how can I interop kotlin code in an existing SBT scala project - scala

Question

I have an existing scala project using SBT which has several modules.
I'd like to start adding new modules in kotlin - I don't require the ability to add kotlin to existing modules (but it could be nice if possible)
I can create new dedicated modules for the new kotlin code if that is a necessity as long as it is possible for the existing scala code to call out to the newly added kotlin modules (visa versa is nice to have but can live without that "kotlin calling scala" if impossible to do that)
Is this feasible and practical thing to do? If possible, how would this be done?
.
├── build.sbt
............
├── Module1ScalaWithJava (EXISTING)
│   ├── src
│   │   ├── main
│   │   │   ├── java
│   │   │   ├── resources
│   │   │   └── scala
├── Module2ScalaOnly (EXISTING)
│   ├── src
│   │   ├── main
│   │   │   └── scala
│   │   └── test
│   │   └── scala
├── NewModuleKotlinOnly (I WANT THIS)
│   ├── src
│   │   ├── main
│   │   │   └── ???KOTLIN????

Answer 1

As mentioned in comments, you can add kotlin module using kotlin-plugin.
Add line to your project/plugins.sbt file (or create it):
addSbtPlugin("com.hanhuy.sbt" % "kotlin-plugin" % "2.0.0")
And you will be able to add kotlin modules to your sbt-project. I would advice you using only build.sbt file for adding modules. I will demonstrate you how to do that below.
I created simple multi-module project with scala and kotlin modules which are depends on each other.
Here my build.sbt:
name := "kotlin-scala"
version := "0.1"
scalaVersion := "2.13.4"
lazy val scalaFirst =
project
.in(file("scala-first"))
lazy val kotlinFirst =
project
.in(file("kotlin-first"))
.settings(
libraryDependencies ++= Seq(
"org.junit.jupiter" % "junit-jupiter-api" % "5.7.0"
).map(_ % Test)
)
lazy val scalaSecond =
project
.in(file("scala-second"))
.dependsOn(kotlinFirst % "compile->compile;test->test")
.settings(
libraryDependencies ++= Seq(
"org.scalatest" %% "scalatest" % "3.2.3"
).map(_ % Test)
)
lazy val kotlinSecond =
project
.in(file("kotlin-second"))
.dependsOn(scalaFirst % "compile->compile;test->test")
.settings(
libraryDependencies ++= Seq(
"org.junit.jupiter" % "junit-jupiter-api" % "5.7.0"
).map(_ % Test)
)
lazy val kotlinScalaSubmodule =
project
.in(file("kotlin-scala-submodule"))
.dependsOn(kotlinFirst % "compile->compile;test->test")
.dependsOn(scalaFirst % "compile->compile;test->test")
.settings(
libraryDependencies ++= Seq(
"org.junit.jupiter" % "junit-jupiter-api" % "5.7.0"
).map(_ % Test)
)
lazy val scalaKotlinSubmodule =
project
.in(file("scala-kotlin-submodule"))
.dependsOn(scalaFirst % "compile->compile;test->test")
.dependsOn(kotlinFirst % "compile->compile;test->test")
.settings(
libraryDependencies ++= Seq(
"org.scalatest" %% "scalatest" % "3.2.3"
).map(_ % Test)
)
build.properties contains:
sbt.version = 1.3.2
My project structure:
Here I have some dependencies between modules:
+---------------+ +--------------+
| kotlin-first | | scala-first |
+---------------+ +--------------+
^ \ / ^
| \/ |
+--------------+ || +---------------+
| scala-second | || | kotlin-second |
+--------------+ || +---------------+
||
/ \
+------------------+ +------------------+
| scala-kotlin-sub | | kotlin-scala-sub |
+------------------+ +------------------+
full project you can find on github
Also I write just some unit-tests to demonstrate that it works properly.
Tested on:
java open-jdk 1.8,
scala version 2.13.4.
sbt version 1.3.2
Intellij IDEA build 2020.2.3 with jetBrains scala plugin works properly with this project.