枚举：
enum CompassPoint{
case north
case south
case east
case west
}
directionToHead = .south
switch directionToHead {
case .north:
print("Lots of planets have a north")
case .south:
print("Watch out for penguins")
case .east:
print("Where the sun rises")
case .west:
print("Where the skies are blue")
}
当不需要匹配每个枚举成员的时候，你可以提供一个default分支来涵盖所有未明确处理的枚举成员
enum Planet: Int {
case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
}
enum CompassPoint: String {
case north, south, east, west
}
递归枚举：
它有一个或多个枚举成员使用该枚举类型的实例作为关联值。使用递归枚举时，编译器会插入一个间接层，你可以在枚举成员前加上indirect 来表示该成员可递归
enum Arth {
case Number(Int)
indirect case Add(Arth,Arth)
indirect case Mul(Arth,Arth)
}
let five = Arth.Number(5)
let four = Arth.Number(4)
let sum = Arth.Add(five, four)
let result = Arth.Mul(sum, Arth.Number(2))
print(result)
print(evaluate(express: result))
func evaluate(express:Arth) -> Int {
switch express {
case .Number(let value):
return value
case .Add(let left, let right):
return evaluate(express: left) + evaluate(express: right)
case .Mul(let left, let right):
return evaluate(express: left) * evaluate(express: right)
}
}
