try catch

do_try_catch.swift

enum CustomError : ErrorType {
    case ErrorWithMessage(message: String)
}

func loginUserWithName(username: String?) throws -> String {
    guard let username = username where username.characters.count != 0 else {
        throw CustomError.ErrorWithMessage(message: "No username provided")
    }
    return "token:" + username
}

func login() {
    
    do {
        let token = try loginUserWithName(nil)
        print("user logged in \(token)")
    } catch {
        print("An error occurred \(error)")
    }
}

login()