Why can't the compiler find implicit ExecutionContext with Implicits.global imported? - scala

Question

I have the following piece of code:
import java.util.concurrent.Executor
import scala.concurrent.Future
trait Storage {
def store(location: String, content: Array[Byte])(implicit exec: Executor): Future[String]
}
object LocalStorage extends Storage {
override def store(location: String, content: Array[Byte])(implicit exec: Executor): Future[String] =
Future {
... do some stuff ...
"Hello"
}
}
And here comes the code for testing:
object LocalStorageTest extends App{
import scala.concurrent.ExecutionContext.Implicits.global
val testImage = AsyncWebClient get "some url"
val result = testImage.flatMap{content => LocalStorage.store("logo.png",content)}
val status =Await.result(result, 30 seconds)
println(status)
AsyncWebClient.shutDown
}
Whenever I try to run the code, I am getting the following error:
Cannot find an implicit ExecutionContext.
Why? Isn't scala.concurrent.ExecutionContext.Implicits.global in scope already? When this import is added directly to LocalStorage object, it works (!)
BUT I am loosing a possibility to alter the context. Since I do not want to run this code on global every time. As this is a part of an akka application and for "production" runtime it is supposed to run on some dispatcher. Just for testing I would like to run it on a global execution context.
Am I missing some important concept here or is my design wrong that I am loosing such flexibility? Where is the issue?!

Answer 1

An ExecutionContext is not the same thing as an Executor. You should make the method accept an implicit exec: ExecutionContext (scala.concurrent.ExecutionContext) and then it will behave as you expect.