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STM monad for cats-effect

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# stm4cats
![Maven Central](https://img.shields.io/maven-central/v/com.olegpy/stm4cats_2.12.svg?color=06C)
[![Build Status](https://travis-ci.org/oleg-py/stm4cats.svg?branch=master)](https://travis-ci.org/oleg-py/stm4cats)
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An implementation of STM for any cats-effect compatible effect type.

Current stable version is `0.1.0-M1`, available for Scala 2.11 and 2.12 and Scala.JS 0.6:
```scala
// Use %%% for Scala.JS
libraryDependencies += "com.olegpy" %% "stm4cats" % "0.1.0-M1"
```

Or, if you're feeling adventurous, a snapshot is build from `master` on each commit.
```scala
resolvers += Resolver.sonatypeRepo("snapshots")
libraryDependencies += "com.olegpy" %% "stm4cats" % "0.1.0-SNAPSHOT"
```

### Try it
```scala
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.update(_ - amt)
    _ <- toR.update(_ + amt)
  } yield ()
  
def freeMoney(toR: TRef[Int]): IO[Unit] = STM.atomically[IO] {
  toR.update(_ + 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](https://www.youtube.com/watch?v=d6WWmia0BPM) by @jdegoes and @wi101 on STM in ZIO
- An [alternative implementation](https://github.com/TimWSpence/cats-stm) by @TimWSpence
- And last, but not least,
[Beautiful concurrency paper](https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/beautiful.pdf)
by Simon Peyton Jones

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