How does this function call this closure when each returns just a single Int?

How does this function call this closure when each returns just a single Int?

Problem Description:

I find this code very confusing:

let myClosure = { (number1: Int, number2: Int) -> Int in
    return number1 * number2
}

func useClosure(closure: (Int, Int) -> Int) {
    let result = closure(2, 3)
    print(result) // prints 6
}

useClosure(closure: myClosure)

The func useClosure takes 2 Ints with the parameter name closure which is the closure myClosure but myClosure has only 1 returned Int, so how can it be the parameter of useClosure?

And where is myClosure getting number1 and number2 from? From useClosure?

On the surface, it seems 2 and 3 are being passed to myClosure but it’s not clear how that is happening.

It all seems circular to me and makes little sense.

I haven’t really tried anything to "resolve" this as it doesn’t really need resolution. The code works fine. I’m just struggling to understand it – from where I am in my journey as a developer.

Solution – 1

If you look at the declaration of myClosure – or even better, option+click on it – you’ll see that its type is (Int, Int) -> Int. Which is exactly the type of the input argument useClosure expects.

That type signature represents a closure, which takes two Ints as input and returns an Int.

The func useClosure takes 2 Ints with the parameter name closure

useClosure takes a closure as its input, not a pair of Ints as you claim.

You are passing the values inside useClosure to the input argument, closure.

When looking at type signatures, everything on the left-hand side of -> represents the input arguments to the function, while everything on its right hand side represents the return type.