flatten.swift

func flattenGen<G : GeneratorType, T where G.Element == T?>(var gen: G) -> [T] {
  return gen.next().map { ($0.map { [$0] } ?? []) + flattenGen(gen) } ?? []
}

func flatten<S : SequenceType, T where S.Generator.Element == T?>(seq: S) -> [T] {
  return flattenGen(seq.generate())
}

func flatAr<T>(ar: ArraySlice<T?>) -> ArraySlice<T> {
  return ar.last.map {flatAr(dropLast(ar)) + ($0.map{[$0]} ?? [])} ?? []
}

func squeezle<
  C : RangeReplaceableCollectionType, T where
  C.Index : BidirectionalIndexType,
  C.Generator.Element == T?
  >(var c: C) -> [T] {
    return c.isEmpty ? [] : {squeezle(c) + ($0.map {[$0]} ?? [])}(removeLast(&c))
}

let jo: [Int?] = [1, 2, nil, 3]

flatAr(ArraySlice(jo))  // [1, 2, 3]
flatten(jo)             // [1, 2, 3]
squeezle(jo)            // [1, 2, 3]