Understand swift Dictionary syntax

Question

I work with legacy code and i find following code:

let actions = [PrinterReportType.z: {
                        printer.printZReport($0)
                        DeviceFabric.lifehubTerminal().reconciliation()
                    }, .x: printer.printXReport, .openSession: printer.openSession}]

"Actions" is declared like this:

let actions: [PrinterReportsModel.PrinterReportType : ((String?) -> ()) -> ()]

Key is enum value, but i can't figure out what is value here. I have new enum type of PrinterReportsModel.PrinterReportType , and i simply want to add new value to that dictionary. I suppose this is some kind of function. So, i want to declare that function, add it here, but i can't figure out how. And i can't figure out wha type is - ((String?) -> ()) -> ()

Answer 1

String is a collection of characters.

String? means the same as Optional<String> , so it is either Optional.none or Optional.some(String) .

(String?) -> () is a function that takes a String? and returns nothing ( () also called Void , the zero-element tuple). Let's call this as a String? -consumer : it takes a String? and does something with it. Maybe it just throws the String? away. Maybe it prints it. Maybe it stores the String? in a variable or a database or sends it over the network.

You could define a String? -consumer closure like this:

let action: (String?) -> () = { (_ s: String?) -> () in
    print(s ?? "(none)")
}

(I'm fully specifying the types above, but you could omit some of the types and let the compiler infer them.)

You could define a String? -consumer function like this:

func test(_ s: String?) -> () { print(s ?? "(none)" }

And pass it around like this:

let action: (String?) -> () = test

You could define a String? -consumer method in a class (or struct) like this:

class MyObject {
    func test(_ s: String?) -> () { print(s ?? "(none)") }
}

And pass it around like this:

let myObject = MyObject()
let action: (String?) -> () = myObject.test

((String?) -> ()) -> () is a function that takes a String? -consumer and returns nothing. You can think of this as a String? -consumer-consumer. Maybe it throws the String? -consumer away. Maybe it stores the String? -consumer in a variable for later. Maybe it calls the String? -consumer once, or ten times, or once for each element in some array of strings. Maybe it schedules a call to the String? -consumer every second until the program exits.

You could define a String? -consumer-consumer closure like this:

let action: ((String?) -> ()) -> () = { (_ sc: (String?) -> ()) -> () in
    sc("hello")
    sc(nil)
    sc("world")
}

You could call the closure like this:

action({ (_ s: String?) -> () in print(s ?? "(none)")}

Or like this:

let printString: (String?) -> () = { print($0) }
action(printString)

You could define a String? -consumer-consumer function like this:

func test(_ sc: (String?) -> ()) -> () {
    sc("hello")
    sc(nil)
    sc("world")
}

And pass it around like this:

let action: ((String?) -> ()) -> () = test

You could define a String? -consumer-consumer method in a class (or struct) like this:

class MyObject {
    func test(_ sc: (String?) -> ()) -> () {
        sc("hello")
        sc(nil)
        sc("world")
    }
}

And pass it around like this:

let myObject = MyObject()
let action: ((String?) -> ()) -> () = myObject.test