stm4cats
An implementation of STM for any cats-effect compatible effect type.
Experiment goals:
- Radically different approach, lock-free instead of a global lock, and ThreadLocals instead of Reader-like structure.
- It's still to be figured out if it's faster than conventional implementation with a global lock, or not.
- Keep everything as minimal as humanly possible. Right now there are only about 300 lines of code.
- However, you need to import
cats.implicits._for all syntax to work
- However, you need to import
- Scala.JS support is a must.
Add it
resolvers += Resolvers.sonatypeRepo("snapshots")
// Use %%% for Scala.JS
libraryDependencies += "com.olegpy" %% "stm4cats" % "0.1.0-SNAPSHOT"
Try it
import cats.implicits._
import cats.effect.IO
import com.olegpy.stm._
import scala.concurrent.ExecutionContext.global
import scala.concurrent.duration._
implicit val cs = IO.contextShift(global)
implicit val timer = IO.timer(global)
def transfer(fromR: TRef[Int], toR: TRef[Int], amt: Int): STM[Unit] =
for {
from <- fromR.get
if from >= amt // Or STM.check(from >= amt)
_ <- fromR.modify(_ - amt)
_ <- toR.modify(_ + amt)
} yield ()
def freeMoney(toR: TRef[Int]): IO[Unit] = STM.atomically[IO] {
toR.modify(_ + 10)
}
val io = for {
fromR <- TRef(0).commit[IO]
// Or shorter syntax:
toR <- TRef.in[IO](0)
amt = 100
f1 <- transfer(fromR, toR, amt).commit[IO].start
f2 <- (freeMoney(fromR) >> IO.sleep(1.second)).foreverM.start
// In 10 seconds, the transfer succeeds
_ <- f1.join
_ <- f2.cancel
res <- toR.get.commit[IO]
_ <- IO(assert(res == amt))
} yield ()
io.unsafeRunSync() // Well, not on JS
Acknowledgements
My interest in STM, as well as some of API in stm4cats was influenced by:
- Talk by @jdegoes and @wi101 on STM in ZIO
- An alternative implementation by @TimWSpence
- And last, but not least, Beautiful concurrency paper by Simon Peyton Jones
