Generate new scala sequence from another sequence

Question

I have a sequence of ids and a function which return an Option[workItem] for each id. All I want to do is generate a sequence of workItem from these two things. I tried various combinations of maps, foreach and for comprehension and nothing works.

def getWorkItems(ids: Future[Seq[Id]]): Future[Seq[Option[WorkItem]]] = {
   ids.map {
        id: workItemId => id.map(getWorkItem(_)
  }
}

This gives me error - Expression of type Seq[Option[WorkItem] doesn't conform to expected type _B

I tried foreach -

def getWorkItems(ids: Future[Seq[Id]]): Future[Seq[Option[WorkItem]]] = {
   ids.map {_.foreach(getWorkItem(_)}      
}

This gives me error - Expression of type Unit does not conform to expected type _B

I am not sure what this _B type is and how do I go about this transformation.

@RamonJ.RomeroyVigil I am not deciding that, just going to return a sequence of Option. — ocwirk, Nov 10 '15 at 22:49
Possible duplicate of [How to flatten list of options using higher order functions?](http://stackoverflow.com/questions/2895069/how-to-flatten-list-of-options-using-higher-order-functions) — Ramón J Romero y Vigil, Nov 10 '15 at 22:51

score 1 · Accepted Answer · answered Nov 10 '15 at 22:51

Will work almostly as you expected.

trait o {

  class Id  {}
  class WorkItem { }

  def getWorkItem(id: Id): Option[WorkItem]

  def getWorkItems(ids: Future[Seq[Id]]): Future[Seq[Option[WorkItem]]] = {
    val res = ids.map { idsSeq =>
      idsSeq.map { id =>
        getWorkItem(id)
      }
    }
    res
  }



}

But as for me getWorkItems should return Future[Seq[WorkItem]] than instead of internal map you can use flatMap

  def getWorkItems(ids: Future[Seq[Id]]): Future[Seq[WorkItem]] = {
    val res = ids.map { idsSeq =>
      idsSeq.flatMap { id =>
        getWorkItem(id)
      }
    }
    res
  }

Generate new scala sequence from another sequence

1 Answers1