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 have 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...