(Swift) nested function, function as return, argument

Question

Regarding the code below, I was wondering how nested functions such as stepForward, stepBackward, when they are actually called through 'moveNearerToZero(currentValue), take currentValue as their arguments.

According to the definition of the function chooseStepFunction, the return type is (Int) -> Int. Then, is it because of the parenthesis around Int that the nested functions are able to take currentValue as their arguments?

I can't undesrtand how the value of -4 of currentValue is captured in two nested functions.

func chooseStepFunction(backward: Bool) -> (Int) -> Int {
    func stepForward(input: Int) -> Int { return input + 1 }
    func stepBackward(input: Int) -> Int { return input - 1 }
    return backward ? stepBackward : stepForward
}
var currentValue = -4
let moveNearerToZero = chooseStepFunction(backward: currentValue > 0)

while currentValue != 0 {
    print("\(currentValue)... ")
    currentValue = moveNearerToZero(currentValue)
}
print("zero!")

It isn’t captured. It is passed directly: currentValue = moveNearerToZero(currentValue). — matt
– matt, Commented Jun 10, 2020 at 12:53

Michael Schinis · Accepted Answer · 2020-06-10 11:55:32Z

3

Functions in Swift are first class citizens. This means that functions can be passed around or stored like a variable, just like any other swift type.

Anything that follows the first -> is the type being returned by that method. In this case chooseStepFunction returns another method that takes one parameter (Int) and returns one value (Int). The parenthesis around the first Int, mean that it's an input parameter.

With that in mind, moveNearerToZero is function that takes one Int and returns another Int. To call that method, you then simply do moveNearerToZero(1) and so on.

I highly recommend you read Apple's section on functions to better understand how functions can be used or passed around. https://docs.swift.org/swift-book/LanguageGuide/Functions.html

answered Jun 10, 2020 at 11:55

Michael Schinis

7071 gold badge7 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

a.palo · Accepted Answer · 2020-06-10 16:57:25Z

0

chooseStepFunction(backward: currentValue > 0) returns "stepForward" function. In "stepForward" function as the value is incremental by 1. So, first -4 is getting printed than -3(-4+1) is getting printed and so on until currentValue reaches 0 where the while loop terminates.

edited Jun 10, 2020 at 16:57

answered Jun 10, 2020 at 12:01

a.palo

2883 silver badges13 bronze badges

2 Comments

Michael Schinis Over a year ago

it doesn't return a reference, but the actual method.

a.palo Over a year ago

Yes. I mean to say the reference of the method.

Collectives™ on Stack Overflow

(Swift) nested function, function as return, argument

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related