Add items to Future[List] inside recursion - scala

Question

I'm having an issue with Future List inside a recursion.
When i implemented this method without Futures i used ListBuffer and then adding items to the list.
val filtered = ListBuffer.empty[PostMD]
filtered ++= postMd.filter(_.fromID == userID)
Now i'm trying to implement it with Futures but i can't find a similar solution
What will be the best way to work with a Future List.
def getData(url: String, userID: String) = {
val filtered: (List[PostMD]) => Future[List[PostMD]] = Future[List[PostMD]]
def inner(url: String): Unit = {
val chunk: Future[JsValue] = BusinessLogic.Methods.getJsonValue(url)
val postMd: Future[List[PostMD]] = for {
x <- chunk.map(_.\("data").as[List[JsValue]])
y <- x.map(_.\("data").as[PostMD])
} yield y
filtered = postMd.map(_.filter(_.fromID == userID)) // <- returned Future[List[PostMD]]
val next: String = (chunk.map(_.\("paging").\("next"))).toString
if (next != null) inner(next)
}
inner(url)
filtered
}
thanks,
miki

Answer 1

I tried to do what you want with random number generation.
import scala.concurrent.{Await, Future}
import scala.util.Random
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
val RANDOM = new Random()
def futureRec(num: Int, f: Future[List[Integer]]): Future[List[Integer]] = {
if(num == 0) {
f
} else {
f.flatMap(l => {
futureRec(num - 1, Future.successful(RANDOM.nextInt() :: l))
})
}
}
val futureResult = futureRec(5, Future.successful(Nil))
Await.result(futureResult, 5 minutes)
So I would do, what you want something like this:
def getData(url: String, userID: String):Future[List[PostMD]] = {
def inner(url: String, f: Future[List[PostMD]]): Future[List[PostMD]] = {
val chunk: Future[JsValue] = ???
chunk.flatMap(ch => {
val postMd = (ch \ "data").\\("data").map(_.as[PostMD]).toList
val relatedPostMd = postMd.filter(_.fromID == userID)
val next: String = (ch.\("paging").\("next")).as[String]
if (next != null)
inner(next, f.map(l => l ++ relatedPostMd))
else
f.map(l => l ++ relatedPostMd)
})
}
inner(url, Future.successful(Nil))
}