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Indirect enums in iOS (Swift) allow creating enumerations that can contain associated values, including recursive structures. With their help, complex data types such as trees or expressions can be created.

Example: a recursive enum for an arithmetic expression:

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

// Usage:
let five = ArithmeticExpression.number(5)
let four = ArithmeticExpression.number(4)
let sum = ArithmeticExpression.addition(five, four)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))

Thus, indirect enums enable creating recursive and complex data types that cannot be expressed with regular enums.