NullOrdering.swift

import Foundation

protocol OptionalType {
    associatedtype Wrapped

    var value: Wrapped? { get }
}

extension Optional: OptionalType {
    var value: Wrapped? {
        return self
    }
}

enum NullOrdering {
    case first, last
}

extension Sequence where Element: OptionalType {
    func sorted(by isLessThan: (Element.Wrapped, Element.Wrapped) -> Bool, nulls nullOrdering: NullOrdering) -> [Element] {
        return sorted(by: { (first, second) -> Bool in
            guard let first = first.value else {
                return nullOrdering == .first
            }
            guard let second = second.value else {
                return nullOrdering == .last
            }
            return isLessThan(first, second)
        })
    }
}

extension Sequence where Element: OptionalType, Element.Wrapped: Comparable {
    func sorted(nulls nullOrdering: NullOrdering) -> [Element] {
        return self.sorted(by: <, nulls: nullOrdering)
    }
}

var seq = AnySequence([2, nil, 1, 3, nil])

let a = seq.sorted(nulls: .last)

print(a)