Discarded non-Unit value Error When Using Akka

Question

I have Scala Play application and I am invoking the Scala Akka Scheduler in the class like this:

@Singleton
class Foo @Inject() (
  actorSystem: ActorSystem,
)(implicit
  executionContext: ExecutionContext,
 ) extends Logging {

  actorSystem.scheduler.schedule(delay, interval)(poller())

  def poller(): Unit ...
}

But I get a compiler warning like this

discarded non-Unit value
[error]   actorSystem.scheduler.schedule(delay, interval)(poller())

Is there a way to suppress this or code this in some way to eliminate the compiler error?

score 2 · Accepted Answer · answered Jun 16 '20 at 22:12

This happens because the Scheduler.schedule method returns a Cancellable instance to allow the job to be cancelled, but this code ignores the return value. When the compiler flag -Ywarn-value-discard or -Wvalue-discard is set, this issues a warning. If the compiler also has -Xfatal-warnings or -Werror set, these warnings become errors.

One workaround is to explicitly ignore the return value, rather than implicitly:

val _ = actorSystem.scheduler.schedule(delay, interval)(poller())

Discarded non-Unit value Error When Using Akka

1 Answers1