What is the best way to merge two Future[Map[T1, T2]] in Scala - scala

Question

I have a list of fileNames and I want to load the correlated pages in batches (and not all at once). In order to do so, I'm using FoldLeft and I'm writing an aggregate function which aggregates a Future[Map[T1,T2]].
def loadPagesInBatches[T1, T2](fileNames: Set[FileName]): Future[Map[T1, T2]] = {
val fileNameToPageId: Map[FileName, PageId] = ... //invokes a function that returns the pageId correlated to the fileName.
val batches: Iterator[Set[FileName]] = fileNames.grouped(10) //batches of 10;
batches.foldLeft(Future(Map.empty[T1, T2]))(aggregate(fileNameToPageId))
}
And the signature of aggregate is as follows:
def aggregate(fileNameToPageId: Map[FileName, PageId]): (Future[Map[T1, T2]], Set[FileName]) => Future[Map[T1, T2]] = {..}
I'm trying to make sure what is the best way to merge these Future[Map]s.
Thanks ahead!
P.S: FileName and PageId are just Types of string.

Answer 1

In case you have exactly 2 futures, zipWith would probably be the most idiomatic.
val future1 = ???
val future2 = ???
future1.zipWith(future2)(_ ++ _)
Which is a shorter way of writing a for comprehension:
for {
map1 <- future1
map2 <- future2
} yield map1 ++ map2
Although zipWith could potentially implement some kind of optimization.

Answer 2

My solution was putting the two maps into a list and using Future.reduceLeft.
def aggregate(fileNameToPageId: Map[FileName, PageId]): (Future[Map[T1, T2]], Set[FileName]) => Future[Map[T1, T2]] = {
case (all, filesBatch) =>
val mapOfPages: Future[Map[NodeId, T]] = for {
... //Some logic
} yield "TheBatchMap"
Future.reduceLeft(List(all, mapOfPages))(_ ++ _)
}