
        IO.raiseError(new RuntimeException("runtime error"))
      )
    )
    .whenA(config.port % 3 == 0)
  _ <-
    if (validConfig(config)) ().pure[Stack]
    else ReaderT.liftF[EitherApp, Config, Unit](EitherT.leftT(InvalidConfig))
  result <- ReaderT.liftF(
    EitherT.liftF[IO, AppError, Result](serviceCall(config))
  )
  _ <- ReaderT.liftF(EitherT.liftF[IO, AppError, Unit](IO.println(result)))
} yield result
// program: Stack[Result]

object MyApp extends IOApp {
  override def run(args: List[String]): IO[ExitCode] =
    val rand = new scala.util.Random
    program
      .run(Config(port = rand.nextInt(3)))
      .foldF(
        err => IO.println(err.msg),
        result => IO.println(result)
      )
      .onError(ex => IO.println(s"Unexpected exception: $ex"))
      .as(ExitCode.Success)
}