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Commandant is a Swift framework for parsing command-line arguments, inspired by Argo (which is, in turn, inspired by the Haskell library Aeson).


With Commandant, a command and its associated options could be defined as follows:

struct LogCommand: CommandProtocol {
    typealias Options = LogOptions

    let verb = "log"
    let function = "Reads the log"

    func run(_ options: Options) -> Result<(), YourErrorType> {
        // Use the parsed options to do something interesting here.
        return ()

struct LogOptions: OptionsProtocol {
    let lines: Int
    let verbose: Bool
    let logName: String

    static func create(_ lines: Int) -> (Bool) -> (String) -> LogOptions {
        return { verbose in { logName in LogOptions(lines: lines, verbose: verbose, logName: logName) } }

    static func evaluate(_ m: CommandMode) -> Result<LogOptions, CommandantError<YourErrorType>> {
        return create
            <*> m <| Option(key: "lines", defaultValue: 0, usage: "the number of lines to read from the logs")
            <*> m <| Option(key: "verbose", defaultValue: false, usage: "show verbose output")
            <*> m <| Argument(usage: "the log to read")

Then, each available command should be added to a registry:

let commands = CommandRegistry<YourErrorType>()

After which, arguments can be parsed by simply invoking the registry:

var arguments = CommandLine.arguments

// Remove the executable name.
arguments.remove(at: 0)

if let verb = arguments.first {
    // Remove the command name.
    arguments.remove(at: 0)

    if let result = commands.run(command: verb, arguments: arguments) {
        // Handle success or failure.
    } else {
        // Unrecognized command.
} else {
    // No command given.

For real-world examples, see the implementation of the Carthage command-line tool.


Commandant is released under the MIT license.