Chapter 8 - Capturing from the enclosing scope

Books Swift Apprentice
1

Why does this work:

func countingClosure() -> () -> Int {
  var counter = 0
  let incrementCounter: () -> Int = {
    counter += 1
    return counter
  }
  return incrementCounter
}

and this not?

func countingClosure() -> () -> Int {
    var counter = 0
    let incrementCounter: () -> Int = {
        return counter += 1 // not working
    }
    return incrementCounter
}

Thanks

2

In the first example, the countingClosure function works because it returns a closure that directly modifies the value of counter by incrementing it by 1. In the second example, the countingClosure function does not work because the expression return counter += 1 attempts to modify the value of counter and return it at the same time. In Swift, it is not possible to modify a value and return it at the same time, which is why the expression in the closure fails.