As I think you already understand, these Sets are functions that test whether a value meets the criteria for each Set.
The union of such a Set must also be a function that returns a Boolean (as shown by the type signature)
def union(f: Set, s: Set): Set
which (because Set
is a type alias) is equivalent to:
def union(f: Int => Boolean, s: Int => Boolean): Int => Boolean
In plain English, union of two sets A and B means: "is the item in A or B".
Your task is to write a function that carries out that plain English specification.
You cannot "add" two functions together (at least, not in a way that is applicable to this question), but you can combine their results.