Optional Default Parameter in Swift

Try this:

 test(string: string ?? "", middleString: middleString ?? "", endString: endString ?? "")

You'll have to use Optional strings String? as your argument type, with default values of nil. Then, when you call your function, you can supply a string or leave that argument out.

func test(string: String? = nil, middleString: String? = nil, endString: String? = nil) -> Void {
    let s = string ?? ""
    let mS = middleString ?? ""
    let eS = endString ?? ""
    // do stuff with s, mS, and eS, which are all guaranteed to be Strings
}

Inside your function, you'll have to check each argument for nil and replace with a default value there. Using the ?? operator makes this easy.

You can then call your function by supplying all arguments, no arguments, or only the ones you want to include:

test(string: "foo", middleString: "bar", endString: "baz")
test()
test(string: "hello", endString: "world")