Combining List, Future and Option in for-comprehension - scalaz

Question

I\'ve got following problem:

val sth: Future[Seq[T, S]] = for {
  x <- whatever: Future[List[T]]
  y <- x: List[T]
  z <- f(y): Future[Option[S]]
           


        

Answer 1

You could do something like what you need using the scalaz ListT monad transformer

 object Test {
   import scalaz._
   import ListT._
   type T = String
   type S = Int
   val whatever: Future[List[T]] = ??? // you get this somewhere
   def f(y: T): Future[Option[S]] = ??? // function that returns future of option

   val sth: Future[List[(T, S)]] = (for {
     y <- listT(whatever) 
     // you cannot mix list and option, but you can convert the option to a list of 1 item
     n <- listT(f(y).map(_.toList)) 
   } yield y -> n).run
 }

N.B.: Since you start with a future, you cannot return a Seq[(T,S)], you can only have a future. You have to call Await.result if you want to block and get the result.