Can't gain the `pipeTo` method on [[scala.concurrent.Future]]

Question

As the doc of akka explains, you should be able to gain the pipeTo method on [[scala.concurrent.Future]] this way:

import akka.pattern.pipe
val future = ...
future pipeTo sender()

Unfortunately, I can't do that, i'm getting an error "can't resolve symbol pipeTo" in my IDE.

As a workaround, I had to use the syntax this way

pipe(future) pipeTo sender()

But it still disturb me to not figure out why (i'm quite newby in scala BTW). Thanks a lot to help understand this puzzle.

scala 2.12.2 akka 2.5.3

Answer 1

You need to have an implicit ExecutionContext in scope, here is an example:

import akka.actor.{Actor, ActorSystem, Props}
import akka.pattern.pipe

import scala.concurrent.Future

// Get the implicit ExecutionContext from this import
import scala.concurrent.ExecutionContext.Implicits.global

object Hello extends App {

  // Creating a simple actor
  class MyActor extends Actor {
    override def receive: Receive = {
      case x => println(s"Received message: ${x.toString}")
    }
  }

  // Create actor system
  val system = ActorSystem("example")
  val ref = system.actorOf(Props[MyActor], "actor")

  // Create the future to pipe
  val future: Future[Int] = Future(100)

  // Test
  future pipeTo ref
}

Console:

sbt run
[info] <stuff here>
[info] Running example.Hello 
Received message: 100

---

The reason you have to do that is because pipeTo is a instance function on a PipeableFuture, and your regular Future has to be "enhanced" to a PipeableFuture. Here is the constructor for PipeableFuture, note the implicit executionContext: ExecutionContext parameter:

final class PipeableFuture[T](val future: Future[T])(implicit executionContext: ExecutionContext)

The full class is here, where you can see the pipeTo function:

final class PipeableFuture[T](val future: Future[T])(implicit executionContext: ExecutionContext) {
  def pipeTo(recipient: ActorRef)(implicit sender: ActorRef = Actor.noSender): Future[T] = {
    future andThen {
      case Success(r) ⇒ recipient ! r
      case Failure(f) ⇒ recipient ! Status.Failure(f)
    }
  }
  def pipeToSelection(recipient: ActorSelection)(implicit sender: ActorRef = Actor.noSender): Future[T] = {
    future andThen {
      case Success(r) ⇒ recipient ! r
      case Failure(f) ⇒ recipient ! Status.Failure(f)
    }
  }
  def to(recipient: ActorRef): PipeableFuture[T] = to(recipient, Actor.noSender)
  def to(recipient: ActorRef, sender: ActorRef): PipeableFuture[T] = {
    pipeTo(recipient)(sender)
    this
  }
  def to(recipient: ActorSelection): PipeableFuture[T] = to(recipient, Actor.noSender)
  def to(recipient: ActorSelection, sender: ActorRef): PipeableFuture[T] = {
    pipeToSelection(recipient)(sender)
    this
  }
}

Since pipe(future) was not an instance function on a Future, it works in your example.