Kotlin --argparser

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This is a library for parsing command-line arguments. It can parse both options and positional arguments. It aims to be easy to use and concise yet powerful and robust.


The main class in this library is ArgParser. It handles the parsing of command-line arguments, and also acts as a factory for creating property delegates. These delegates help to keep client code clear and concise.

Typical usage is to create a class to represent the set of parsed arguments, which are in turn each represented by properties that delegate to an ArgParser:

import com.xenomachina.argparser.ArgParser

class MyArgs(parser: ArgParser) {
    val v by parser.flagging(help = "enable verbose mode")

    val widgetName by parser.storing("name of the widget")

    val size by parser.storing("size of the plumbus") { toInt() }

The name of an option is inferred from the name of the property it is bound to. The options above are named “-v”, “–widget-name” and “–size”, respectively.

Direct control over an option’s name is also possible, and for most types of options it is also possible to have multiple names (typically used to have both a short and long name):

class MyArgs(parser: ArgParser) {
    val verbose by parser.flagging("-v", "--verbose",
                                   help = "enable verbose mode")

    val name by parser.storing("-w", "--widget-name",
                               help = "name of the widget")

    val size by parser.storing("-s", "--size",
                               help = "size of the plumbus") { toInt() }

Option Types

Various types of options can be parsed from the command line arguments:

Boolean Flags

Boolean flags are created by asking the parser for a flagging delegate. One or more option names, either short or long style, must be provided:

val verbose by parser.flagging("-v", "--verbose",
                               help = "enable verbose mode")

Here the presence of either -v or --verbose options in the arguments will cause the Boolean property verbose to be true, otherwise it will be false.

Storing a Single Argument

Single argument options are created by asking the parser for a storing delegate.

val name by parser.storing("-N", "--name",
                           help = "name of the widget")

Here either -N or --name with an argument will cause name to have that argument as its value.

A function can also be supplied to transform the argument into the desired type. Here the size property will be an Int rather than a String:

val size by parser.storing("-s", "--size",
                           help = "size of the plumbus") { toInt() }

Adding to a Collection

Options that add to a Collection each time they appear in the arguments are created with using the adding delegate. Just like storing delegates, a transform function may optionally be supplied:

val includeDirs by parser.adding(
        "-I", help = "directory to search for header files") { File(this) }

Now each time the -I option appears, its argument is appended to includeDirs.

Mapping from an option to a fixed value

For choosing between a fixed set of values (typically, but not necessarily, from an enum), a mapping delegate can be used:

val mode by parser.mapping(
        "--fast" to Mode.FAST,
        "--small" to Mode.SMALL,
        "--quiet" to Mode.QUIET,
        help = "mode of operation")

Here the mode property will be set to the corresponding com.xenomachina.argparser.ArgParser.Mode value depending on which of --fast, --small, and --quiet appears (last) in the arguments.

mapping is one of the few cases where it is not possible to infer the option name from the property name.

More advanced options

For all other types of options, the option method should be used. The methods mentioned above are, in fact, convenience methods built on top of the option method.

For example, it is possible to create an option that has multiple arguments:

  fun ArgParser.putting(vararg names: String, help: String) =
          option<MutableMap<String, String>>(*names,
                  argNames = listOf("KEY", "VALUE"),
                  help = help) {
              value.orElse { mutableMapOf<String, String>() }.apply {
                  put(arguments.first(), arguments.last()) }

Note that the option method does not have an auto-naming overload. If you need this capability, create a DelegateProvider that creates your Delegate:

  fun ArgParser.putting(help: String) =
          ArgParser.DelegateProvider { identifier ->
              putting(identifierToOptionName(identifier), help = help) }

Positional Arguments

Positional arguments are collected by using the positional and positionalList methods.

For a single positional argument:

val destination by parser.positional("DEST",
                                     help = "destination filename")

The name (“DEST”, here) is used in error handling and help text.

For a list of positional arguments:

val sources by parser.positionalList("SOURCE", 1..Int.MAX_VALUE,
                                     help = "source filename")

The range indicates how many arguments should be collected, and actually defaults to the value shown in this example. As the name suggests, the resulting property will be a List.

Both of these methods accept an optional transform function for converting arguments from String to whatever type is actually desired:

val destination by parser.positional("DEST",
                                     help = "...") { File(this) }

val sources by parser.positionalList("SOURCE", 1..Int.MAX_VALUE,
                                     help = "...") { File(this) }

Modifying Delegates

The delegates returned by any of these methods also have a few methods for setting optional attributes:

  • Some types of delegates (notably storing, mapping, and positional) have no default value, and hence will be required options unless a default value is provided. This is done with the default method:

    val name by parser.storing("-N", "--name", help = "...").default("John Doe")

    Note that it is possible to use null for the default:

    val name by parser.storing("-N", "--name", help = "...").default(null)

    The resulting value will be nullable (a String? in this case).

  • Sometimes it’s easier to validate an option at the end pf parsing, in which case the addValidator method can be used.

    val percentages by parser.adding("--percentages", help = "...") { toInt() }
            .addValidator {
                  if (value.sum() != 100)
                      throw InvalidArgumentException(
                              "Percentages must add up to 100%")

Error Handling

Exceptions caused by user error will all derive from SystemExitException, and include a status code appropriate for passing to exitProcess. It is recommended that transform functions (given to storing, positionalList, etc.) throw a SystemExitException when parsing fails.

Additional post-parsing validation can be performed on a delegate using addValidator.

As a convenience, these exceptions can be handled by using the mainBody function:

class ParsedArgs(parser: ArgParser) {
    val name by positional("The user's name").default("world")

fun main(args: Array<String>) = mainBody("hello") {
        ParsedArgs(ArgParser(args)).run {
            println("Hello, {name}!")

Note that parsing does not take place until at least one delegate is read, or force is called manually. It may be desirable to call force on the parser in the init of your args object after declaring all of your parsed properties.


Parsing of command-line arguments is performed sequentially. So long as option-processing is enabled, each not-yet-processed command-line argument that starts with a hyphen (-) is treated as an option.

Short Options

Short options start with a single hyphen. If the option takes an argument, the argument can either be appended:

# "-o" with argument "ARGUMENT"
my_program -oARGUMENT

or can be the following command-line argument:

# "-o" with argument "ARGUMENT"
my_program -o ARGUMENT

Zero argument short options can also be appended to each other without intermediate hyphens:

# "-x", "-y" and "-z" options
my_program -xyz

An option that accepts arguments is also allowed at the end of such a chain:

# "-x", "-y" and "-z" options, with argument for "-z"
my_program -xyzARGUMENT

Long Options

Long options start with a double hyphen (--). An argument to a long option can either be delimited with an equal sign (=):

# "--foo" with argument "ARGUMENT"
my_program --foo=ARGUMENT

or can be the following command-line argument:

# "--foo" with argument "ARGUMENT"
my_program --foo ARGUMENT

Multi-argument Options

Multi-argument options are supported, though currently not by any of the convenience methods. Option-arguments after the first must be separate command-line arguments, for both an long and short forms of an option.

Positional Arguments

In GNU mode (the default), options can be interspersed with positional arguments, but in POSIX mode the first positional argument that is encountered disables option processing for the remaining arguments. In either mode, if the argument “–” is encountered while option processing is enabled, then option processing is for the rest of the command-line. Once the options and option-arguments have been eliminated, what remains are considered to be positional arguments.

Each positional argument delegate can specify a minimum and maximum number of arguments it is willing to collect.

The positional arguments are distributed to the delegates by allocating each positional delegate at least as many arguments as it requires. If more than the minimum number of positional arguments have been supplied then additional arguments will be allocated to the first delegate up to its maximum, then the second, and so on, until all arguments have been allocated to a delegate.

This makes it easy to create a program that behaves like grep:

  class Args(parser: ArgParser) {
      // accept 1 regex followed by n filenames
      val regex by parser.positional("REGEX",
              help = "regular expression to search for")
      val files by parser.positionalList("FILE",
              help = "file to search in")

And equally easy to create a program that behaves like cp:

  class Args(parser: ArgParser) {
      // accept n source files followed by 1 destination
      val sources by parser.positionalList("SOURCE",
              help = "source file")
      val destination by parser.positional("DEST",
              help = "destination file")

Forcing Parsing

Parsing normally does not begin until a delegate’s value is accessed. Sometimes this is not desirable, so it is possible to enforce the parsing of arguments into a class of values. This ensures that all arguments that are required are provided, and all arguments provided are consumed.

Forcing can be done in a separate step using the force method:

  val parser = ArgParser(args)
  val parsedArgs = ParsedArgs(parser)
  // now you can use parsedArgs

Alternatively, forcing can be done inline via the parseInto method:

  val parsedArgs = ArgParser(args).parseInto(::ParsedArgs)
  // now you can use parsedArgs

In both cases exceptions will be thrown where parsing or validation errors are found.

Help Formatting

By default, ArgParser will add a --help option (short name -h) for displaying usage information. If this option is present the program will halt and print a help message like the one below, based on the ArgParser configuration:

usage: program_name [-h] [-n] [-I INCLUDE]... -o OUTPUT
                    [-v]... SOURCE... DEST

This is the prologue. Lorem ipsum dolor sit amet, consectetur
adipiscing elit. Aliquam malesuada maximus eros. Fusce
luctus risus eget quam consectetur, eu auctor est
ullamcorper. Maecenas eget suscipit dui, sed sodales erat.

required arguments:
  -o OUTPUT,          directory in which all output should
  --output OUTPUT     be generated

optional arguments:
  -h, --help          show this help message and exit

  -n, --dry-run       don't do anything

  -I INCLUDE,         search in this directory for header
  --include INCLUDE   files

  -v, --verbose       increase verbosity

positional arguments:
  SOURCE              source file

  DEST                destination file

This is the epilogue. Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Donec vel tortor nunc. Sed eu
massa sed turpis auctor faucibus. Donec vel pellentesque
tortor. Ut ultrices tempus lectus fermentum vestibulum.

The creation of the --help option can be disabled by passing null as the helpFormatter when constructing the ArgParser, or configured by manually constructing a HelpFormatter instance. In the above example a DefaultHelpFormatter was created with the prologue and epilogue.


  • This library should be considered to be very beta. While there are no plans to make any breaking changes to the API, it’s possible that there may be some until it is mature.

  • Upon reading the value any of the delegated properties created by an ArgParser, the arguments used to construct that ArgParser will be parsed. This means it’s important that you don’t attempt to create delegates on an ArgParser after any of its existing delegated properties have been read. Attempting to do so will cause an IllegalStateException. It would be nice if Kotlin had facilities for doing some of the work of ArgParser at compile time rather than run time, but so far the run time errors seem to be reasonably easy to avoid.

Configuring Your Build

Kotlin-argparser binaries are hosted on Bintray’s JCenter. In Gradle, use something like this in your build.gradle:

buildscript {
    repositories {

dependencies {
    compile "com.xenomachina:kotlin-argparser:$kotlin_argparser_version"

More information on setting up your Gradle, Maven, or Ivy dependencies can be found under the “Maven build settings” heading on Kotlin-argparser’s Bintray page, as well as the version of the latest release.

A Complete Example

See kotlin-argparser-example for a complete example of how to use Kotlin-argparser.


Thanks to the creators of Python’s argparse module, which provided the initial inspiration for this library.

Thanks also to the team behind Kotlin.

Finally, thanks to all of the contributors and folks who have contributed issues.

compile "com.xenomachina:kotlin-argparser:2.0.4"

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