To which data category does the Indirect Enum type correspond in programming language?

In Swift language, the Indirect Enum type belongs to the category of recursive enumerations. These are enumerations that can contain cases referencing themselves, allowing the creation of data structures with a recursive nature, such as trees or lists.

The indirect keyword indicates to the compiler that the associated values of this enumeration should be stored indirectly (via a reference) to avoid infinite recursion when calculating the size of the type.

Example:

indirect enum BinaryTree {
    case empty
    case node(Int, left: BinaryTree, right: BinaryTree)
}

let tree = BinaryTree.node(5, left: .empty, right: .node(10, left: .empty, right: .empty))

Thus, Indirect Enum is a way to define recursive data structures in Swift.