Funtional Concurrency in Scala 101

git clone git@github.com:Avasil/fp-concurrency-101.git
or https://github.com/Avasil/fp-concurrency-101.git

git checkout exercises

sbt exercises/compile
sbt client/fastOptJS
sbt server/compile

class Cafe {
    def buyCoffee(cc: CreditCard): Coffee = {
        val cup = new Coffee()
        cc.charge(cup.price)
        cup
    }
}

class Cafe {
    def buyCoffee(cc: CreditCard): (Coffee, Charge) = {
        val cup = new Coffee()
        (cup, new Charge(cc, cup.price))
    }
}

val a: Int = 10
val b: Int = a + a // 20

val c: Int = 10 + 10 // 20

val r = scala.util.Random

val a: Int = r.nextInt // 7
val b: Int = a + a     // 7 + 7

val c: Int = r.nextInt + r.nextInt // 7 + 5

import monix.eval.Task
import cats.implicits._

val r = scala.util.Random
// when run, generates 7
val a: Task[Int] = Task(r.nextInt) 
a.runSyncUnsafe // 7

val b: Task[Int] = (a, a).mapN(_ + _) 
b.runSyncUnsafe // 7 + 5

val c: Task[Int] = (Task(r.nextInt), Task(r.nextInt)).mapN(_ + _) 
c.runSyncUnsafe // 7 + 5

val d: Task[Int] = a.map(x => x + x)
d.runSyncUnsafe // 7 + 7

import monix.eval.Task

val task = Task { println("hello!") }

// nothing happens yet, it's just a description 
// of evaluation that will produce `Unit`, end
// with error or never complete
val program: Task[Unit] =
  for {
     _ <- task 
     _ <- task 
  } yield ()

// prints "hello!" twice
program.runSyncUnsafe()

import monix.eval.Task
import cats.syntax.flatmap._

val program: Task[Unit] = 
  taskA.flatMap(_ => taskB)

val program: Task[Unit] = 
  taskA >> taskB

import monix.eval.Task
import scala.concurrent.duration._

// waits 100 millis (without blocking any Thread!) 
// then prints "woke up"
Task.sleep(100.millis) >> Task(println("woke up"))


// never finishes
Task.never >> Task(println("woke up"))

// will print A, B and C
val program: Task[Int] =
  for {
    _ <- taskA
    // in case of error executes the Task in argument
    _ <- taskB.handleErrorWith(_ => Task(println("B")))
    _ <- taskC
  } yield 42

def retryBackoff[A](source: Task[A])
                   (maxRetries: Int, delay: FiniteDuration): Task[A] = {
  source.onErrorHandleWith { ex =>
    if (maxRetries > 0)
      retryBackoff(source)(maxRetries - 1, delay * 2)
        .delayExecution(delay)
    else
      Task.raiseError(ex)
  }
}

import monix.eval.Task
import cats.implicits._

val taskA = Task(println("A"))
val taskB = Task(println("B"))

val listOfTasks: List[Task[Unit]] = List(taskA, taskB)

val taskOfList: Task[List[Unit]] = Task.gather(listOfTasks)

// using Parallel type class instance from Cats
val parTask: Task[Unit] = 
  (taskA, taskB).parTupled

val taskInParallel: Task[Unit] =
  for {
    // run in "background"
    fibA <- taskA.start 
    fibB <- taskB.start
    // wait for ioa result
    _ <- fibA.join 
    _ <- fibB.join
  } yield ()


// run both in parallel 
// and cancel the loser
val raceTask: Task[Either[Unit, Unit]] = 
  Task.race(taskA, taskB)

  import monix.execution.schedulers.TestScheduler
  import scala.concurrent.duration._

  def retryBackoff[A](source: Task[A])
                     (maxRetries: Int, delay: FiniteDuration): Task[A] = {
    source.onErrorHandleWith { ex =>
      if (maxRetries > 0)
        retryBackoff(source)(maxRetries - 1, delay * 2)
          .delayExecution(delay)
      else
        Task.raiseError(ex)
    }
  }

  val sc = TestScheduler()

  val failedTask: Task[Int] = Task.raiseError[Int](new Exception("boom"))
  val task: Task[Int] = retryBackoff(failedTask)(5, 2.hours)

  val f: CancelableFuture[Int] = task.runToFuture(sc)
  
  println(f.value) // None
  sc.tick(10.hours)
  println(f.value) // None
  sc.tick(1000.days)
  println(f.value) // Some(Failure(java.lang.Exception: boom))

trait Observable[+A] {
  def subscribe(o: Observer[A]): Cancelable
}

trait Observer[-T] {
  def onNext(elem: T): Future[Ack]

  def onError(ex: Throwable): Unit

  def onComplete(): Unit
}

val list: Observable[Long] = 
    Observable.range(0, 1000)
      .take(100)
      .map(_ * 2)
       
val task: Task[Long] =
    list.foldLeftL(0L)(_ + _)

val task: Task[List[Int]] = 
  for {
    queue     <- ConcurrentQueue.unbounded[Task, Int]()
    // feeds queue in the background
    _         <- runProducer(queue).start 
    list      <- Observable
                   .repeatEvalF(queue.poll())
                   .throttleLast(150.millis)
                   .takeWhile(_ > 100)
                   .toListL
  } yield list

import monix.eval.Task
import monix.execution.schedulers.TestScheduler
import monix.reactive.Observable

import scala.concurrent.duration._

// using `TestScheduler` to manipulate time
implicit val sc = TestScheduler()

val stream: Task[Unit] = {
  Observable
    .intervalWithFixedDelay(2.second)
    .mapEval(l => Task.sleep(2.second).map(_ => l))
    .foreachL(println)
}

stream.runToFuture(sc)

sc.tick(2.second) // prints 0
sc.tick(4.second) // prints 1
sc.tick(4.second) // prints 2
sc.tick(4.second) // prints 3

abstract class Ref[F[_], A] {
  def get: F[A]
  def set(a: A): F[Unit]
  def modify[B](f: A => (A, B)): F[B]
  // ... and more
}

import monix.eval.Task
import cats.effect.concurrent.Ref

val stringsRef: Task[Ref[Task, List[String]]] =
  Ref.of[IOTaskList[String]](List())

def take(ref: Ref[Task, List[String]]): Task[Option[String]] =
  ref.modify(list => (list.drop(1), list.headOption))

def put(ref: Ref[Task, List[String]], s: String): Task[Unit] =
  ref.modify(list => (s :: list, ()))

val program: Task[Unit] =
  for {
    _ <- stringsRef.flatMap(put(_, "Scala"))
    element <- stringsRef.flatMap(take)
  } yield println(element)

program.runSyncUnsafe() // prints None

val program: Task[Unit] =
  for {
    ref <- stringsRef
    _ <- put(ref, "Scala")
    element <- take(ref)
  } yield println(element)

program.runSyncUnsafe() // prints "Some(Scala)"

abstract class Deferred[F[_], A] {
  def get: F[A]
  def complete(a: A): F[Unit]
}

val program =
  for {
    stopRunning <- Deferred[Task, Unit]
    _           <- runImportantService(healthcheck).start
    _           <- Task.race(stopRunning.get, runOtherService.loopForever)
  } yield ()

// somewhere in ImportantService
stopRunning.complete(())