ConsensusApp.scala

import scala.util.Random
import scala.concurrent.duration._
import cats.effect.{ExitCode, IOApp, IO}
import cats.effect.concurrent.MVar
import cats.instances.list._
import cats.syntax.apply._
import cats.syntax.parallel._

object ConsensusApp extends IOApp {

  class Consensus(val mvar: MVar[IO, Int]) {
    def propose(value: Int): IO[Int] =
      mvar.tryPut(value) *> mvar.read
  }

  def propose(consensus: Consensus, value: Int): IO[Int] =
    for {
      delay  <- IO(Random.nextInt(1000))
      _      <- IO.sleep(FiniteDuration.apply(delay, MILLISECONDS))
      result <- consensus.propose(value)
      _      <- IO(println(s"Proposed $value, got $result, delay $delay ms"))
    } yield result

  def run(args: List[String]): IO[ExitCode] =
    for {
      mvar      <- MVar.empty[IO, Int]
      consensus = new Consensus(mvar)
      _         <- (1 to 10).toList.parTraverse(value => propose(consensus, value))
    } yield ExitCode.Success
}