How to write mutating function for enum in Swift - swift

Question

I want to write mutating function like the following. But this code cannot be compiled because x and y are immutable and copied value. I want to get the reference of x and y. How to get the reference of variables wrapped by enum?
enum Fruit {
case apple(Int)
case banana(Int, Int, Int)
mutating func increment() {
switch self {
case let .apple(x):
x += 1
case let .banana(x, y, z):
x += 1
}
}
}
var a = Fruit.banana(100, 200, 300)
a.increment()
I know the following code can do the same thing. But I think this solution is redundant because we have to write each variable twice.
enum Fruit {
case apple(Int)
case banana(Int, Int, Int)
mutating func increment() {
switch self {
case let .apple(x):
self = .apple(x + 1)
case let .banana(x, y, z):
self = .banana(x + 1, y, z)
}
}
}
var a = Fruit.banana(100, 200, 300)
a.increment()

Answer 1

Using self = .apple(x) will do the trick. It is customary, but you already knew this.
enum Fruit {
case apple(Int)
case banana(Int, Int, Int)
mutating func increment() {
switch self {
case let .apple(x):
self = .apple(x + 1)
case let .banana(x, y, z):
self = .banana(x + 1, y + 1, z + 1)
}
}
}
var a = Fruit.banana(100, 200, 300)
a.increment()
You could also add support for generic operators:
enum Fruit {
case apple(Int)
case banana(Int, Int, Int)
mutating func operate(_ op: (Int, Int) -> Int,_ num: Int) {
switch self {
case let .apple(x):
self = .apple(op(x, num))
case let .banana(x, y, z):
self = .banana(op(x, num), op(y, num), op(z, num))
}
}
}
var a = Fruit.banana(100, 200, 300)
a.operate(+, 1) // adds all by 1
a.operate(*, 500) // multiplies all by 500
You could also add support for an array of generic operators:
enum Fruit {
case apple(Int)
case banana(Int, Int, Int)
mutating func operate(_ nest: [((Int, Int) -> Int, Int)]) {
for i in nest {
switch self {
case let .apple(x):
self = .apple(i.0(x, i.1))
case let .banana(x, y, z):
self = .banana(i.0(x, i.1), i.0(y, i.1), i.0(z, i.1))
}
}
}
}
var a = Fruit.banana(100, 200, 300)
a.operate([(+, 500), (*, 2)]) // Adds all by 500, then multiply all by 2
Here's a nice simplification
If it were up to me, this is what I would do to get rid of repeating self =. You can simplify this further if you wish.
enum FruitList: String { case apple, banana }
struct Fruit {
static func apple(_ one: Int) -> Fruit {
return Fruit.init(.apple, [one])
}
static func banana(_ one: Int,_ two: Int,_ three: Int) -> Fruit {
return Fruit.init(.apple, [one, two, three])
}
var picked: (FruitList, [Int])
init(_ fruit: FruitList,_ list: [Int]) {
picked = (fruit, list)
}
mutating func operate(_ nest: [((Int, Int) -> Int, Int)]) {
for i in nest {
for j in 0..<picked.1.count {
picked.1[j] = i.0(picked.1[j], i.1)
}
}
}
}
var a = Fruit.apple(100)
a.operate([(+, 500), (*, 2)]) // Add all by 500, then multiply all by 2