UnfailableFuture is a subset type of Future that can not result in a failure.

UnfailableFuture.scala

import scala.concurrent.{ExecutionContext, Future}
import scala.util.control.NonFatal

// private constructor guarantees that we can not build a semantically incorrect UnfailableFuture
class UnfailableFuture[A] private (val value: Future[A]) extends AnyVal

object UnfailableFuture {
  def apply[A](fut: Future[A], fallback: Throwable => A)(implicit ec: ExecutionContext): UnfailableFuture[A] = {
    val futWithFallback = fut.recover {
      case NonFatal(failure) => fallback(failure)
    }
    new UnfailableFuture(futWithFallback)
  }

  def successful[A](a: A) = new UnfailableFuture(Future.successful(a))
}

// Example: allow to safely wrap a processing function to pass it in failure-sensitive context (like .pipeTo in an actor)
// with type-checking.
def handleFailure(userProcessingFunction: Input => Future[Output]): Input => UnfailableFuture[Output] = {
  ...
}

class MyActor(userProcessingFunction: Input => UnfailableFuture[Output]) extends Actor {
  ...
}