Protocol Composition

protocol Named {
    var name: String { get }
}
protocol Aged {
    var age: Int { get }
}
struct Person: Named, Aged {
    var name: String
    var age: Int
}
func describe(_ value: Named & Aged) -> String {
    return "\(value.name) is \(value.age) years old"
}
let person = Person(name: "Lee", age: 40)
print(describe(person))

protocol Named { var name: String { get } }
protocol Aged { var age: Int { get } }
protocol Employed { var job: String { get } }
struct Worker: Named, Aged, Employed {
    var name: String
    var age: Int
    var job: String
}
func introduce(_ value: Named & Aged & Employed) {
    print("\(value.name), \(value.age), works as \(value.job)")
}
introduce(Worker(name: "Kai", age: 33, job: "Engineer"))

1. Trying to use inheritance-style syntax to require multiple protocols; composition uses & between protocol names, not commas in a type annotation.
2. Forgetting a composed type (ProtocolA & ProtocolB) requires an instance to conform to BOTH protocols, not just one of them.
3. Overcomplicating a function signature with many composed protocols when a single custom protocol combining the requirements would be clearer.

• ProtocolA & ProtocolB describes a type conforming to both protocols at once.
• This is commonly used as a parameter type to require multiple capabilities.
• Composition differs from class inheritance, which only allows one superclass.
• You can compose any number of protocols together with &.