Cats STM

An implementation of Software Transactional Memory for Cats Effect, inspired by Beautiful Concurrency.

For more information, see the documentation.

Usage

libraryDependencies += "io.github.timwspence" %% "cats-stm" % "0.2.0"

I haven't setup cross-building yet so I'm afraid this is only available on 2.12 for the moment.

The core abstraction is the TVar (transactional var), which exposes operations in the STM monad. Once constructed, STM actions can be atomically evaluated in the IO monad.

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

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] = for {
    _ <- STM.atomically[IO] {
      for {
        balance <- accountForTim.get
        _       <- STM.check(balance > 100)
        _       <- accountForTim.modify(_ - 100)
        _       <- accountForSteve.modify(_ + 100)
      } yield ()
    }
  } yield ()

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

  private def printBalances(accountForTim: TVar[Long], accountForSteve: TVar[Long]): IO[Unit] = for {
    _ <- accountForTim.get.commit[IO].flatMap(b => IO(println(s"Tim: $b")))
    _ <- accountForSteve.get.commit[IO].flatMap(b => IO(println(s"Tim: $b")))
  } yield ()

}

Documentation

The documentation is built using sbt microsites. You can generate it via sbt makeMicrosite. You can view it locally via cd target/site && jekyll serve.

You can also publish to Github pages via sbt publishMicrosite.