folds.swift

struct Reducer<Element, A> {
    let empty: A
    let combine: (A, Element) -> A
}

func sum() -> Reducer<Int, Int> {
    return Reducer(empty: 0, combine: +)
}

func count<A>() -> Reducer<A, Int> {
    return Reducer(empty: 0, combine: { x, _ in x + 1})
}

extension Array {
    func reduce<A>(_ reducer: Reducer<Element,A>) -> A {
        return reduce(reducer.empty, reducer.combine)
    }
}

func zip<El,A,B>(_ l: Reducer<El, A>, _ r: Reducer<El, B>) -> Reducer<El,(A,B)> {
    return Reducer(empty: (l.empty, r.empty), combine: { x, y in
        (l.combine(x.0, y), r.combine(x.1, y))
    })
}