CommandConfiguration = .init( subcommands: [Sum.self, Average.self], defaultSubcommand: Sum.self ) } struct Sum: ParsableCommand { @Argument() var numbers: [Int] = [] mutating func run() throws { print(numbers.reduce(0, +)) } } struct Average: ParsableCommand { @Argument() var numbers: [Int] = [] func validate() throws { if numbers.count == 0 { throw ValidationError("Please specify numbers.") } } mutating func run() throws { print(numbers.reduce(0, +) / numbers.count) } } SampleSubCommands.main()