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Kotlin is a programming language developed by JetBrains. Originally shaped as a programming language based on the Java platform and JVM, Kotlin was designed with Java interoperability in mind, meaning that a Kotlin program can use not just the Kotlin Standard Library but the Java SE API and all Java libraries too, and Java programs can call useful helpers in the Kotlin Standard Library and all sorts of other Kotlin code as well. Later, Kotlin has been expanded with Android support, JavaScript support through Kotlin/JS, and support for native machine code as compiler target via Kotlin/Native.

Like virtually all other GNU/Linux distributions, Gentoo does not provide Kotlin in its official ebuild repository yet. A few users' personal ebuild repositories may contain an ebuild for Kotlin that unpacks the compiler Zip archive made by the upstream to the file system. During Google Summer of Code 2021, ebuilds that can build the Kotlin core libraries were created, so Gentoo users can install the Kotlin libraries from source instead of binaries pre-compiled by the upstream. A blog post documenting how the ebuilds were created is available.


Available packages

The following packages for Kotlin are available:

Package Maven Central Artifact Description Status Comments
Kotlin Standard Library
dev-java/kotlin-stdlib org.jetbrains.kotlin:kotlin-stdlib
Kotlin Standard Library for JVM Works
dev-java/kotlin-stdlib-jdk7 org.jetbrains.kotlin:kotlin-stdlib-jdk7 Kotlin Standard Library JDK 7 extension Works
dev-java/kotlin-stdlib-jdk8 org.jetbrains.kotlin:kotlin-stdlib-jdk8 Kotlin Standard Library JDK 8 extension Works
dev-java/kotlin-stdlib-js org.jetbrains.kotlin:kotlin-stdlib-js Kotlin Standard Library for JavaScript Unchecked There is not a known method to reproduce a JAR that is equivalent to the upstream's pre-built JAR from Portage. The upstream provides a Mocha test suite, which is yet to be integrated with the ebuild.
dev-java/kotlin-test org.jetbrains.kotlin:kotlin-test
Kotlin Test Multiplatform library Works
dev-java/kotlin-test-junit org.jetbrains.kotlin:kotlin-test-junit Kotlin Test Support for JUnit 4 Works
dev-java/kotlin-test-js org.jetbrains.kotlin:kotlin-test-js Kotlin Test for JavaScript Unchecked There is not a known method to reproduce a JAR that is equivalent to the upstream's pre-built JAR from Portage, and the upstream does not have a test suite that can be used to test this package.
Other Library Components
dev-java/kotlin-reflect org.jetbrains.kotlin:kotlin-reflect Kotlin Full Reflection Library Should Work There is not a known method to reproduce a JAR that is structurally identical to the upstream's pre-built JAR from Portage, and the upstream does not have a test suite that can be used to test this package. However, when this package is used with the Kotlin compiler, no undesired or unexpected behavior would be exhibited.
dev-java/kotlin-annotations-jvm org.jetbrains.kotlin:kotlin-annotations-jvm Kotlin annotations for JVM Works
dev-lang/kotlin-bin org.jetbrains.kotlin:kotlin-compiler Binary package for the Kotlin compiler Works

Versioning and package slotting

The Kotlin packages strive to stay consistent with the upstream's versioning scheme. Here is the glossary of terms the upstream uses to define the versioning scheme and describe different kinds of Kotlin releases:

Feature releases
e.g. 1.3, 1.4, 1.5...
Incompatible API changes are expected. For example, several compiler warnings have been promoted to compiler errors in Kotlin 1.5[1].
Incremental releases
e.g. 1.5.0, 1.5.10, 1.5.20, 1.5.30...
Small but non-trivial changes are expected; however, they should be API compatible with prior versions in the same feature release series. For example, in the upstream's Kotlin compiler Zip archive for Kotlin 1.5.20, a new Lombok compiler plugin JAR was added[2].
Bug fix releases
e.g. 1.5.30, 1.5.31, 1.5.32...
As the name suggests, this kind of releases usually contains minor fixes only.

All Kotlin packages are slotted based on the feature releases, so users can specify to install Kotlin 1.4.x, 1.5.x, etc. to use multiple feature releases in parallel.


Ebuild repository

The Kotlin ebuilds are currently located in the Spark overlay. Before installing the Kotlin ebuilds, please add the Spark overlay to the system using the instructions in the overlay's README.

USE flags

dev-java/kotlin-* packages

Each of the Kotlin library packages provides some or all of the USE flags listed below:

  • binary: Use the binary JAR pre-built by the upstream instead of compile the package from source
  • source: Install an archive of the source files under /usr/share/${PN}-${SLOT}/sources, so the package's source files can be viewed from some IDEs
  • test: Enable dependencies to run the package's tests, which are controlled by FEATURES=test


The first step to install any Kotlin packages - both the Kotlin library packages and third-party Kotlin packages - is to install the Kotlin compiler, which is provided by dev-lang/kotlin-bin.

root #emerge --ask dev-lang/kotlin-bin

If this is the first time dev-lang/kotlin-bin is being installed, bootstrap packages for Kotlin libraries required by the Kotlin compiler will be pulled as dependencies. After the Kotlin compiler is installed, these libraries can be rebuilt from source using it, and the bootstrap packages will be replaced:

root #emerge --ask --oneshot dev-java/kotlin-stdlib dev-java/kotlin-reflect

The process described here is applicable only to Kotlin 1.4 and above. A set of packages for Kotlin 1.3 are offered, but these packages do not build the Kotlin libraries from source, so they can be merged directly without bootstrapping:

root #emerge --ask dev-lang/kotlin-bin:1.3

The Kotlin 1.3 packages are considered legacy and will not receive the same level of support as the packages for the latest Kotlin feature releases.

Using the upstream's pre-built binaries

If use of the upstream's pre-built binaries is acceptable for some or all of the Kotlin library packages, the binary USE flag can be enabled for them so they will not be built from source. The following example demonstrates how to enable the USE flag for select Kotlin packages:

FILE /etc/portage/package.useUse the pre-built binary for dev-java/kotlin-reflect and dev-java/kotlin-stdlib-js
dev-java/kotlin-reflect binary
dev-java/kotlin-stdlib-js binary


There is not a known way to reproduce a package for kotlin-stdlib-js that is identical to the upstream's pre-built artifact from Portage, and no adequate testing on the ebuild has been conducted yet. Therefore, the dev-java/kotlin-stdlib-js package built from source might have issues or might not work at all. Should any problem arises, please fall back to the binary pre-built by the upstream with the method described above.

For Kotlin/JS support, please install dev-java/kotlin-stdlib-js.

root #emerge --ask dev-java/kotlin-stdlib-js

Installing an old feature release

The steps listed above install the latest version of Kotlin available in the Spark overlay. The overlay might provide packages for multiple feature releases of Kotlin, and an older release can be installed using instructions in this section.

If no versions of dev-java/kotlin-stdlib and dev-java/kotlin-reflect are installed on the system, an older release of the Kotlin compiler can be installed directly by specifying the feature release's version as the slot of dev-lang/kotlin-bin. Bootstrap packages for kotlin-stdlib and kotlin-reflect for the same release will be pulled together, and they can be rebuilt after the Kotlin compiler is installed:

root #emerge --ask dev-lang/kotlin-bin:1.4
root #emerge --ask --oneshot dev-java/kotlin-stdlib:1.4 dev-java/kotlin-reflect:1.4

If any version of dev-java/kotlin-stdlib or dev-java/kotlin-reflect is already installed on the system, the above commands would not work. Portage would attempt to install dev-java/kotlin-stdlib and dev-java/kotlin-reflect for the older feature release directly, but these Kotlin libraries must be re-bootstrapped for the release using the same version of the Kotlin compiler.

root #emerge --ask dev-lang/kotlin-bin:1.4
 * Error: circular dependencies:

(dev-java/kotlin-stdlib-1.4.32:1.4/1.4::spark-overlay, ebuild scheduled for merge) depends on
 (virtual/kotlin-1.4:1.4/1.4::spark-overlay, ebuild scheduled for merge) (buildtime)
  (dev-lang/kotlin-bin-1.4.32:1.4/1.4::spark-overlay, ebuild scheduled for merge) (runtime)
   (dev-java/kotlin-stdlib-1.4.32:1.4/1.4::spark-overlay, ebuild scheduled for merge) (buildtime)

In this case, please explicitly request the bootstrap packages for kotlin-stdlib and kotlin-reflect for the older release to be installed before running the commands above:

root #emerge --ask --oneshot dev-java/kotlin-stdlib-bootstrap:1.4 dev-java/kotlin-reflect-bootstrap:1.4

Managing multiple versions of Kotlin

The slotting of Kotlin packages makes it possible to install and use more than one version of Kotlin on a single system without conflicts. To facilitate use of multiple Kotlin versions, an eselect module for Kotlin, app-eselect/eselect-kotlin, is provided with the Kotlin packages for selecting the default Kotlin version backing the kotlin, kotlinc and kapt tools. For each of those Kotlin tools, versioned executables like kotlinc1.4, kotlinc1.5, etc. are provided too for users who want to use a different Kotlin version without changing the default settings.

Finding the Kotlin tools' version

The option recognized by various Kotlin tools for querying their version is -version.

user $kotlinc -version
info: kotlinc-jvm 1.5.20 (JRE 1.8.0_292-b10)


Similar to the java-vm eselect module, the Kotlin eselect module supports independent Kotlin compiler version selection for both the user and the system. Additionally, if multiple packages of the Kotlin compiler are installed for the same Kotlin feature release, then the Kotlin eselect module provides the functionality to choose the package to use for the feature release.

Getting information for installed Kotlin compiler packages

Like common eselect modules, the list action is used for printing a list of installed Kotlin compilers:

user $eselect kotlin list
Available Kotlin compilers:
  [1]   kotlin-bin-1.4 1.4 user
  [2]   kotlin-1.5 system
  [3]   kotlin-bin-1.5 1.5

Next to each compiler package's name are indicators showing if the package has been chosen as the default compiler for the user, the system, or a Kotlin feature release. The default version selections can be obtained with the show action as well:

user $eselect kotlin show
Current Kotlin compiler for 1.4
Current Kotlin compiler for 1.5
Current Kotlin compiler for system
Current Kotlin compiler for user

Setting a default Kotlin compiler for the user or the system

The set action of the Kotlin eselect module changes the Kotlin compiler preferences. Like most other eselect modules, the target Kotlin compiler can be specified using either its full name or its ordinal in the list returned by eselect kotlin list.

For example, if eselect kotlin list prints the list shown in the previous section, then both of these commands can set the default Kotlin compiler for the user to kotlin-bin-1.5:

user $eselect kotlin set user kotlin-bin-1.5
user $eselect kotlin set user 3

The default Kotlin compiler for the system can be set similarly:

root #eselect kotlin set system kotlin-bin-1.5
root #eselect kotlin set system 3

Setting a default Kotlin compiler for a feature release

The feature described in this section is provisioned for addition of alternative Kotlin compiler packages like dev-lang/kotlin in the future. Since there is only one Kotlin compiler package available currently, this feature does not have any use cases yet.

A default Kotlin compiler package can also be set for each Kotlin feature release. This will link the versioned Kotlin tool executables like kotlin1.5 and kotlinc1.5 to the executables for that package and cause any ebuilds depending on that feature release to use that package for compilation.

To find all Kotlin compiler packages for a specific feature release, specify the feature release number as an argument to eselect kotlin list:

user $eselect kotlin list 1.5
Available Kotlin compilers for Kotlin 1.5:
  [1]   kotlin-1.5 system
  [2]   kotlin-bin-1.5 1.5

The default compiler preference for a feature release can be changed with eselect kotlin set too, but please note that if an ordinal is being used to specify the compiler package, then the ordinal needs to come from the list of compilers for the feature release. For example, the same kotlin-bin-1.5 package's ordinal should be 2 instead of 3:

root #eselect kotlin set 1.5 kotlin-bin-1.5
root #eselect kotlin set 1.5 2

Selecting Kotlin feature release used to build a Kotlin package

Every Kotlin package has a set of KOTLIN_SINGLE_TARGET USE_EXPAND flags that can be used to select the Kotlin feature release used to build it. The format of values in KOTLIN_SINGLE_TARGET is kotlin1-x for Kotlin 1.x.

A global default value of KOTLIN_SINGLE_TARGET is set for all Kotlin packages that support more than one Kotlin feature release. It can be overridden both globally and on a per-package basis in /etc/portage/package.use.

FILE /etc/portage/package.useOverriding KOTLIN_SINGLE_TARGET
# Unset any global default value, then choose Kotlin 1.5 for all Kotlin packages
# Only one value can be set for KOTLIN_SINGLE_TARGET, hence '-*'
*/* KOTLIN_SINGLE_TARGET: -* kotlin1-5

# KOTLIN_SINGLE_TARGET for Kotlin library packages would be affected by the
# above line too, which will prevent Kotlin library packages not for Kotlin
# 1.5 from being installed, so the proper KOTLIN_SINGLE_TARGET value for any
# other Kotlin feature release that is needed must be re-enabled explicitly
dev-java/kotlin-*:1.4 KOTLIN_SINGLE_TARGET: -* kotlin1-4
dev-java/kotlin-*:1.6 KOTLIN_SINGLE_TARGET: -* kotlin1-6

# Per-package settings should be applied after any global settings;
# otherwise, they will be overridden by the global settings

# Use Kotlin 1.4 to build dev-java/okio
dev-java/okio KOTLIN_SINGLE_TARGET: -* kotlin1-4
# Use Kotlin 1.6 to build dev-java/kotlinx-cli
dev-java/kotlinx-cli KOTLIN_SINGLE_TARGET: -* kotlin1-6
After adjusting KOTLIN_SINGLE_TARGET, and before performing a world update, please remember to manually install additional slots of dev-lang/kotlin-bin for every Kotlin release that has been selected for at least one package by following the instructions in #Installing an old feature release (which are also applicable to the case where a newer feature release needs to be installed).

Versioned Kotlin tool executables

To run a Kotlin tool for a specific feature release without changing the preferences via the Kotlin eselect module, the versioned Kotlin tool executables can be used.

user $kotlinc1.4 -version
info: kotlinc-jvm 1.4.32 (JRE 1.8.0_292-b10)
user $kotlinc1.5 -version
info: kotlinc-jvm 1.5.20 (JRE 1.8.0_292-b10)
The versioned executables are exclusive on Gentoo (at least as of now); they are non-standard in the upstream's Kotlin compiler distribution. Therefore, the versioned executables should not be used in programs or scripts that are intended to be run on other GNU/Linux distributions or operating systems.

Tools that are not available

If the default Kotlin compiler package does not provide all Kotlin tools, then calling the executable for an absent tool will result in an error message like the following:

user $kotlinc-js
 * kotlinc-js is not available in /opt/kotlin-bin-1.4/bin

In this case, users may either temporarily use the versioned Kotlin tool executable for a Kotlin feature release that provides the tool (e.g. kotlinc-js1.5) or permanently change the default package to another one that provides kotlinc-js.


Compile and run the most simple "hello, world" program

A simple Kotlin "hello, world" program can be written as follows:

CODE A "hello, world" program for Kotlin
fun main() {
    println("hello, world")

Assuming this program's source file is saved to hello.kt, it can be compiled with kotlinc, the main executable for the Kotlin compiler:

user $kotlinc hello.kt

In this case, the compiled Java class file's name will be HelloKt.class, which can be launched with kotlin:

user $kotlin HelloKt
hello, world

If the Kotlin program does not use any class or method from the Kotlin Standard Library, which is true for this version of "hello, world" for Kotlin, it can also be run with java:

user $java HelloKt
hello, world

Compile and run a "hello, world" program which uses the Kotlin Standard Library

Consider a fancy version of the Kotlin "hello, world" program, which creates a list for all the words in the phrase with the listOf method and generates the phrase from the list with the joinToString method, both of which are members of the Kotlin Standard Library:

CODE A "hello, world" program for Kotlin which uses the Kotlin Standard Library
fun main() {
    val words = listOf("hello", "world")

This program can be compiled with kotlinc and run with kotlin as normal. However, it cannot be run directly with java:

user $kotlinc hello.kt
user $kotlin HelloKt
hello, world
user $java HelloKt
Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/collections/CollectionsKt
	at HelloKt.main(hello.kt:2)
	at HelloKt.main(hello.kt)
Caused by: java.lang.ClassNotFoundException: kotlin.collections.CollectionsKt
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
	... 2 more

This is because kotlin adds the Kotlin Standard Library to the classpath automatically, whereas java does not. The program can still be run with java if the classpath is manually set. The Kotlin Standard Library's classpath can be obtained through the java-config tool. For example, the following command can be used to run the program with java and Kotlin Standard Library 1.5:

user $java -classpath ".:$(java-config -dp kotlin-stdlib-1.5)" HelloKt
hello, world

Compile and run a "hello, world" program which uses additional libraries

Consider an even fancier version of the Kotlin "hello, world" program, which first creates a stream of words using the Stream.of(T...) method from Java, then converts the stream to a list with the kotlin.streams.toList method in kotlin-stdlib-jdk8, and finally joins the elements in the list into a string.

CODE A "hello, world" program for Kotlin which uses methods from kotlin-stdlib-jdk8
import java.util.stream.Stream
import kotlin.streams.toList

fun main() {
    val stream = Stream.of("hello", "world")
    val words = stream.toList()

If this program is compiled directly using kotlinc, there will be errors suggesting that the kotlin.streams.toList method cannot be found:

user $kotlinc hello.kt
hello.kt:2:15: error: unresolved reference: streams
import kotlin.streams.toList
hello.kt:6:24: error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
public inline fun <T> Enumeration<TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin.collections
public fun <T> Array<out TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin.collections
public fun BooleanArray.toList(): List<Boolean> defined in kotlin.collections
public fun ByteArray.toList(): List<Byte> defined in kotlin.collections
public fun CharArray.toList(): List<Char> defined in kotlin.collections
public fun CharSequence.toList(): List<Char> defined in kotlin.text
public fun DoubleArray.toList(): List<Double> defined in kotlin.collections
public fun FloatArray.toList(): List<Float> defined in kotlin.collections
public fun IntArray.toList(): List<Int> defined in kotlin.collections
public fun LongArray.toList(): List<Long> defined in kotlin.collections
public fun <T> Pair<TypeVariable(T), TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin
public fun ShortArray.toList(): List<Short> defined in kotlin.collections
public fun <T> Triple<TypeVariable(T), TypeVariable(T), TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin
public fun <T> Iterable<TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin.collections
public fun <K, V> Map<out TypeVariable(K), TypeVariable(V)>.toList(): List<Pair<TypeVariable(K), TypeVariable(V)>> defined in kotlin.collections
public fun <T> Sequence<TypeVariable(T)>.toList(): List<TypeVariable(T)> defined in kotlin.sequences
    var words = stream.toList()

This is caused by kotlinc not automatically including kotlin-stdlib-jdk8.jar in the classpath. Again, it can be added manually:

user $kotlinc -classpath "$(java-config -dp kotlin-stdlib-jdk8-1.5)" hello.kt
user $kotlin -classpath ".:$(java-config -dp kotlin-stdlib-jdk8-1.5)" HelloKt
hello, world

Compile and run a Java program which uses the Kotlin Standard Library

Kotlin code can be called from Java programs, so it is possible to use Kotlin libraries within Java. For example, the following program is the Java equivalent for the Kotlin "hello, world" program which uses the Kotlin Standard Library shown in a previous section.

CODE A "hello, world" program for Java which uses the Kotlin Standard Library
import java.util.List;

import static kotlin.collections.CollectionsKt.listOf;
import static kotlin.collections.CollectionsKt.joinToString;

public final class Hello {
    private Hello() {

    public static void main(String[] args) {
        List<String> words = listOf("hello", "world");
        System.out.println(joinToString(words, ", ", "", "", -1, "...", null));

This program can be compiled with javac and run with java, provided that the Kotlin Standard Library is in the classpath:

user $javac -classpath "$(java-config -dp kotlin-stdlib-1.5)" Hello.java
user $java -classpath ".:$(java-config -dp kotlin-stdlib-1.5)" Hello
hello, world

Compile and run programs with libraries for a newer feature release

The Kotlin compiler will emit a warning if the classpath contains a library for a newer feature release. For example, the following messages would given by Kotlin compiler 1.4 when Kotlin Standard Library 1.5 is used:

user $kotlinc1.4 -classpath "$(java-config -dp kotlin-stdlib-1.5)" hello.kt
warning: runtime JAR files in the classpath should have the same version. These files were found in the classpath:
    /usr/share/kotlin-stdlib-1.5/lib/kotlin-stdlib.jar (version 1.5)
    /opt/kotlin-bin/lib/kotlin-stdlib.jar (version 1.4)
    /opt/kotlin-bin/lib/kotlin-script-runtime.jar (version 1.4)
    /opt/kotlin-bin/lib/kotlin-reflect.jar (version 1.4)
warning: consider providing an explicit dependency on kotlin-reflect 1.5 to prevent strange errors
warning: some runtime JAR files in the classpath have an incompatible version. Consider removing them from the classpath

The warning can be eliminated by not letting kotlinc include kotlin-stdlib.jar and kotlin-reflect.jar it depends on in the classpath for compilation. This is controlled by the -no-stdlib option.

user $kotlinc1.4 -no-stdlib -classpath "$(java-config -dp kotlin-stdlib-1.5)" hello.kt

Compile and run programs with libraries for an older feature release

If libraries for a feature release older than the compiler's version is used, then kotlinc will emit an additional warning even if the -no-stdlib option is enabled. The following warning would appear when Kotlin compiler 1.5 is used with Kotlin Standard Library 1.4:

user $kotlinc1.5 -no-stdlib -classpath "$(java-config -dp kotlin-stdlib-1.4)" hello.kt
warning: runtime JAR files in the classpath have the version 1.4, which is older than the API version 1.5. Consider using the runtime of version 1.5, or pass '-api-version 1.4' explicitly to restrict the available APIs to the runtime of version 1.4. You can also pass '-language-version 1.4' instead, which will restrict not only the APIs to the specified version, but also the language features
/usr/share/kotlin-stdlib-1.4/lib/kotlin-stdlib.jar: warning: runtime JAR file has version 1.4 which is older than required for API version 1.5

Specifying the library's version with the -api-version option, as instructed by the warning, is sufficient to eliminate it.

user $kotlinc1.5 -api-version 1.4 -no-stdlib -classpath "$(java-config -dp kotlin-stdlib-1.4)" hello.kt

Version upgrade

New incremental or bug fix release

All small updates that do not involve migration to a new feature release should be installable with the normal system update method without any issues.

New feature release

To upgrade to a newer Kotlin feature release, kotlin-stdlib and kotlin-reflect must be re-bootstrapped before the remaining components for the new release can be updated. This is necessary because the upstream bootstraps each new feature release with an RC version of that release instead of the latest version in the previous feature release series. For example, Kotlin 1.5.0 is bootstrapped with version 1.5.0-RC-556[3] instead of 1.4.32.

root #emerge --ask --oneshot dev-java/kotlin-stdlib-bootstrap dev-java/kotlin-reflect-bootstrap
root #emerge --ask --update --deep --newuse @world
root #emerge --ask --oneshot dev-java/kotlin-stdlib dev-java/kotlin-reflect
root #emerge --ask --depclean

Staying on a feature release

Users who installed the Kotlin packages without specifying the slot but would like to stay on the installed feature release of Kotlin can re-add dev-lang/kotlin-bin with a slot to the "world" favorites file. If the current installed feature release is 1.4, then please run the following commands:

root #emerge --deselect dev-lang/kotlin-bin
root #emerge --ask --noreplace dev-lang/kotlin-bin:1.4

See also

External resources


  1. JetBrains. Compatibility guide for Kotlin 1.5, Kotlin docs, July 7th, 2021. Retrieved on July 7th, 2021.
  2. Yuan Liao. dev-lang/kotlin-bin-1.5.20: Add new JAR to compiler library list (Git commit), July 4th, 2021. Retrieved on July 7th, 2021.
  3. gradle.properties, JetBrains/kotlin GitHub repository, April 4th, 2021. Retrieved on July 7th, 2021.