Swift: assigning function to variable

Question

I have a class in swift with the following variable:

var pendingFunction = ((Double, Double) -> Double) 

Swift then tells me:

Expected member name or constructor call after type name

It insists that I change my code to:

var pendingFunction = ((Double, Double) -> Double).self 

I have no clue what this .self thing is doing (sorry I'm new to Swift)

I then try to assign pendingFunction to a new function:

pendingFunction = function 

where function takes two Doubles and returns a Double.

However, I'm presented with the following error:

Cannot assign value of type '(Double,Double) -> Double' to type '((Double,Double)->Double).Type'

So my question is: what is the .self thing doing and how can I properly assign a function to the variable?

Answer 1

The postfix .self expression just refers to the object it's an instance of. Kind of like .this in other languages. For types in particular, it tells the compiler that you're referring to the type itself, rather than instructing it to create a new instance of that type. If you'd like to know more, you can read all about it in the docs here. While useful in a lot of cases, it's not really needed here.

As for your problem, when you assign:

var pendingFunction = ((Double, Double) -> Double).self 

You're assigning the type of a particular sort of function as the value of the variable. From that, Swift infers that the type of the variable should be Type. Then later when you try to assign an actual function fitting that type as the value, it throws an error because it's expecting a type, not a function.

Instead of assigning a type as value, you want to declare the variable with that type as its type:

var pendingFunction: ((Double, Double) -> Double) 

Here's an example of the whole thing:

var pendingFunction: ((Double, Double) -> Double)  func myAdditionFunction (first: Double, second: Double) -> Double {     return first + second }  pendingFunction = myAdditionFunction  print(pendingFunction(1,2)) // prints "3.0"