Where is the indirect enum typically located, and how is it implemented?

In Swift, an indirect enum is used when one of the enum cases contains an associated value of the same type as the enum itself, allowing the creation of recursive data structures.

The keyword indirect is placed before the entire enum declaration or before a specific case to inform the compiler that the value should be stored indirectly (via a reference), rather than directly, to avoid infinite recursion.

Typically, an indirect enum is located in the same file where it is used, for example, to describe trees or expressions:

indirect enum Expression {
    case number(Int)
    case addition(Expression, Expression)
    case multiplication(Expression, Expression)
}

// Usage:
let expr = Expression.addition(.number(2), .multiplication(.number(3), .number(4)))

This declaration allows creating recursive structures with arbitrary depth.