Run a Ruby function from Scala and get the result.

demo.rb

#!ruby

def entrypoint(arg)
  "I received #{arg}"
end

dependencies.scala

libraryDependencies ++= Seq(
  "org.jruby" % "jruby" % "9.2.7.0",
)

RubyRunner.scala

import org.jruby.embed.ScriptingContainer

object RubyRunner {
  def rubyScript = io.Source.fromFile("demo.rb").mkString

  def main(args: Array[String]): Unit = {
    val result = runScript(rubyScript, "hello world")
    println(s"Ruby returned: $result")
  }

  // this runs a Ruby script, calling into a method called "entrypoint" with one argument.
  def runScript(script: String, args: String): String = {
    // Boot up a scripting container:
    val container = new ScriptingContainer()

    // Load the script from file into Ruby interpreter:
    container.runScriptlet(script)

    // Call a method in Ruby with an argument, and expect a string reply:
    // receiver is Ruby class; null = top-level self
    container.callMethod[String](null, "entrypoint", args, classOf[String])
  }

}