Aqui estão alguns códigos de gatos Scala usando oIO Monad:

import java.util.concurrent.{ExecutorService, Executors}

import cats.effect.IO

import scala.concurrent.{ExecutionContext, ExecutionContextExecutor}
import scala.util.control.NonFatal

object Program extends App {

  type CallbackType = (Either[Throwable, Unit]) => Unit

  // IO.async[Unit] is like a Future that returns Unit on completion.
  // Unlike a regular Future, it doesn't start to run until unsafeRunSync is called.
  def forkAsync(toRun: () => Unit)(executor: ExecutorService): IO[Unit] = IO.async[Unit] { callback: CallbackType =>
    // "callback" is a function that either takes a throwable (Left) or whatever toRun returns (Right).
    println("LalalaAsync: " + Thread.currentThread().getName)
    executor.execute(new Runnable {
      def run(): Unit = {
        val nothing: Unit = toRun() // Note: This line executes the body and returns nothing, which is of type Unit.
        try {
          callback(Right(nothing)) // On success, the callback returns nothing
        } catch {
          case NonFatal(t) => callback(Left(t)) // On failure, it returns an exception
        }
      }
    })
  }

  def forkSync(toRun: () => Unit)(executor: ExecutorService): IO[Unit] = IO.apply {
    println("LalalaSync: " + Thread.currentThread().getName)
    executor.execute(new Runnable {
      def run(): Unit = {
        toRun()
      }
    })
  }

  val treadPool: ExecutorService = Executors.newSingleThreadExecutor()
  val mainThread: Thread = Thread.currentThread()

  val Global: ExecutionContextExecutor = ExecutionContext.global

  /*
  Output:
    1 Hello World printed synchronously from Main.main
    LalalaSync: scala-execution-context-global-12
    Hello World printed synchronously from thread pool.pool-1-thread-1
    LalalaAsync: scala-execution-context-global-12
    Hello World printed asynchronously from thread pool.pool-1-thread-1
    2 Hello World printed synchronously from Global .scala-execution-context-global-12
   */
  val program = for {
    _ <- IO {
      println("1 Hello World printed synchronously from Main." + Thread.currentThread().getName) // "main" thread
    }
    _ <- IO.shift(Global) // Shift to Global Execution Context
    _ <- forkSync { () =>
      println("Hello World printed synchronously from thread pool." + Thread.currentThread().getName) // "pool-1-thread-1" thread
    }(treadPool)
    _ <- forkAsync { () =>
      println("Hello World printed asynchronously from thread pool." + Thread.currentThread().getName) // "pool-1-thread-1" thread
    }(treadPool)
    _ <- IO.shift(Global) // Shift to Global Execution Context
    _ <- IO {
      println("2 Hello World printed synchronously from Global ." + Thread.currentThread().getName) // "scala-execution-context-global-13" thread
    }
  } yield ()

  program.unsafeRunSync()
}

Para executá-lo, você precisará adicionar:

libraryDependencies ++= Seq(
  "org.typelevel" %% "cats" % "0.9.0",
  "org.typelevel" %% "cats-effect" % "0.3"
),

Para o seu arquivo build.sbt.

Observe a saída:

  /*
  Output:
    1 Hello World printed synchronously from Main.main
    LalalaSync: scala-execution-context-global-12
    Hello World printed synchronously from thread pool.pool-1-thread-1
    LalalaAsync: scala-execution-context-global-12
    Hello World printed asynchronously from thread pool.pool-1-thread-1
    2 Hello World printed synchronously from Global .scala-execution-context-global-12
 */

Basicamente, não entendo como o IO.shift (Global) ou como o IO.async funciona.

Por exemplo, por que depois que eu chamo "forkAsync", se eu não chamo "IO.shift (Global)", os objetos síncronos subsequentes são executados em "pool-1-thread-1". Além disso, qual é a diferença entre forkAsync e forkSync neste exemplo? Os dois iniciam no ExecutionContext.global e, em seguida, executam um Runnable em "pool.pool-1-thread-1".

Como forkAsync e forkSync estão fazendo exatamente a mesma coisa ou forkAsync está fazendo algo diferente? Se eles estão fazendo a mesma coisa, qual é o sentido de agrupar o código no IO.async? Se eles não estão fazendo a mesma coisa, como eles são diferentes?