OxApp
To properly handle application interruption and clean shutdown, Ox provides a way to define application entry points
using OxApp
trait. The application’s main run
function is then executed on a virtual thread, with a root Ox
and
IO
capabilities provided.
Here’s an example:
import ox.*
import scala.concurrent.duration.*
object MyApp extends OxApp:
def run(args: Vector[String])(using Ox, IO): ExitCode =
forkUser {
sleep(500.millis)
println("Fork finished!")
}
println(s"Started app with args: ${args.mkString(", ")}!")
ExitCode.Success
When the application receives a SIGINT/SIGTERM, e.g. due to a CTRL+C, the root scope (and hence any child scopes and
forks) are interrupted. This allows for a clean shutdown: any resources that are attached to scopes, or managed using
try-finally
blocks, are released. Application shutdown is handled by adding a Runtime.addShutdownHook
.
In the code below, the resource is released when the application is interrupted:
import ox.*
object MyApp extends OxApp:
def run(args: Vector[String])(using Ox, IO): ExitCode =
releaseAfterScope:
println("Releasing ...")
println("Waiting ...")
never
The run
function receives command line arguments as a Vector
of String
s, a given Ox
and IO
capabilities and
has to return an ox.ExitCode
value which translates to the exit code returned from the program. ox.ExitCode
is
defined as:
enum ExitCode(val code: Int):
case Success extends ExitCode(0)
case Failure(exitCode: Int = 1) extends ExitCode(exitCode)
There’s also a simplified variant of OxApp
for situations where you don’t care about command line arguments.
The run
function doesn’t take any arguments beyond the root Ox
and IO
capabilities, expects no ExitCode
and will
handle any exceptions thrown by printing a stack trace and returning an exit code of 1
:
import ox.*
object MyApp extends OxApp.Simple:
def run(using Ox, IO): Unit = println("All done!")
OxApp
has also a variant that integrates with either
blocks for direct-style error handling called OxApp.WithEitherErrors[E]
. Here, E
is the type of errors from the
run
function that you want to handle. The interesting bit is that run
function in OxApp.WithEitherErrors
receives
an either
block token of type EitherError[E]
(which itself is an alias for Label[Either[E, ExitCode]]
as either
operates on boundary/break mechanism). Therefore, it’s possible to use .ok()
combinators directly in the run
function scope. OxApp.WithEitherErrors
requires that one implements a function that translates application errors
into ExitCode
instances. Here’s an example that always fails and exits with exit code 23
:
import ox.*
import ox.either.*
sealed trait MyAppError
case class ComputationError(msg: String) extends Exception(msg) with MyAppError
object MyApp extends OxApp.WithEitherErrors[MyAppError]:
def doWork(): Either[MyAppError, Unit] = Left(ComputationError("oh no"))
def handleError(myAppError: MyAppError): ExitCode = myAppError match {
case ComputationError(_) => ExitCode.Failure(23)
}
def run(args: Vector[String])(using Ox, EitherError[MyAppError], IO): ExitCode =
doWork().ok() // will end the scope with MyAppError as `doWork` returns a Left
ExitCode.Success
Additional configuration
All ox.OxApp
instances can be configured by overriding the def settings: Settings
method. Settings include:
interruptedExitCode
: what exit code should be returned by the application once it gracefully shutdowns after it was interrupted (for example Ctrl+C was pressed by the user). By default0
(graceful shutdown)handleException
andhandleInterruptedException
: callbacks for exceptions that occur when evaluating the application’s body, or that are thrown when the application shuts down due to an interruption (SIGINT/SIGTERM). By default, the stack traces are printed to stderr, unless a default uncaught exception handler is defined.
Settings can be overridden:
import ox.*
import scala.concurrent.duration.*
object MyApp extends OxApp:
override def settings: OxApp.Settings = OxApp.Settings.Default.copy(
interruptedExitCode = ExitCode.Failure(130)
)
def run(args: Vector[String])(using Ox, IO): ExitCode =
sleep(60.seconds)
ExitCode.Success