tailrec.swift

indirect enum TailRec<T> {
    case Done(T)
    case More(() -> TailRec<T>)
    
    func run() -> T {
        var tailRec = self
        while true {
            switch tailRec {
            case .More(let next): tailRec = next()
            case .Done(let t): return t
            }
        }
    }
}

let done = TailRec.Done(10)
done.run()
let more = TailRec.More({ _ in .Done(10) })
more.run()

func tri(n: UInt, acc: UInt = 0) -> UInt {
    func loop(n: UInt, acc: TailRec<UInt>) -> TailRec<UInt> {
        if n < 2 {
            return .Done(n + acc.run())
        } else {
            return .More({ _ in
                loop(n: n - 1, acc: .Done(acc.run() + n))
            })
        }
    }
    return loop(n: n, acc: .Done(0)).run()
}

tri(n: 100000)