Generics in a protocol-oriented design

Now that we have seen how to use generics, let's see how we can use them in a protocol-oriented design. In a previous example in this chapter, we created a generic List type, however, we can greatly improve on this design by using what we learned throughout this chapter. We will include only a small subset of the actual requirements for a List type so we can focus on the design rather than all the requirements.

With a protocol-oriented design, we always start with the protocol. The following code shows the List protocol:

protocol List {
  associatedtype T
  subscript<E: Sequence>(indices: E) -> [T]
      where E.Iterator.Element == Int { get }
  mutating func add(_ item: T)
  func length() -> Int
  func get(at index: Int) -> T?
  mutating func delete(at index: Int)
}

We start the List protocol by defining the associated type T. This associated type will be the type of data stored in the list. We use the T type as the parameter for the add...