Advanced and Practical Enum usage in Swift

Recursive / Indirect Types

Recursive / Indirect Types

Indirect types allow you to define enums where the associated value of a case is the very same enum again.

As an example, consider that you want to define a file system representations with files and folders containing files. If File and Folder were enum cases, then the Folder case would need to have an array of File cases as it's associated value. Since this is a recursive operation, the compiler has to make special preparations for it. Quoting from the Swift documentation:

Enums and cases can be marked indirect, which causes the associated value for the enum to be stored indirectly, allowing for recursive data structures to be defined.

So to implement our FileNode enum, we'd have to write it like this:

enum FileNode {
  case file(name: String)
  indirect case folder(name: String, files: [FileNode])
}

The indirect keyword tells the compiler to handle this enum case indirectly. You can also add the keyword for the whole enum. As an example imagine mapping a binary tree:

indirect enum Tree<Element: Comparable> {
    case empty
    case node(Tree<Element>,Element,Tree<Element>)
}

This is a very powerful feature that allows you to map complex relationships in a very clean way with an enum.