Sets of a given size as a Swift type - swift

Question

In Swift, is there a way to construct a type of “sets of a given size?”(Despite the absence of dependent types in Swift, is such a construction nonetheless possible without excessive “contortions?”)
As an example, I would like to be able to define a parameterized type SetOfSizeTwo<T> whose instances are sets comprising of exactly two objects of (Hashable) type T.
Currently, I am using a poor man’s proxy:
struct SetOfSizeTwo<T> where T: Hashable {
var set = Set<T>(minimumCapacity: 2)
}
However, this type does not force the property set to be of size of 2.
Update
The blog article A hack for fixed-size arrays in Swift, by Ole Begemann, leads me to believe that a robust construction of a fixed-sized-set type in Swift 4 is non-trivial, if at all possible.

Answer 1

Here is one approach, as closest as I could get, to suit your needs. It is a bit rough around the edges and needs some polishing, but I think you'll get the idea:
class Size {
let size: Int
init(size: Int) {
self.size = size
}
required init() {
self.size = 0
}
}
class SizeOne: Size {
private override init(size: Int) {
super.init(size: size)
}
required init() {
super.init(size: 1)
}
}
class SizedSet<S, T> where S: Size, T: Hashable {
private var set: Set<T>
private let maximumSize: S
init() {
set = Set<T>()
maximumSize = S()
}
func insert(item: T) {
if !set.contains(item) && set.count + 1 <= maximumSize.size {
set.insert(item)
}
}
func remove(item: T) {
set.remove(item)
}
func contents() -> Set<T> {
return set
}
}
Usage:
let set: SizedSet<SizeOne, Int> = SizedSet()
print(set.contents())
// []
set.insert(item: 1)
print(set.contents())
// [1]
set.insert(item: 2)
print(set.contents())
// [1]