Optional parameter in enum case in swift

Question

In swift a function can have a optional parameters that have default values such as:

func f(a:Int, b:Int?=nil) {}

f(1);
f(1,2);

I'd like to do this with associated values with an enum. Following from this post about type safe url routes, I'd like to have a route which can take an optional parameter, such as:

enum StopPoint {
    case Search(query:String, limit:Int?=nil)
}

However it says I can't have an default value for a parameter in a tuple. However it is possible to have a case such as case Arrivals(stopId:Int), yet it is not possible in general to have a tuple with one named parameter.

So is it possible to have an enum with a default parameter, and is the associated value a tuple or not?

Answer 1

What you can do is:

enum StopPoint {
    case Search(query: String, limit: Int?)

    init(query: String, limit: Int? = nil) {
        self = .Search(query: query, limit: limit)
    }
}

let foo = StopPoint(query: "default")             // Search("default", nil)
let bar = StopPoint(query: "special", limit:  42) // Search("special", Optional(42))

Answer 2

You can do this by creating static functions on the enum that matches the enum case that you want to allow optional parameters for.

enum StopPoint {
    case search(query: String, limit: Int?)

    static func search(query: String) -> StopPoint {
        return .search(query: query, limit: nil)
    }
}

This makes it so that you can both provide the limit, or not, using the expected syntax:

let a = StopPoint.search(query: "Foobar")
let b = StopPoint.search(query: "Foobar", limit: 5)
let c = StopPoint.search(query: "Foobar", limit: nil)

func foo(_ stop: StopPoint) { /* ... */ }

foo(.search(query: "Foobar"))
foo(.search(query: "Foobar", limit: 5))
foo(.search(query: "Foobar", limit: nil))

Tested and confirmed working with Swift 4.2