EarlyReturn.swift

enum ReturnType<T> {
    case Normal(T)
    case Early(T)
}

extension SequenceType {
    func rreduce<T>(initial: T, combine: (T, Self.Generator.Element) -> ReturnType<T>) -> T {
        var result = initial
        for el in self {
            switch combine(result, el) {
            case .Early(let res):
                return res
            case .Normal(let res):
                result = res
            }
        }
        return result
    }
}

let addUntil3: (Int, Int) -> ReturnType<Int> = { total, el in
    let total = total + el
    if el == 3 {
        return .Early(total)
    }
    return .Normal(total)
}

[1,2,3,4,5].reduce(0, combine: +) // 1+2+3+4+5 -> 15
[1,2,3,4,5].rreduce(0, combine: addUntil3) // 1+2+3 -> 6