A simple, modular command line argument parser in Swift.

What's New

Swift 5


Add support for Swift 5, and keep supporting Swift 3 and 4.

 Run shell commands   |   Parse command line arguments | Handle files and directories

Build Status Platforms Version Carthage compatible


Moderator is a simple Swift library for parsing commandline arguments.


  • Modular, easy to extend.
  • Supports a cross platform syntax.
  • Generates help text automatically.
  • Handles arguments of the type '--option=value'.
  • Optional strict parsing, where an error is thrown if there are any unrecognised arguments.
  • Any arguments after an "--" argument are taken literally, they are not parsed as options and any '=' are left untouched.

See also why Moderator was created.


from linuxmain-generator:

import Moderator
import FileSmith

let arguments = Moderator(description: "Automatically add code to Swift Package Manager projects to run unit tests on Linux.")
let overwrite = arguments.add(.option("o","overwrite", description: "Replace <test directory>/LinuxMain.swift if it already exists."))
let testdirarg = arguments.add(Argument<String?>
	.optionWithValue("testdir", name: "test directory", description: "The path to the directory with the unit tests.")
_ = arguments.add(Argument<String?>
	.singleArgument(name: "directory", description: "The project root directory.")
	.map { (projectpath: String) in
		let projectdir = try Directory(open: projectpath)
		try projectdir.verifyContains("Package.swift")
		Directory.current = projectdir

do {
	try arguments.parse()

	let testdir = try Directory(open: testdirarg.value)
	if !overwrite.value && testdir.contains("LinuxMain.swift") {
		throw ArgumentError(errormessage: "\(testdir.path)/LinuxMain.swift already exists. Use -o/--overwrite to replace it.")
} catch {

Automatically generated help text:

Automatically add code to Swift Package Manager projects to run unit tests on Linux.

Usage: linuxmain-generator
      Replace <test directory>/LinuxMain.swift if it already exists.
  --testdir <test directory>:
      The path to the directory with the unit tests. Default = 'Tests'.
      The project root directory. Default = './'.


Moderator works by having a single Moderator object which you add individual argument parsers to. When you start parsing it goes through each argument parser in the order they were added. Each parser takes the array of string arguments from the command line, finds the arguments it is responsible for, processes them and throws any errors if anything is wrong, removes the arguments from the array, returns its output (which for some parsers may be nil if the argument was not found) and passes the modified array to the next parser.

This keeps the code simple and each parser only has to take care of its own arguments. The built-in parsers can easily be customised, and you can create your own parsers from scratch.

Built-in parsers


func option(_ names: String..., description: String? = nil) -> Argument<Bool>

Handles option arguments like -h and --help. Returns true if the argument is present and false otherwise.

Option with value

func optionWithValue(_ names: String..., name valuename: String? = nil, description: String? = nil)
-> Argument<String?>

Handles option arguments with a following value, like --help <topic>. It returns the value as a String, or nil if the option is not present.

Single argument

func singleArgument(name: String, description: String? = nil) -> Argument<String?>

Returns the next argument, or nil if there are no more arguments or the next argument is an option. Must be added after any option parsers.



Can be used on parsers returning optionals, to replace nil with a default value.


Can be used on parsers returning optionals, to throw an error on nil.


Takes the output of any argument parser and converts it to something else.


Looks for multiple occurrences of an argument, by repeating an optional parser until it returns nil.

let m = Moderator()
let options = m.add(Argument<String?>.optionWithValue("b").repeat())
let multiple = m.add(Argument<String?>.singleArgument(name: "multiple").repeat())

try m.parse(["-b", "b1", "-b", "b2", "-b", "b3", "one", "two", "three"])

options.value is now ["b1", "b2", "b3"] and multiple.value is ["one", "two", "three"]


Counts the number of times an option argument occurs.

let m = Moderator()
let option = m.add(Argument<Bool>.option("b").count())

try m.parse(["-b", "-b", "some", "other", "-b", "arguments"])

option.value returns 3  

Add new parsers

If the built in parsers and customisations are not enough, you can easily create your own parsers. As an example here is the implementation of the singleArgument parser:

extension Argument {
	public static func singleArgument (name: String, description: String? = nil) -> Argument<String?> {
		return Argument<String?>(usage: { ("<"+name+">", $0) }) { args in
			let index = args.first == "--" ? args.index(after: args.startIndex) : args.startIndex
			guard index != args.endIndex, !isOption(index: index, args: args) else { return (nil, args) }
			var args = args
			return (args.remove(at: index), args)

In the Argument initialiser you return a tuple with the output of the parser and the arguments array without the processed argument(s).


Add .Package(url: "", "0.5.0") to your Package.swift:

import PackageDescription

let package = Package(
	name: "somename",
	dependencies: [
		.Package(url: "", "0.5.0")

and run swift build.

Add github "kareman/Moderator" to your Cartfile, then run carthage update and add the resulting framework to the "Embedded Binaries" section of the application. See Carthage's README for further instructions.

Add Moderator to your Podfile.

pod "Moderator", git: ""

Then run pod install to install it.


Released under the MIT License (MIT),

Kåre Morstøl, NotTooBad Software


  • Swift Tools 4.0.0
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Last updated: Thu Apr 04 2024 22:20:26 GMT-0900 (Hawaii-Aleutian Daylight Time)