I wanted a version of zip(_:_:) that padded the end of the shorter sequence with Optionals.

no_u.swift

import Foundation

func paddedZip<T, U, Sequence1, Sequence2>(
    _ sequence1: Sequence1,
    _ sequence2: Sequence2
    ) -> Zip2Sequence<[T?], [U?]> where
    Sequence1 : Sequence, Sequence1.Element == T,
    Sequence2 : Sequence, Sequence2.Element == U {
        var array1: [T?] = []
        var array2: [U?] = []
        
        var iterator1 = sequence1.makeIterator()
        var iterator2 = sequence2.makeIterator()
        
        var shouldContinue = true
        
        while shouldContinue {
            switch (iterator1.next(), iterator2.next()) {
            case (.some(let element1), .some(let element2)):
                array1.append(.some(element1))
                array2.append(.some(element2))
            case (.some(let element1), .none):
                array1.append(.some(element1))
                array2.append(.none)
            case (.none, .some(let element2)):
                array1.append(.none)
                array2.append(.some(element2))
            case (.none, .none):
                shouldContinue = false
            }
        }
        
        return zip(array1, array2)
}