Not clear syntax - swift

Question

In the book "Learning Swift" (Wagner) there's this example about the closures ( closures as parameters in particular):
func firstInNumbers(numbers: [Int], passingTest: (number: Int) -> Bool -> Int? {
for number in numbers {
if passingTest(number: number) {
return number
}
}
return nil
}
let numbers = [1,2,3,4,5]
func greaterThanThree(number: Int) -> Bool {
return number > 3
}
var firstNumber = firstInNumbers(numbers, greaterThanThree)
println(firstNumber)
<code>
So, why the (number: number) syntax in the if statement ?

Answer 1

The second number:
passingTest(number: number)
^^^^^^^
was added because the type of the closure passingTest is (number: Int) -> Bool. See the number: in the type? That specifies the argument label you need to put when invoking the closure. The reason why the parameter label was put there was probably because the author tries to be clearer, but IMO it is already quite clear what the parameter of that closure is doing. Plus, the label number doesn't really add much meaning... So the first number: was really just for "aesthetics". Maybe it makes it easier to understand for beginners?
However, in Swift 4, you become unable to add parameter labels to closure types. In Swift 4, the method would be like this:
func firstInNumbers(numbers: [Int], passingTest: (Int) -> Bool) -> Int? {
for number in numbers {
if passingTest(number) {
return number
}
}
return nil
}

Answer 2

It seems the thing confusing you is that the closure's parameter name, "number", is the same as the variable you are passing into the closure.
//The parameter name "number" here -----------------V
func firstInNumbers(numbers: [Int], passingTest: (number: Int) -> Bool -> Int? {
//And the variable "number" here -\
// V---------------------------/
for number in numbers {
if passingTest(number: number) {
return number
}
}
return nil
}
Perhaps it's easier to tell the difference if you use a different name, like this:
func firstInNumbers(numbers: [Int], passingTest: (number: Int) -> Bool -> Int? {
for n in numbers {
if passingTest(number: n) {
return n
}
}
return nil
}
In Swift 4 closure parameters can't be labeled. Additionally there were a couple small syntax errors in the original post. Correcting the syntax errors, and bringing the closure examples up to Swift 4, the full example would look like this:
func firstInNumbers(numbers: [Int], passingTest: (_ number: Int)->Bool) -> Int? {
for number in numbers {
if passingTest(number) {
return number
}
}
return nil
}
let numbers = [1,2,3,4,5]
func greaterThanThree(_ number: Int) -> Bool {
return number > 3
}
var firstNumber = firstInNumbers(numbers: numbers, passingTest: greaterThanThree)
print(firstNumber)