Function "accessWashroom" currentTime code is incomplete. Something like: def accessWashroom(person: String, sem: Semaphore[IO]): IO[Unit] = for { t0 <- IO(System.currentTimeMillis()) _ <- IO(s"[+ 0 ms] $person wants to access the washroom, waiting for getting the access").trace _ <- sem.acquire t1 <- IO(System.currentTimeMillis()) _ <- IO(s"[+ \({t1 - t0} ms] \)person got access to washroom and using it now").trace _ <- IO.sleep(5.second) t2 <- IO(System.currentTimeMillis()) _ <- IO(s"[+ \({t2 - t0} ms] \)person left the washroom").trace _ <- sem.release } yield () does the job.

Using Scala 3.8.3 (Windows 10): I get the warning: Compiling project (Scala 3.8.3, JVM (temurin:21)) [warn] .\yadu9.sc:51:38 [warn] method onError in class IO is deprecated since 3.6.0: Use onError with PartialFunction argument [warn] val loggedFailedError_1: IO[Int] = failedIO.onError(ex => IO.println("It failed with message: "+ex.getMessage)) [warn] ^^^^^^^^^^^^^^^^ Compiled project (Scala 3.8.3, JVM (temurin:21)) [io-compute-1] 500 [io-compute-blocker-1] -2 [io-compute-2] -2 [io-compute-blocker-1] 0 [io-compute-blocker-1] 0 [io-compute-blocker-1] -9 It failed with message: Boom! Exception in thread "main" java.lang.RuntimeException: Boom! ... Code section is: val errorIO = IO(100/0) val recoveredFailedIO_1: IO[Int] = errorIO.recover { case ex: ArithmeticException => -9 } // deprecated: method onError in class IO is deprecated since 3.6.0: Use onError with PartialFunction argument val loggedFailedError_1: IO[Int] = failedIO.onError(ex => IO.println("It failed with message: "+ex.getMessage)) // new: val loggedFailedError: IO[Int] = failedIO.onError {case ex => IO.println(s"It failed with message: ${ex.getMessage}")}

Wouldn't it be better to use _ <- IO.println(... instead of _ = println( ... ?

extension [A](io: IO[A]) { def trace: IO[A] = for { res <- io _ = println(s"[${Thread.currentThread().getName}] " + res) } yield res } Should read: extension [A](io: IO[A]) def trace: IO[A] = for { res <- io _ <- IO.println(s"[${Thread.currentThread.getName}] " + res) } yield res

In the last code example, the indentation is incorrect.

def example6 = val io1 = IO("Scala6") val io2 = IO("Cats6") val listTraverse = Traverse[List] // a typeclass instance -> import cats.implicits.toTraverseOps val ioList: List[IO[String]] = List(io1, io2) val insideOutIOs: IO[List[String]] = listTraverse.sequence(ioList) // is equal to the sequence-method: // val seqAsTraverse: IO[List[String]] = listTraverse.traverse(ioList)(identity) val traversedList: IO[List[String]] = listTraverse.traverse(ioList)(io => io.map(_ + "!")) traversedList.trace Due to the import, no typeclass instance is needed anymore! def example7 = import cats.implicits.toTraverseOps val io1 = IO("Scala7") val io2 = IO("Cats7") val ioList: List[IO[String]] = List(io1, io2) val insideOutIOs: IO[List[String]] = ioList.sequence val traversedList: IO[List[String]] = ioList.traverse(io => io.map(_ + "!")) traversedList.trace

// deprecated: method onError in class IO is deprecated since 3.6.0: Use onError with PartialFunction argument: val loggedFailedError: IO[Int] = failedIO.onError(ex => IO.println("It failed with message: "+ex.getMessage)) // new: val loggedFailedError: IO[Int] = failedIO.onError {case ex => IO.println(s"It failed with message: ${ex.getMessage}")}

We can use the method attempt to lift the value of an IO into an Either. If the IO failed, it lifts the value as Right, otherwise into a Left: vice versa!