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

The problem with for comprehension is that it's not some kind of magic monadic "unwrapper", it's just a sequence of map, flatMap and filter.

As you may know map and flatMap operate only on "inner" type, leaving "outer" type of monad unchanged. This means you can't do this:

for {
  x <- whatever: Future[List[T]]
  y <- x: List[T]
} yield y

inside single for. Instead, you can do something like this:

for (x <- whatever: Future[List[T]])
  yield for (y <- x: List[T]) yield y

Which looks kinda ugly.

Back to your case, I's easier to write whole transformation explicitly using map and flatMap, as it gives you greater visibility and control:

whatever.flatMap {
  x: List[T] =>
    Future.sequence(x.map {
      y: T => f(y).map(y -> _)
    }).map(_.collect {
      case (y, Some(n)) => y -> n
    })
}

Also, @trustnoone mentioned, you can't get rid of the Future without explicitly calling Await.