Extended capability for a more restrictive generic - swift

Question

I have a generic binary search tree based on Comparable:
public class BSTree<T: Comparable> {
public func insert(_ val: T, _ n: Int) {
// ...
}
#discardableResult
public func delete(_ val: T, _ n: Int) -> Int {
// ...
}
}
I want to add the ability to provide the sum of the values, if T is an arithmetic type. I tried the following:
public class BSTree<T: Comparable> {
private var sumStorage: T?
public func insert(_ val: T, _ n: Int) {
if let arithVal = val as? AdditiveArithmetic {
for _ in 0 ..< n { sumStorage += arithVal }
}
// ...
}
#discardableResult
public func delete(_ val: T, _ n: Int) -> Int {
// ...
numDeleted = ...
if let arithVal = val as? AdditiveArithmetic {
for _ in 0 ..< numDeleted { sumStorage -= arithVal }
}
}
}
extension BSTree where T: AdditiveArithmetic {
public var sum: T {
sumStorage as? T ?? T.zero
}
}
Of course, when I try to cast val as AdditiveArithmetic I get “Protocol 'AdditiveArithmetic' can only be used as a generic constraint because it has Self or associated type requirements”. Plus sumStorage isn’t AdditiveArithmetic, so I can’t add to it, and I can’t make it a stored property of the extension, because ... you just can’t.
What I finally came up with was to use inheritance:
class SummedBSTree<T>: BSTree<T> where T: AdditiveArithmetic & Comparable {
public var sum = T.zero
override public func insert(_ val: T, _ n: Int) {
super.insert(val, n)
for _ in 0 ..< n { sum += val }
}
#discardableResult
override public func delete(_ val: T, _ n: Int) -> Int {
let numDeleted = super.delete(val, n)
for _ in 0 ..< numDeleted { sum -= val }
return numDeleted
}
}
This works, but it seems like it’s a case of using a sledgehammer where a jeweler’s screwdriver should be able to do the trick. It’s frustrating that something that would be so easy to do in Objective-C (and other less strongly typed languages) is so difficult in Swift. Can someone come up with a way of adding the summing capability without subclassing?

Answer 1

import UIKit
//https://stackoverflow.com/questions/61784548/swift-extended-capability-for-a-more-restrictive-generic
protocol ON1Speedable {
associatedtype Item: Comparable
var sumStorage: Item? { get set }
}
public class BSTree<T: Comparable> {
var sumStorage: T?
init(sumStorage: T? = nil) {
self.sumStorage = sumStorage
}
}
extension ON1Speedable where Item: AdditiveArithmetic, Item: Strideable, Item.Stride: SignedInteger {
mutating func insert(_ val: Item, _ n: Int) {
sumStorage = sumStorage ?? Item.zero
for _ in 0..<n { sumStorage! += val }
}
#discardableResult
mutating func delete(_ val: Item, _ n: Int) -> Item? {
sumStorage = sumStorage ?? Item.zero
for _ in Item.zero..<val { sumStorage! -= val }
return sumStorage
}
}
extension BSTree: ON1Speedable { }
var g2 = BSTree<Int>(sumStorage: 0)
g2.sumStorage
g2.insert(5, 5)
g2.sumStorage // 25
g2.delete(5, 5) // 0
var g3 = BSTree<String>()
g3.sumStorage // nil
//g3.insert("foo", 5) // Error: Referencing instance method 'insert' on 'ON1Speedable' requires that 'String.Stride' conform to 'SignedInteger'
g3.sumStorage // nil
//g3.delete("bar", 5) // Error: Referencing instance method 'delete' on 'ON1Speedable' requires that 'String.Stride' conform to 'SignedInteger'