And autoclosure saves(?) the day for generic recursive enums

gistfile1.swift

// Creating a generic recursive data structure with autoclosure. (READ ALL NOTES; THIS MAY NOT DO WHAT YOU WANT.)
// Closures are reference types, so the size is known (? I think ?)
// Note that this is just because of unimplemented features in the compiler in Beta5
// There's no reason to think this is a long-term requirement.

// IMPORTANT: the closure will run every time you access this value, so if that has
// side effects, this won't work. It's only possible on pure value types.

// But the fact that this works as expected is actually kind of incredible.
// Think about what is required for it to work out the type for NestedList.Elem("first").
// Yet Xcode still has no idea where to put the }.

enum NestedList<A> {
  case Elem(@autoclosure ()->A) // <--- The magic
  case List([NestedList])

  var description: String {
    switch self {
    case let Elem(x): return reflect(x()).summary
    case let List(list): return "[" + join(", ", list.map{ $0.description }) + "]"
      }
  }
}

let first = NestedList.Elem("first")
let second = NestedList.Elem("second")
let list = NestedList.List([first, second])
let outerlist = NestedList.List([first, list])
println(first.description)
println(list.description)
println(outerlist.description)

// I (correctly) won't compile because of type mismatch
// let bad = NestedList.List([first, NestedList.Elem(2)])


// For more fun, we can replace the array with a varadic,
// but notice how it becomes a tuple of an array rather than an array (https://devforums.apple.com/thread/239744)

enum NestedList {
  case Elem(String)
  case List(NestedList...) // <===

  var description: String {
    switch self {
    case let Elem(text): return text
    case let List(list): return "[" + join(", ", list.0.map{ $0.description }) + "]"
      }
  }
}

let first = NestedList.Elem("first")
let second = NestedList.Elem("second")
let list = NestedList.List(first, second) // <=== no more []
let outerlist = NestedList.List(first, list)
first.description
list.description
outerlist.description