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| //: Playground - noun: a place where people can play | |
| // Got most of it from: | |
| // https://developer.apple.com/sample-code/wwdc/2015/downloads/Crustacean.zip | |
| // This is in the Protocol-Oriented Programming in Swift talk | |
| // Pay special attention to the Heterogeneous Equality section, on page 2. | |
| // | |
| // Most comments directly copy/pasted from Crustacean Playground | |
| protocol Account { | |
| var accountID: String { get } | |
| func isEqualTo(other: Account) -> Bool | |
| } | |
| struct FooAccount: Account { | |
| var accountID = "foo" | |
| } | |
| struct BarAccount: Account { | |
| var accountID = "bar" | |
| } | |
| // Value types should always be Equatable. Mostly this is just rote comparison | |
| // of the corresponding sub-parts of the left-and right-hand arguments, but | |
| // things get interesting below where handle heterogeneous comparison. | |
| extension FooAccount: Equatable {} | |
| func == (lhs: FooAccount, rhs: FooAccount) -> Bool { | |
| return lhs.accountID == rhs.accountID | |
| } | |
| extension BarAccount: Equatable {} | |
| func == (lhs: BarAccount, rhs: BarAccount) -> Bool { | |
| return lhs.accountID == rhs.accountID | |
| } | |
| // Asking every Account to support a heterogeneous binary operation imposes | |
| // quite a burden—one that is usually solved by adding a Self requirement to the | |
| // protocol. However, equality is a special case, because there's usually a | |
| // meaningful result when the types don't match: the instances can be assumed to | |
| // be not-equal. We extend Equatable (which supports homogeneous equality | |
| // comparison) to provide an isEqualTo for Account that returns false when the | |
| // types don't match. | |
| extension Account where Self: Equatable { | |
| func isEqualTo(other: Account) -> Bool{ | |
| guard let o = other as? Self else { return false } | |
| return self == o | |
| } | |
| } | |
| // Use isEqualTo() to implement Equatable conformance to Account | |
| func == (lhs: Account, rhs: Account) -> Bool { | |
| return lhs.isEqualTo(rhs) | |
| } | |
| let foo = FooAccount() | |
| let bar = BarAccount() | |
| print(foo == bar) // False | |
| var accounts: [Account] = [Account]() | |
| accounts.append(foo) | |
| accounts.append(bar) | |
| print(accounts) |
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