Heterogeneous List in Swift

heterogeneous.swift

//: Playground - noun: a place where people can play

protocol HList {}
struct HNil: HList {}
struct HCons<Element, List: HList>: HList {
  let head: Element
  let tail: List
}

infix operator |*| { associativity right }
func |*| <E, L: HList>(element: E, rest: L) -> HCons<E, L> {
  return HCons(head: element, tail: rest)
}

// Example

struct Key {
  let value: Int
  init(_ value: Int) {
    self.value = value
  }
}

struct Name {
  let value: String
  init(_ value: String) {
    self.value = value
  }
}

protocol Breed {}
struct Cow: Breed {}

struct Price {
  let value: Double
  init(_ value: Double) {
    self.value = value
  }
}

typealias Animal = HCons<Key, HCons<Name, HCons<Breed, HCons<Price, HNil>>>>
let angus: Animal =  Key(42)
                 |*| Name("Angus")
                 |*| Cow()
                 |*| Price(75.5)
                 |*| HNil()