Starlark interoperation to JVM

demo.scala

package example

import com.google.common.collect.{ImmutableCollection, ImmutableList, ImmutableMap}
import net.starlark.java.annot.{Param, StarlarkBuiltin, StarlarkMethod}
import net.starlark.java.eval.{Dict, EvalException, Module, Mutability, Starlark, StarlarkFunction, StarlarkInt, StarlarkList, StarlarkSemantics, StarlarkThread, Structure, Tuple}
import net.starlark.java.syntax.{FileOptions, ParserInput}

import scala.collection.mutable.ListBuffer

object StarlarkDemo extends App {
  if (args.length < 1) {
    println("run BUILD.example")
    sys.exit(1)
  }
  val fileName = args(0)
  val input = ParserInput.readFile(fileName)
  val buf = ListBuffer.empty[Definition]
  val env = makeEnvironment(buf)
  val module = Module.withPredeclared(StarlarkSemantics.DEFAULT, env)
  val ops = Module.create()
  module.setGlobal("ops", ops)
  withMutability(input.getFile) { mu =>
    val thread = new StarlarkThread(mu, StarlarkSemantics.DEFAULT)
    Starlark.execFile(input, FileOptions.DEFAULT, module, thread)
    val isAdult = module.getGlobal("is_adult")
    println(isAdult)
    println(module.getGlobal("recall_op"))
    println(module.getGlobal("ops"))

    val user = new User("a", 1)
    val userAttrs = Starlark.dir(mu, StarlarkSemantics.DEFAULT, user)
    println(s"dir(user) = $userAttrs")
    val age = Starlark.getattr(mu, StarlarkSemantics.DEFAULT, user, "age", null)
    println(s"user.age = $age")
    println(Starlark.call(thread, isAdult, java.util.List.of(user), java.util.Map.of()))
  }
  val defs = buf.toList
  println(defs.mkString("\n"))


  // close mutability to freeze the values
  def withMutability(fileName: String)(f: Mutability => Unit): Unit = {
    val mu = Mutability.create(fileName)
    try {
      f(mu)
    } finally {
      mu.close()
    }
  }

  def makeEnvironment(buf: ListBuffer[Definition]): ImmutableMap[String, AnyRef] = {
    val env = ImmutableMap.builder[String, AnyRef]()
    Starlark.addMethods(env, new Functions(buf), StarlarkSemantics.DEFAULT)
    env.build()
  }
}

@StarlarkBuiltin(name = "User")
class User(name: String, age: Int) {
  @StarlarkMethod(name = "name", structField = true)
  def getName() = name
  @StarlarkMethod(name = "age", structField = true)
  def getAge() = age
}


class Functions(buf: ListBuffer[Definition]) {
  
  @StarlarkMethod(
    name = "op",
    parameters = Array(
      new Param(name = "name", named = true),
      new Param(name = "args", named = true, defaultValue = "{}")
    ),
    doc = "Define an operation."
  )
  def op(name: String, args: Dict[String, Object]): String = {
    println(s"Define operation: $name $args")
    name
  }
}

predicate.star

def is_adult(user):
    return user.age >= 18