swift--枚舉

枚舉

enum CompassPoint {

    case north

    case south

    case east

    case west

}

var directionToHead = CompassPoint.west

directionToHead = .east

遞歸枚舉indirect

indirect enum ArithmeticExpression {

    case number(Int)

    case addition(ArithmeticExpression, ArithmeticExpression)

    case multiplication(ArithmeticExpression, ArithmeticExpression)

}

let five = ArithmeticExpression.number(5)

let four = ArithmeticExpression.number(4)

let sum = ArithmeticExpression.addition(five, four)

let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))

func evaluate(_ expression: ArithmeticExpression) -> Int {

    switch expression {

    case let .number(value):

        return value

    case let .addition(left, right):

        return evaluate(left) + evaluate(right)

    case let .multiplication(left, right):

        return evaluate(left) * evaluate(right)

    }

}

print(evaluate(product))//18