ProtocolWithSelfOrPAT.swift

protocol Record: Equatable {
    func save()
}

extension Record {
    func save() {
        // save to disk
        print("Record: Disk")
    }
}

struct User: Record {
    let name: String
    
    func save() {
        // save to server
        print("User: Server")
    }
    
    // Some User capability
    func doUserThing() {
        print("Doing thing")
    }
}

func expectARecord<R: Record>(_ record: R) {
    record.save()
}

// Problem: user her is only a Record, not a User
let user: some Record = User(name: "Jane Doe")
// error: 'some' types are only implemented for the declared type of properties and subscripts and the return type of functions
//let makeNoSense = User(name: "Paul") as some Record

user.save()  // user's static type is Record, its dynamic type is User, so save will do according to user.
//user.doUserThing()      // <=== no can do!

// This let user be a User and a Record fully
let userFully = User(name: "Jane Doe")
userFully.save()  // Save according to User's implementation
expectARecord(userFully)
userFully.doUserThing()