Indirect.swift

/// Property wrapper to introduce indirection into your property.
///
/// Indirection is very useful to introduce recursion into `structs`, which don't usually support it. This is very similar to using the `indirect` modifier on enum cases.
///
///     struct Person: Identifiable {
///         let id = UUID()
///
///         @Indirect var father: Person?
///         @Indirect var mother: Person?
///     }
@propertyWrapper
enum Indirect<Value> {
    indirect case value(Value)
    
    var wrappedValue: Value {
        get {
            guard case .value(let value) = self else { fatalError("Can't reach indirect value") }
            return value
        }
        set {
            self = .value(newValue)
        }
    }
    
    init(wrappedValue: Value) {
        self = .value(wrappedValue)
    }
}

import Foundation

struct Person: Identifiable {
    let id = UUID()
    
    @Indirect var mother: Person?
    @Indirect var father: Person?
}

let person1: Person = .init(mother: Person(mother: Person(mother: nil, father: nil), father: nil), father: Person(mother: nil, father: Person(mother: Person(mother: nil, father: nil), father: nil)))

dump(person1)

//  person1
//      mother
//          mother
//              nil
//              nil
//          nil
//      father
//          nil
//          father
//              mother
//                  nil
//                  nil
//              nil
        
person1.father = nil // Cannot assign to property: 'person1' is a 'let' constant -> advantage of using structs over classes