sumRecursively based on http://natashatherobot.com/functional-swift-tail-recursion/

sumRecursively.swift

func sumRecursively(numbers: [Int], _ acc:Int = 0) -> Int {
    if let head = numbers.first {
        let tail = Array(dropFirst(numbers))
        return sumRecursively(tail, head + acc)
    } else {
        return acc
    }
}

let myNumbers = [1,2,3,4,5]
sumRecursively(myNumbers) // 15