How to convert a Twitter Future[Map[A, Future[C]]] to a Future[Map[A, C]]

Question

I am trying to fix the following issue:

I have a Future[Map[A, B]]. For all B, I need to apply a method that convert B to a Future[C] and I want to give back a Future[Map[A, C]]

Here is the code I have so far:

def getClients(clientIds: Seq[Int]): Future[Map[Int, ClientData]] = {

  def getClientData(clientInfo: ClientInfo): Future[ClientData] = 
    clientInfo match {
      case ValidInfo(info) => getData(info)
      case _ => throw new Exception
    }

  client.getClients(clientIds) map {
    _.toMap map {
      case (clientId: Int, clientInfo: ClientInfo) =>
        getClientData(clientInfo) map {
          clientData => (clientId, clientData)
      }
    }
  }
}

This code is wrong as it returns a Iterable[Future[(Int, ClientData)]]

For info getClients is a thrift method that returns Future[Map[A, B]] where the Map is mutable, so I need to convert it to an immutable map first with toMap.

Thank you in advance for your help!

Answer 1

scala> def f: Future[Map[String, Future[Int]]] = ???
f: Future[Map[String,Future[Int]]]

scala> def x = for {
     |   m <- f
     |   i = m.map{ case (k, fv) => fv.map{ k -> _ } }
     |   l <- Future.sequence(i)
     | } yield l.toMap
x: Future[Map[String,Int]]

Step by step:

Convert Future[Map[A, Future[B]]] to Future[Iterable[Future[(A, B)]]]:

scala> def x1 = f.map{ _.map{ case (k, fv) => fv.map{ k -> _ } } }
x1: Future[Iterable[Future[(String, Int)]]]

Convert Iterable[Future[(A, B)]] to Future[Iterable[(A, B)]] and flatten Future[Future[...]] using flatMap:

scala> def x2 = x1.flatMap{ Future.sequence(_) }
x2: Future[immutable.Iterable[(String, Int)]]

Convert Iterable[(A, B)] to Map[A, B]:

scala> def x = x2.map{ _.toMap }
x: Future[Map[String,Int]]

For com.twitter.util.Future you should use collect instead of sequence and toSeq before collect since it accepts Seq:

def x = for {
  m <- f
  i = m.map{ case (k, fv) => fv.map{ k -> _ } }
  l <- Future.collect(i.toSeq)
} yield l.toMap