[swift-evolution] zip3, zip4, ...

Donnacha Oisín Kidney oisin.kidney at gmail.com
Sun Dec 6 17:49:18 CST 2015

An implementation of this is actually pretty complicated, since you aren’t supposed to call a generator once it’s returned nil.

public struct NilPaddedZipGenerator<G0: GeneratorType, G1: GeneratorType> : GeneratorType {
  private var (g0, g1): (G0?, G1?)
  public mutating func next() -> (G0.Element?, G1.Element?)? {
    let (e0,e1) = (g0?.next(),g1?.next())
    switch (e0,e1) {
    case (nil,nil): return nil
    case (  _,nil): g1 = nil
    case (nil,  _): g0 = nil
    default: break
    return (e0,e1)

public struct NilPaddedZip<S0: SequenceType, S1: SequenceType> : LazySequenceType {
  private let (s0, s1): (S0, S1)
  public func generate() -> NilPaddedZipGenerator<S0.Generator, S1.Generator> {
    return NilPaddedZipGenerator(g0: s0.generate(), g1: s1.generate())

public func zipWithPadding<S0: SequenceType, S1: SequenceType>(s0: S0, _ s1: S1)
  -> NilPaddedZip<S0, S1> {
    return NilPaddedZip(s0: s0, s1: s1)

> On 6 Dec 2015, at 23:44, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> Is there an implementation in the stdlib for (T?, T?) like this?
>> On Dec 6, 2015, at 4:37 PM, Dmitri Gribenko <gribozavr at gmail.com <mailto:gribozavr at gmail.com>> wrote:
>> On Sun, Dec 6, 2015 at 3:34 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> It's pretty easy to build your own Zips. Not sure the language really needs this. For example, I recently built a zip that produces (T?, T?) which fills one of the two with nil until both lists are consumed:
>> func longZip<S0: SequenceType, S1: SequenceType>(seq0: S0, _ seq1: S1) ->
>>     AnyGenerator<(S0.Generator.Element?, S1.Generator.Element?)> {
>> Just wanted to point out that AnyGenerator has an inherent cost from the type erasure.  The implementation in the standard library uses generics and is fully optimizable.
>> Dmitri
>> -- 
>> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
>> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com <mailto:gribozavr at gmail.com>>*/
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