tuple.swift

protocol _AnyTuple : CustomStringConvertible { var tDesc: String { get } }

struct EmptyTuple: _AnyTuple {
  var description: String { return "()" }
  var tDesc: String { return  ")" }
}

struct NonEmptyTuple<Element, Tail : _AnyTuple> { var (first, tail): (Element, Tail) }

extension NonEmptyTuple : Tuple, _AnyTuple {
  var description: String { return "(" + String(reflecting: first) + tail.tDesc }
  var tDesc: String { return ", " + String(reflecting: first) + tail.tDesc }
}

infix operator | { associativity right precedence 90 }

func |<E, T:_AnyTuple>(lhs: E, rhs: T) -> NonEmptyTuple<E, T> {
  return NonEmptyTuple(first: lhs, tail: rhs)
}

func |<E, T>(lhs: E, rhs: T) -> NonEmptyTuple<E, NonEmptyTuple<T, EmptyTuple>> {
  return NonEmptyTuple(first: lhs, tail: NonEmptyTuple(first: rhs, tail: EmptyTuple()))
}

protocol Tuple : _AnyTuple {
  typealias First
  typealias Tail : _AnyTuple
  var first : First { get }
  var tail : Tail { get }
}

extension Tuple where Tail : Tuple {
  typealias Second = Tail.First
  var second: Tail.First { return self.tail.first }
}
extension Tuple where Tail : Tuple, Tail.Tail : Tuple {
  typealias Third = Tail.Second
  var third: Third { return self.tail.second }
}
extension Tuple where Tail : Tuple, Tail.Tail : Tuple, Tail.Tail.Tail : Tuple {
  typealias Fourth = Tail.Third
  var fourth: Fourth { return self.tail.third }
}

let triple = (1 | 2.0 | "A")
let quadle = (1 | 2.0 | "A" | [1, 2])

triple.first
triple.second
triple.third

// triple.fourth

quadle.fourth