EitherReality.swift

import SwiftUI

struct ContentView: View {
  var body: some View {
    VStack {
      Text("Hello, world!")
      Reality {
        Anchor {
          Model()
        }
      }
    }
    .padding()
  }
}

// MARK: -

protocol EntityRepresentable {}

struct Reality: UIViewRepresentable {
  let tree: BinaryTree<EntityRepresentable>
  
  func makeCoordinator() {
    Coordinator()
  }
  
  init(@RealityBuilder _ builder: () -> BinaryTree<EntityRepresentable>) {
    self.tree = builder()
  }
  
  func makeUIView(context: Context) -> UIView {
    .init()
  }
  
  func updateUIView(_ uiView: UIViewType, context: Context) {
    //TODO: compare `self.tree` with `context.coordinator.tree`
  }
}

class Coordinator {
  var tree: BinaryTree<EntityRepresentable> = .empty
  init() {}
}

struct Anchor: EntityRepresentable {
  let body: BinaryTree<EntityRepresentable>
  init(@RealityBuilder _ builder: () -> BinaryTree<EntityRepresentable>) {
    self.body = builder()
  }
}
struct Model: EntityRepresentable {
  let body: BinaryTree<EntityRepresentable>
  init(@RealityBuilder _ builder: () -> BinaryTree<EntityRepresentable>) {
    self.body = builder()
  }
}

@resultBuilder
struct RealityBuilder {
  static func buildBlock(
    _ components: EntityRepresentable
  ) -> BinaryTree<EntityRepresentable> {
    .node(components, .empty)
  }
}

enum BinaryTree<T> {
  case empty
  indirect case node(BinaryTree, BinaryTree)
}

// enum Either<Left, Right> {
//     case left(Left)
//     case right(Right)
// }