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Why Clikt?

There are existing Kotlin libraries for creating command line interfaces, and many Java libraries work in Kotlin as well. However, none of them had all the following features:

  • Unrestricted composability of commands
  • Fully static type safety for parameters
  • Composable parameter customization that doesn’t require registering converter objects.
  • Full support for Unix command line conventions
  • Capable of reading parameter values from environment variables out of the box
  • Included support for common functionality (keyboard interactivity, line ending normalization, launching editors, etc.)
  • Built-in support for multi-token command aliases

Clikt is focused on making writing robust, posix-compliant command line interfaces as easy as possible. A good CLI does more than just parse argv. It allows users to specify values in environment variables, and in some cases prompts for additional input, or opens an editor. Clikt supports all of this out of the box.

Sometimes you need to make a CLI that doesn’t follow Unix conventions. You might be writing for windows, or you want to use the Java style of long options with a single dash. Maybe you need to use a bunch of required options instead of arguments, or you want the help page formatted differently. “Best practices” might not be the best for you, so Clikt tries to make implementing uncommon use-cases as easy as possible.

Why not a Kotlin library like kotlin-argparser or kotlinx.cli?

Clikt isn’t the only Kotlin CLI library. kotlin-argparser and kotlinx.cli both predate Clikt’s creation.

Both, like Clikt, use property delegates to define parameters, but they’re missing most of Clikt features and its extensible design.

kotlinx.cli was written by JetBrains and mostly copied kotlin-argparser‘s design (and, later, some of Clikt’s).

kotlin-argparser works well for simple cases. It’s missing a lot of features that Clikt has, but features could be added. Its real drawback is that it fundamentally does not support composition of commands or parameter values. The lack of subcommand support was already a non-starter, but there are other design decisions that make it unsuitable.

In the simple cases, the two libraries are similar. Here’s an example from its README:

class MyArgs(parser: ArgParser) {
    val v: Boolean by parser.flagging(help="enable verbose mode")
    val username: String? by parser.storing(help="name of the user")
    val count: Int? by parser.storing(help="number of the widgets") { toInt() }
    val source: List<String> by parser.positionalList(help="source filenames")
    val destination: String by parser.positional(help="destination")

fun main(args: Array<String>) = mainBody {
    ArgParser(args).parseInto(::MyArgs).run {
        println("Hello, $username!")
        println("Moving $count widgets from $source to $destination.")

Here’s the same thing with Clikt:

class Cli : CliktCommand() {
    val v: Boolean by option(help = "enable verbose mode").flag()
    val username: String? by option(help = "name of the user")
    val count: Int? by option(help = "number of the widgets").int()
    val source: List<String> by argument(help = "source filenames").multiple()
    val destination: String by argument(help = "destination")
    override fun run() {
        println("Hello, $name!")
        println("Moving $count widgets from $source to $destination.")

fun main(args: Array<String>) = Cli().main(args)

Both work fine, although you may find Clikt more consistent and a bit less verbose. The differences become more pronounced once you try to do anything that isn’t built in to kotlin-argparser.

Maybe you need an option to take two values. Here’s another example from the kotlin-argparser README showing how to do that:

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()) }

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

class MyArgs(parser: ArgParser) {
    val v by parser.putting(help="this takes two values")

Clikt has that functionality built in as option().pair(), but you could implement it yourself like this:

class Cli : CliktCommand() {
    val v by option(help="this takes two values").transformValues(2) { it[0] to it[1] }

The Clikt version is of course much simpler, but there are more fundamental issues with the kotlin-argparser version that drove the creation of Clikt:

  • Its inheritance-based design means that if you wanted to change the type of each value, you would have to copy all the code for each type. With Clikt, you could just do option().int().transformValues(2) { it[0] to it[1] }
  • Its inheritance-based design means that supporting types, multiple values, and multiple option occurrences would require a combinatorial number of copies of the above code. With Clikt, these are all orthogonal.
  • You have to do all error checking yourself. The argparser example silently discards extra values, or copies the single value, rather than inform the user of the mistake. You could write more code to do so, but Clikt takes care of it for you.
  • Option name inference is not automatic, requiring you to wrap the delegate with yet another function.
  • Each delegate function has a different name, with no indication of whether it’s creating an option or positional argument. With Clikt, all options are created with option(), and all arguments with argument().

Some of these problems can be solved by writing more code, and some can’t. On the other hand, Clikt attempts to have a consistent, intuitive, composable design that does the right thing without forcing you to think about edge cases.

Why not a Java library like JCommander or Picocli?

There are a lot of command line libraries for Java. Most are verbose and not composable. Two popular Java libraries that are usable from Kotlin are JCommander and picocli.

These libraries use annotations to define parameters, and reflection to set fields. This is functional for simple types, but defining your own types requires you to register a type adapter with the library. This means that type errors are not caught until runtime, and many types of customization are not possible.

For example, in JCommander, options that take multiple values cannot be converted to other types. The JCommander docs explain:

… only List is allowed for parameters that define an arity. You will have to convert these values yourself if the parameters you need are of type Integer or other (this limitation is due to Java’s erasure).

You also can’t customize many aspects of parsing in JCommander. It can’t infer parameter names. With JCommander, you can’t have an option with multiple values and multiple occurrences at the same time. You can’t have more than one argument, and it can only take one value or an unlimited number of values. You can’t nest subcommands.

JCommander and piocli are great libraries if you’re writing code in Java, but we can do much better with Kotlin.