Extensions with the Swift standard library

Let's say that, in our application, we needed to calculate the factorial of some integers. A factorial is written as 5!. To calculate a factorial, we take the product of all the positive integers that are less than, or equal to, the number. The following example shows how we would calculate the factorial of five:

5!  =  5*4*3*2*1 
5!  =  120 

We could very easily create a global function to calculate the factorial, and we would do that in most languages. However, in Swift, extensions give us a better way to do this. The Integer type in Swift is implemented as a structure that we can extend to add this functionality directly to the type itself. The following example shows how we can do this:

extension Int { 
    func factorial() -> Int { 
        var answer = 1 
        for x in (1...self).reversed() { 
           answer *= x ...