Swift Programming: The Big Nerd Ranch Guide, 3rd Edition

Generic Composition and Opaque Types

Generic data structures can compose with one another using protocols to build systems of types that elegantly describe more complex ideas. For example, think of an array of dictionaries, which might have a composed type like Array<Dictionary<String,Int>>. Sometimes, you might want to hide some of the gritty implementation details of your compositions.

To see what this looks like in practice, you will model a hungry human ordering toast at a restaurant. Begin with a Food protocol and a food that conforms to it.

Example 21.23. Defining some food

...
protocol Food {
    var menuListing: String { get }
}

struct Bread: Food {
    var kind = "sourdough"
    var menuListing: String {
        "\(kind) bread"
    }
}

Using a protocol to ...