Packages

trait IOApp extends AnyRef

The primary entry point to a Cats Effect application. Extend this trait rather than defining your own main method. This avoids the need to run IO.unsafeRunSync (or similar) on your own.

IOApp takes care of the messy details of properly setting up (and tearing down) the unsafe.IORuntime needed to run the IO which represents your application. All of the associated thread pools (if relevant) will be configured with the assumption that your application is fully contained within the IO produced by the run method. Note that the exact details of how the runtime will be configured are very platform-specific. Part of the point of IOApp is to insulate users from the details of the underlying runtime (whether JVM or JavaScript).

object MyApplication extends IOApp {
  def run(args: List[String]) =
    for {
      _ <- IO.print("Enter your name: ")
      name <- IO.readln
      _ <- IO.println("Hello, " + name)
    } yield ExitCode.Success
}

In the above example, MyApplication will be a runnable class with a main method, visible to Sbt, IntelliJ, or plain-old java. When run externally, it will print, read, and print in the obvious way, producing a final process exit code of 0. Any exceptions thrown within the IO will be printed to standard error and the exit code will be set to 1. In the event that the main Fiber (represented by the IO returned by run) is canceled, the runtime will produce an exit code of 1.

Note that exit codes are an implementation-specific feature of the underlying runtime, as are process arguments. Naturally, all JVMs support these functions, as does Node.js and Scala Native, but some JavaScript execution environments will be unable to replicate these features (or they simply may not make sense). In such cases, exit codes may be ignored and/or argument lists may be empty.

Note that in the case of the above example, we would actually be better off using IOApp.Simple rather than IOApp directly, since we are neither using args nor are we explicitly producing a custom ExitCode:

object MyApplication extends IOApp.Simple {
  val run =
    for {
      _ <- IO.print("Enter your name: ")
      name <- IO.readln
      _ <- IO.println(s"Hello, " + name)
    } yield ()
}

It is valid to define val run rather than def run because IO's evaluation is lazy: it will only run when the main method is invoked by the runtime.

In the event that the process receives an interrupt signal (SIGINT) due to Ctrl-C (or any other mechanism), it will immediately cancel the main fiber. Assuming this fiber is not within an uncancelable region, this will result in interrupting any current activities and immediately invoking any finalizers (see: IO.onCancel and IO.bracket). The process will not shut down until the finalizers have completed. For example:

object InterruptExample extends IOApp.Simple {
  val run =
    IO.bracket(startServer)(
      _ => IO.never)(
      server => IO.println("shutting down") *> server.close)
}

If we assume the startServer function has type IO[Server] (or similar), this kind of pattern is very common. When this process receives a SIGINT, it will immediately print "shutting down" and run the server.close effect.

One consequence of this design is it is possible to build applications which will ignore process interrupts. For example, if server.close runs forever, the process will ignore interrupts and will need to be cleaned up using SIGKILL (i.e. kill -9). This same phenomenon can be demonstrated by using IO.uncancelable to suppress all interruption within the application itself:

object Zombie extends IOApp.Simple {
  val run = IO.never.uncancelable
}

The above process will run forever and ignore all interrupts. The only way it will shut down is if it receives SIGKILL.

It is possible (though not necessary) to override various platform-specific runtime configuration options, such as computeWorkerThreadCount (which only exists on the JVM). Please note that the default configurations have been extensively benchmarked and are optimal (or close to it) in most conventional scenarios.

However, with that said, there really is no substitute to benchmarking your own application. Every application and scenario is unique, and you will always get the absolute best results by performing your own tuning rather than trusting someone else's defaults. IOApp's defaults are very good, but they are not perfect in all cases. One common example of this is applications which maintain network or file I/O worker threads which are under heavy load in steady-state operations. In such a performance profile, it is usually better to reduce the number of compute worker threads to "make room" for the I/O workers, such that they all sum to the number of physical threads exposed by the kernel.

Source
IOApp.scala
See also

IO

run

ResourceApp

IOApp.Simple

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Abstract Value Members

  1. abstract def run(args: List[String]): IO[ExitCode]

    The entry point for your application.

    The entry point for your application. Will be called by the runtime when the process is started. If the underlying runtime supports it, any arguments passed to the process will be made available in the args parameter. The numeric value within the resulting ExitCode will be used as the exit code when the process terminates unless terminated exceptionally or by interrupt.

    args

    The arguments passed to the process, if supported by the underlying runtime. For example, java com.company.MyApp --foo --bar baz or node com-mycompany-fastopt.js --foo --bar baz would each result in List("--foo", "--bar", "baz").

    See also

    IOApp.Simple!.run:cats\.effect\.IO[Unit]*

Concrete Value Members

  1. final def !=(arg0: Any): Boolean
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  2. final def ##: Int
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  3. final def ==(arg0: Any): Boolean
    Definition Classes
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  4. def MainThread: ExecutionContext

    Executes the provided actions on the JVM's main thread.

    Executes the provided actions on the JVM's main thread. Note that this is, by definition, a single-threaded executor, and should not be used for anything which requires a meaningful amount of performance. Additionally, and also by definition, this process conflicts with producing the results of an application. If one fiber calls evalOn(MainThread) while the main fiber is returning, the first one will "win" and will cause the second one to wait its turn. Once the main fiber produces results (or errors, or cancels), any remaining enqueued actions are ignored and discarded (a mostly irrelevant issue since the process is, at that point, terminating).

    This is not recommended for use in most applications, and is really only appropriate for scenarios where some third-party library is sensitive to the exact identity of the calling thread (for example, LWJGL). In these scenarios, it is recommended that the absolute minimum possible amount of work is handed off to the main thread.

    Attributes
    protected
  5. final def asInstanceOf[T0]: T0
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  6. def clone(): AnyRef
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    protected[lang]
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    @throws(classOf[java.lang.CloneNotSupportedException]) @native()
  7. def computeWorkerThreadCount: Int

    Controls the number of worker threads which will be allocated to the compute pool in the underlying runtime.

    Controls the number of worker threads which will be allocated to the compute pool in the underlying runtime. In general, this should be no greater than the number of physical threads made available by the underlying kernel (which can be determined using Runtime.getRuntime().availableProcessors()). For any application which has significant additional non-compute thread utilization (such as asynchronous I/O worker threads), it may be optimal to reduce the number of compute threads by the corresponding amount such that the total number of active threads exactly matches the number of underlying physical threads.

    In practice, tuning this parameter is unlikely to affect your application performance beyond a few percentage points, and the default value is optimal (or close to optimal) in most common scenarios.

    This setting is JVM-specific and will not compile on JavaScript.

    For more details on Cats Effect's runtime threading model please see https://typelevel.org/cats-effect/docs/thread-model.

    Attributes
    protected
  8. final def eq(arg0: AnyRef): Boolean
    Definition Classes
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  9. def equals(arg0: AnyRef): Boolean
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  10. def finalize(): Unit
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    @throws(classOf[java.lang.Throwable])
  11. final def getClass(): Class[_ <: AnyRef]
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    @native()
  12. def hashCode(): Int
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    @native()
  13. final def isInstanceOf[T0]: Boolean
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    Any
  14. def logNonDaemonThreadsEnabled: Boolean

    Controls whether non-daemon threads blocking application exit are logged to stderr when the IO produced by run has completed.

    Controls whether non-daemon threads blocking application exit are logged to stderr when the IO produced by run has completed. This mechanism works by starting a daemon thread which periodically polls all active threads on the system, checking for any remaining non-daemon threads and enumerating them. This can be very useful for determining why your application isn't gracefully exiting, since the alternative is that the JVM will just hang waiting for the non-daemon threads to terminate themselves. This mechanism will not, by itself, block shutdown in any way. For this reason, it defaults to true.

    In the event that your application exit is being blocked by a non-daemon thread which you cannot control (i.e. a bug in some dependency), you can circumvent the blockage by appending the following to the IO returned from run:

    val program: IO[ExitCode] = ???                      // the original IO returned from `run`
    program.guarantee(IO(Runtime.getRuntime().halt(0)))  // the bit you need to add

    This finalizer will forcibly terminate the JVM (kind of like kill -9), ignoring daemon threads and shutdown hooks, but only after all native Cats Effect finalizers have completed. In most cases, this should be a relatively benign thing to do, though it's definitely a bad default. Only use this to workaround a blocking non-daemon thread that you cannot otherwise influence!

    Can also be configured by setting the cats.effect.logNonDaemonThreadsOnExit system property.

    Attributes
    protected
    See also

    logNonDaemonThreadsInterval

  15. def logNonDaemonThreadsInterval: FiniteDuration

    Controls the interval used by the non-daemon thread detector.

    Controls the interval used by the non-daemon thread detector. Defaults to 10.seconds.

    Can also be configured by setting the cats.effect.logNonDaemonThreads.sleepIntervalMillis system property.

    Attributes
    protected
    See also

    logNonDaemonThreadsEnabled

  16. final def main(args: Array[String]): Unit
  17. final def ne(arg0: AnyRef): Boolean
    Definition Classes
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  18. final def notify(): Unit
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    @native()
  19. final def notifyAll(): Unit
    Definition Classes
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    @native()
  20. def reportFailure(err: Throwable): IO[Unit]

    Configures the action to perform when unhandled errors are caught by the runtime.

    Configures the action to perform when unhandled errors are caught by the runtime. By default, this simply delegates to cats.effect.std.Console!.printStackTrace. It is safe to perform any IO action within this handler; it will not block the progress of the runtime. With that said, some care should be taken to avoid raising unhandled errors as a result of handling unhandled errors, since that will result in the obvious chaos.

    Attributes
    protected
  21. def runtime: IORuntime

    The runtime which will be used by IOApp to evaluate the IO produced by the run method.

    The runtime which will be used by IOApp to evaluate the IO produced by the run method. This may be overridden by IOApp implementations which have extremely specialized needs, but this is highly unlikely to ever be truly needed. As an example, if an application wishes to make use of an alternative compute thread pool (such as Executors.fixedThreadPool), it is almost always better to leverage IO.evalOn on the value produced by the run method, rather than directly overriding runtime.

    In other words, this method is made available to users, but its use is strongly discouraged in favor of other, more precise solutions to specific use-cases.

    This value is guaranteed to be equal to unsafe.IORuntime.global.

    Attributes
    protected
  22. def runtimeConfig: IORuntimeConfig

    The configuration used to initialize the runtime which will evaluate the IO produced by run.

    The configuration used to initialize the runtime which will evaluate the IO produced by run. It is very unlikely that users will need to override this method.

    Attributes
    protected
  23. final def synchronized[T0](arg0: => T0): T0
    Definition Classes
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  24. def toString(): String
    Definition Classes
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  25. final def wait(): Unit
    Definition Classes
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    @throws(classOf[java.lang.InterruptedException])
  26. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
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    @throws(classOf[java.lang.InterruptedException])
  27. final def wait(arg0: Long): Unit
    Definition Classes
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    @throws(classOf[java.lang.InterruptedException]) @native()

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