scala concurrent

magnet_pattern.md

• http://blog.madhukaraphatak.com/scala-magnet-pattern/ 에 따르면 정상 실행되야 하지만 나는 안 됨

  scala> :paste
  // Entering paste mode (ctrl-D to finish)
  
  import scala.concurrent.Future
  import scala.concurrent.Await
  import scala.concurrent.duration.Duration
  
  sealed trait FutureMagnet {
    type Result
  
    def apply() : Result
  }
  
  def completeFuture(magnet: FutureMagnet):magnet.Result = magnet()
  
  object FutureMagnet {
    implicit def intFutureCompleter(future:Future[Int]) = new FutureMagnet {
      override type Result = Int
  
      override def apply(): Result = Await.result(future,Duration.Zero)
    }
  
    implicit def stringFutureCompleter(future:Future[String]) = new FutureMagnet {
      override type Result = String
  
      override def apply(): Result = Await.result(future,Duration.Zero)
    }
  
  }
  
  
  scala> completeFuture( Future {1})
                              ^
       error: Cannot find an implicit ExecutionContext. You might pass
       an (implicit ec: ExecutionContext) parameter to your method.
  
       The ExecutionContext is used to configure how and on which
       thread pools Futures will run, so the specific ExecutionContext
       that is selected is important.
  
       If your application does not define an ExecutionContext elsewhere,
       consider using Scala's global ExecutionContext by defining
       the following:
  
       implicit val ec: scala.concurrent.ExecutionContext = scala.concurrent.ExecutionContext.global
  

problem_when_no_magnet_pattern.md

• method overload이 실패. 원인은 type erasure때문

  scala> import scala.concurrent.Future
  import scala.concurrent.Future
  
  scala> import scala.concurrent.Await
  import scala.concurrent.Await
  
  scala> import scala.concurrent.duration.Duration
  import scala.concurrent.duration.Duration
  
  scala> def completeFuture(value: Future[String]): String = Await.result(value, Duration.Zero)
  completeFuture: (value: scala.concurrent.Future[String])String
  
  scala> def completeFuture(value: Future[Int]): Int = Await.result(value, Duration.Zero)
  completeFuture: (value: scala.concurrent.Future[Int])Int
  
  scala> completeFuture(Future {1})
                             ^
       error: Cannot find an implicit ExecutionContext. You might pass
       an (implicit ec: ExecutionContext) parameter to your method.
  
       The ExecutionContext is used to configure how and on which
       thread pools Futures will run, so the specific ExecutionContext
       that is selected is important.
  
       If your application does not define an ExecutionContext elsewhere,
       consider using Scala's global ExecutionContext by defining
       the following:
  
       implicit val ec: scala.concurrent.ExecutionContext = scala.concurrent.ExecutionContext.global
  
  scala> completeFuture(Future {"Hello"})
                              ^
       error: type mismatch;
        found   : String("Hello")
        required: Int
  

• ref; http://blog.madhukaraphatak.com/scala-magnet-pattern/