toy-hlist.swift

func main () {
  let typedHeterogeneousList = true =+= "Hello" =+= 1 =+= HNil()
  
  let first: Bool = typedHeterogeneousList.head()
  let second: String = typedHeterogeneousList.tail().head()
  let third: Int = typedHeterogeneousList.tail().tail().head()
}

protocol HList {
  typealias Head
  typealias Tail
}

struct HNil : HList {
  typealias Head = Void
  typealias Tail = Void
}

class HCons<H, T : HList> : HList {
  typealias Head = H
  typealias Tail = T
  
  
  // Use func indirection to hack around:
  // LLVM ERROR: unimplemented IRGen feature! non-fixed class layout
  let _head: () -> Head
  func head () -> Head { return _head() }

  let _tail: () -> Tail
  func tail () -> Tail { return _tail() }

  init (_ head: H, _ tail: T) {
    self._head = {head}
    self._tail = {tail}
  }
}

operator infix =+= {
  associativity right
}

func =+=<H, L : HList> (lhs: H, rhs: L) -> HCons<H, L> {
  return HCons(lhs, rhs)
}

main()