let nums = [0,1,2,3,4,5].scan(+)
print(nums) //[0, 1, 3, 6, 10, 15]

let chars = ["0","1","2","3","4","5"].scan(+)
print(chars) //["0", "01", "012", "0123", "01234", "012345"]

let range = (1..<5).scan(0, +)
print(range) //[0, 1, 3, 6, 10]

extension Sequence {
    func scan(_ combine: (Element, Element) throws -> Element) rethrows -> [Element] {
        var iterator = makeIterator()
        guard let initial = iterator.next() else {
            return []
        }
        var firstIteration = true
        var accumulator = initial
        return try map { element in
            if firstIteration == true {
                firstIteration = false
                return initial
            } else {
                accumulator = try combine(accumulator, element)
                return accumulator
            }
        }
    }
    
    func scan(_ initial: Element, _ combine: (Element, Element) throws -> Element) rethrows -> [Element] {
        var accumulator = initial
        let sequence = [initial] + self
        return try sequence.map { element in
            accumulator = try combine(accumulator, element)
            return accumulator
        }
    }
}

extension Sequence {
    func scan<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> [Result]
}