In what cases are the pattern for indirect enums used?

The indirect enum pattern is used when you need to implement an enumeration with associated values or behavior that cannot be expressed with a simple enum. This is useful if:

• You need to store additional data with each case.
• You want to implement different data types or structures for different options.
• You need to use an enum with dynamic values or object references.

In Swift, for example, indirect enums allow recursive definitions, which are convenient for building complex data structures.

Example of an indirect enum in Swift:

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

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

Here, the indirect enum allows creating recursive expressions.