The CollectionType protocol methods

All the collections mentioned earlier—array, set and, dictionary—implement a CollectionType protocol. Because of this, they are interchangeable. You can use any of them in places where a CollectionType method is required. An example is a function with a CollectionType parameter:

func useCollection<T: CollectionType>(x: T) {
  print("collection has \(x.count) elements")
}

let array = [1, 2, 3]
let set: Set = [2, 2, 3, 4, 5]
let dic = ["A" : 1, "B" : 2]

useCollection(array)
useCollection(set)
useCollection(dic)

Protocol extensions

The other incredibly useful feature is protocol extensions. With protocol extensions, we can add implementations of methods and properties directly to the protocol. All types that conform to that protocol are able to use those methods for free. Let's add our own property to a CollectionType method:

extension CollectionType {
  var middle: Self.Index.Distance {
    return count / 2
  }
}

array.middle
set.middle
dic.middle

The...