Resources and concurrency

with input = java.util.Scanner(System.in) {
    if (input.hasNextLine()) { sayLine(input.nextLine()); }
}

async fun count() -> Int { return await task { return 7; }; }

with closes resources in reverse order. defer runs cleanup on scope exit. expect, require and ensure remain active without JVM assertion flags. guard lock { ... } synchronizes on a non-null receiver and prohibits await inside the guard.

Tasks run on virtual threads. Task<T> extends CompletionStage<T> for Java callers. await accepts either type, handles cancellation/failure/interruption, and is valid only inside async functions or task bodies.