Example from @cocoaphony at try! Swift NYC (Sep. 1, 2016)

impossible-bugs.swift

struct Token {
    var value: String
}

struct Credential {
    var username: String
    var password: String
}

enum Result<Value> {
    case success(Value)
    case failure(Error)
}

func login(credential: Credential, callback: (token: Token?, error: Error?)) {
    // Now we need to check four cases:
    // 
    //                      result
    //                  nil      value
    //               ┌────────┬────────┐
    //           nil │   ??   │    ✓   │
    //  error        ├────────┼────────┤
    //         value │    ✓   │   ??   │
    //               └────────┴────────┘
    // 
    // ...even though only two make logical sense. Having both
    // an error value and a result value is a case we have to
    // test for.
}

// By using the Result enum type instead of a tuple with two
// optional values, we force either a success value or a 
// failure value.
func login(credential: Credential, callback: Result<Token>) {
    switch callback {
    case .success(let token):
        // Handle success
        break
    case .failure(let error):
        // Handle error
        break
    }
}