Recursive definitions

Some data structures need to be able to refer to instances of themselves within their own definition. You can't do this with a struct, but the lowly enum offers a solution.

Let's look at boxes that can contain either a number of books, or another box. We'll start by defining a Book type:

typealias Book = String 

Yes, I know, the suggestion that a book is nothing more than a mere string of text is a little off the mark, but you know what I mean.

Next, we'll define a Box type, which can either contain Book objects (so an array of type [Book]), or another Box object:

enum Box 
{ 
  case books([Book]) 
  case box(Box) 
} 

And straight away we get a compiler error.

However, this easily fixed, by declaring the .box case to be indirect:

enum Box 
{ 
 case books([Book]) 
 indirectcase box(Box) 
} 

Now we can have books in a box, or a box in a box, or a box of books in a box.