cats_stm_0.7.0_4.md

import cats.effect.{ExitCode, IO, IOApp}
import io.github.timwspence.cats.stm.{TVar, STM}
import scala.concurrent.duration._

object Main extends IOApp {

  override def run(args: List[String]): IO[ExitCode] =
    for {
      accountForTim   <- TVar.of[Long](100).commit[IO]
      accountForSteve <- TVar.of[Long](0).commit[IO]
      _               <- printBalances(accountForTim, accountForSteve)
      _               <- giveTimMoreMoney(accountForTim).start
      _               <- transfer(accountForTim, accountForSteve)
      _               <- printBalances(accountForTim, accountForSteve)
    } yield ExitCode.Success

  private def transfer(accountForTim: TVar[Long], accountForSteve: TVar[Long]): IO[Unit] =
    STM.atomically[IO] {
      for {
        balance <- accountForTim.get
        _       <- STM.check(balance > 100)
        _       <- accountForTim.modify(_ - 100)
        _       <- accountForSteve.modify(_ + 100)
      } yield ()
    }

  private def giveTimMoreMoney(accountForTim: TVar[Long]): IO[Unit] =
    for {
      _ <- IO.sleep(5000.millis)
      _ <- STM.atomically[IO](accountForTim.modify(_ + 1))
    } yield ()

  private def printBalances(accountForTim: TVar[Long], accountForSteve: TVar[Long]): IO[Unit] =
    for {
      (amountForTim, amountForSteve) <- STM.atomically[IO](for {
        t <- accountForTim.get
        s <- accountForSteve.get
      } yield (t, s))
      _ <- IO(println(s"Tim: $amountForTim"))
      _ <- IO(println(s"Steve: $amountForSteve"))
    } yield ()

}