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iOS Swift Closures Functions as First Class Citizens Capturing Variables

Dennis Henley
Dennis Henley
3,421 Points
Posted by Dennis Henley
Dennis Henley
3,421 Points

When to use parentheses

In the code under discussion, we create a function within a function.

func printerFunction () -> (Int) -> Void { var runningTotal = 0 func printInteger(number: Int) -> Void { runningTotal += 10 println ("The running total is: (runningTotal)") } return printInteger }

Then we declare a constant of type printerFunction.

let printAndReturnIntegerFunc = printerFunction()

We use parentheses with this declaration.

In a previous example, we created a function as:

func isEvenNumber(i: Int) -> Bool { return i%2 == 0 }

and declared a constant as:

let ifEven = isEvenNumber

This did not require parentheses.

When should we use parenthesis when we make a constant of a function?

3 Answers

nathankrishnan
nathankrishnan
26,276 Points

Dennis, in:

let printAndIntegerFunc = printerFunction()

We are assigning the constant to what printerFunction returns: a function that takes in an int as a parameter and returns void. In other words, we are assigning the execution of the printerFunction to a constant. The execution of the printerFunction returns the inner function: printInteger. That is why this is possible:

printAndReturnIntegerFunc(2)

We are able to pass in the integer value of 2 into the printInteger function, which is the inner function that is invoked when printerFunction is executed.

In

let ifEven = isEvenNumber

The parenthesis aren't used when declaring the function to the constant because we are assigning the function itself, not the execution of the function, to the constant.

Dylan Shine
Dylan Shine
17,565 Points
Dylan Shine
Dylan Shine
17,565 Points

Hi Dennis,

I don't think I'm going to be able to give you an answer as it relates to your code but the reason we use parentheses is for function/method invocation. If you forget to append the parentheses, you will be return the function object itself, not the functions return value.

Dennis Henley
Dennis Henley
3,421 Points
Dennis Henley
Dennis Henley
3,421 Points

Nathan,

Thanks. That cleared things up.