Scala Code - Immutable Set Issues - algorithm

Question

I'm learning Scala and I would like to convert some of my old algorithms code to Scala.
I have very simple Java code which prints superSet (a combination of all possible sets including empty set and the set itself).
public static Set<Set<Integer>> createSuperSet(Set<Integer> originalSet){
Set<Set<Integer>> superSet = new HashSet<Set<Integer>>();
if (originalSet.size() == 0){
Set<Integer> empty = new HashSet<Integer>();
superSet.add(empty);
return superSet;
}
List<Integer> list = new ArrayList<Integer>(originalSet);
Integer head = list.get(0);
Set<Integer> rest = new HashSet<Integer>(list.subList(1, list.size()));
for (Set<Integer> set : createSuperSet(rest)){
Set<Integer> newSet = new HashSet<Integer>();
newSet.add(head);
newSet.addAll(set);
superSet.add(newSet);
superSet.add(set);
}
return superSet;
}
Now I'm trying to achieve the same functionality in Scala:
def createSuperSet(originalSet: Set[Int]): Set[Set[Int]] ={
val superSet = Set[Set[Int]]()
originalSet.toList match {
case List() => {superSet + Set[Int]()}
case head::restAsList => {
val rest = restAsList.toSet[Int]
val result = createSuperSet(rest)
result.foreach(f=>{
val newSet:Set[Int] = f + head
superSet + f +newSet
})
superSet
}
}
}
but unfortunately this code returns empty Set. I'm suspecting that this issue happens due to Immutable collection usage. I was trying to run it in Debugger and I see that recursive call to function always returns empty set and my code is never getting into foreach function.
Please help. Any thoughts are welcomed.

Answer 1

In idiomatic scala the + (and -, ++, etc) operator as applied to immutable collections creates a new collection - otherwise they would not be immutable. Instead you have to combine the modification with another piece of syntactic sugar under which if you append = to an operator it assigns the result of the operator to the left-hand variable: superSet += f + newSet.

Answer 2

I solved the problem this way:
def createSuperSet(originalSet: Set[Int]): Set[Set[Int]] ={
var superSet = Set[Set[Int]]()
originalSet.toList match {
case List() => {superSet + Set[Int]()}
case head::restAsList => {
val rest = restAsList.toSet[Int]
val result = createSuperSet(rest)
result.map(f=>{
superSet = superSet + f+ (f+head)
})
superSet
}
}
}
running println(createSuperSet(Set[Int](1,2,3))
prints
Set(Set(), Set(3, 1), Set(2), Set(2, 1), Set(3, 2), Set(3), Set(3, 2, 1), Set(1))
but I'll be very glad to find out if there is any more elegant solution