Swift枚举-相关值与递归枚举

代码：

enum ArithmeticExpression {
    // 相关值
    case Number(Int)
    // 递归枚举
    indirect case Addition(ArithmeticExpression, ArithmeticExpression)
    indirect case Multiplication(ArithmeticExpression, ArithmeticExpression)
    indirect case Division(ArithmeticExpression, ArithmeticExpression)
}

func evaluate(expression: ArithmeticExpression) -> Int {
    switch expression {
        case .Number(let value):
            return value
        case .Addition(let left, let right):
            return evaluate(left) + evaluate(right)
        case .Multiplication(let left, let right):
            return evaluate(left) * evaluate(right)
        case .Division(let left, let right):
            return evaluate(left) / evaluate(right)
    }
}

let five = ArithmeticExpression.Number(5)
let four = ArithmeticExpression.Number(4)
let sum = ArithmeticExpression.Addition(five, four)
let product = ArithmeticExpression.Multiplication(sum, ArithmeticExpression.Number(2))
let quotient = ArithmeticExpression.Division(five, four)

print(evaluate(sum))
print(evaluate(product))
print(evaluate(quotient))

输出：

9
18
1