protocol Describable { func describe() -> String } extension Describable { func describe() -> String { "(no description)" } } struct User: Describable { let name: String } struct Car: Describable { let model: String func describe() -> String { "Car: \(model)" } } let u = User(name: "Morgan") let c = Car(model: "SwiftMobile") print(u.describe()) print(c.describe())