protocolequatable.swift

//: Playground - noun: a place where people can play

import Cocoa

protocol Account: Hashable {

    var accountID: String {get}
    var hashValue: Int {get}
}

// https://twitter.com/optshiftk/status/628985834801336320

func ==<T: Account>(lhs: T, rhs: T) -> Bool {

    return lhs.accountID == rhs.accountID
}

class FooAccount: Account {

    let accountID = "foo"
    let hashValue = 1
}

class BarAccount: Account {

    let accountID = "bar"
    let hashValue = 2
}

let foo = FooAccount()
let bar = BarAccount()
var accounts = Set<Account>()
accounts.insert(foo)
accounts.insert(bar)

// Two errors on line 32:
// protocol type 'Account' does not conform to protocol 'Hashable' because 'Account' is not declared @objc
// protocol 'Account' can only be used as a generic constraint because it has Self or associated type requirements