Show how ZIO behave with/without fiber fork.

ZioExecSemantic.scala

/*
 * This test shows that without a fork, execution is purely mono-fiber and sequential.
 */
object TestZioExecSemantic {
  val rt = new DefaultRuntime {}
  trait LOG {
    def apply(s: String): UIO[Unit]
  }
  def makeLog = UIO(new LOG {
    val zero = System.currentTimeMillis()
    def apply(s : String) = UIO(println(s"[${System.currentTimeMillis()-zero}] $s"))
  })

  val semaphore = Semaphore.make(1)
  
  // prog 1 has no 'fork'
  def prog1(c: Clock) = for {
    sem <- semaphore
    log <- makeLog
    _   <- log("sem get 1")
    a   <- sem.withPermit(IO.effect(println("Hello world 1")))
    _   <- log("sem get 2")
    b   <- sem.withPermit(IO.effect({println("sleeping now"); Thread.sleep(2000); println("after sleep: second hello")}))
    _   <- log("sem get 3")
           // at tham point, the semaphore is free because b is fully executed, so no timeout
    c   <- sem.withPermit(IO.effect(println("third hello"))).timeout(Duration(5, java.util.concurrent.TimeUnit.MILLISECONDS)).provide(c)
    _   <- c match {
             case None => log("---- A timeout happened")
             case Some(y) => log("++++ No timeout")
           }
  } yield ()
  
  // prog 2 uses 'fork'
  def prog2(c: Clock) = for {
    sem <- semaphore
    log <- makeLog
    _   <- log("sem get 1")
    a   <- sem.withPermit(IO.effect(println("Hello world 1"))).fork
    _   <- log("sem get 2")
    b   <- sem.withPermit(IO.effect({println("sleeping now"); Thread.sleep(2000); println("after sleep: second hello")})).fork
    _   <- log("sem get 3")
    c   <- sem.withPermit(IO.effect(println("third hello"))).timeout(Duration(5, java.util.concurrent.TimeUnit.MILLISECONDS)).provide(c).fork
    _   <- a.join
    _   <- b.join
    x   <- c.join
    _   <- x match {
             case None => log("---- A timeout happened")
             case Some(y) => log("++++ No timeout")
           }
  } yield ()


  def main(args: Array[String]): Unit = {
    rt.unsafeRunSync(prog1(rt.Environment))
    println("****************")
    rt.unsafeRunSync(prog2(rt.Environment))

/* exec prints:

[2] sem get 1
Hello world 1
[50] sem get 2
sleeping now
after sleep: second hello
[2053] sem get 3
third hello
[2178] ++++ No timeout
****************
[2] sem get 1
Hello world 1
[6] sem get 2
sleeping now
[10] sem get 3
after sleep: second hello
[2015] ---- A timeout happened

Process finished with exit code 0

 */
}