Scala3 type bug when used with java

[divyanshu-labs]

In a new Scala3 types system, if I write two simple classes in Scala as follows

final class ScalaClass(val in: "Hello") {

  override def toString: String = s"ScalaClass($in)"

  def printAllCharacters(): Unit = in.foreach(println)
}

&

final class ScalaClass1(private var in: "Hello") {
  def getIn: "Hello" = in

  def setIn(in1: "Hello"): Unit = {
    in = in1
  }

  def printAllCharacters(): Unit = in.foreach(println)

  override def toString: String = s"ScalaClass1($in)"
}

And then use them in Java as well as in Scala would produce different output, for example:

public class JavaClass {

    public static void main(String[] args) {
        ScalaClass scalaClass = new ScalaClass("jhjhjhjh");
        System.out.println(scalaClass);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>");
        ScalaClass1 scalaClass1 = new ScalaClass1("sasas");
        System.out.println(scalaClass1);
        System.out.println(scalaClass1.getIn());
        scalaClass1.setIn("asasasas");
        System.out.println(scalaClass1.getIn());
        System.out.println(scalaClass1);
    }
}

Would will produce output as:

ScalaClass(Hello)
>>>>>>>>>>>>>>>>>>>>>>>
ScalaClass1(sasas)
sasas
Hello
ScalaClass1(Hello)

And running the same in Scala would be something different:

@main def runScalaClass(): Unit = {
  //  val a = ScalaClass("As") Compilation Error
  val a = ScalaClass("Hello")
  println(a)
}

&

@main def runScalaMain1(): Unit = {

  //  val a = new ScalaClass1("asas") Compilation Error
  val a = new ScalaClass1("Hello")
  println(a)
  println(a.getIn)
  //  a.setIn("sss") Compilation Error
  a.setIn("Hello")
  println(a)
}

So the same simple API works differently for Java as well as in Scala.

• What if someone needs to write simple Scala classes that are interoperable with Java?
• In this case, the same code should produce identical results when used with Java and Scala, which is not the case!
• Such a typing system makes the project more prone to developer bugs in projects that contain code that combines Java and Scala.
• Hard to understand, for example, someone reading the code would see a simple Scala class with getter and setter but when run through java: getter, setter, and constructor all three produce unexpected results, resulting in confusion between what is written and what is output.

[divyanshu-labs]

The above two classes ScalaClass & ScalaClass1 would look something like this

import java.lang.invoke.SerializedLambda;
import scala.Function1;
import scala.Predef$;
import scala.collection.StringOps$;

public final class ScalaClass
{
    private final String in;
    
    public ScalaClass(final String in) {
        this.in = in;
    }
    
    public String in() {
        return this.in;
    }
    
    @Override
    public String toString() {
        return new StringBuilder(17).append("ScalaClass(").append("Hello").append(")").toString();
    }
    
    public void printAllCharacters() {
        StringOps$.MODULE$.foreach$extension(Predef$.MODULE$.augmentString("Hello"), (Function1)(x -> Predef$.MODULE$.println(x)));
    }
}

&

import java.lang.invoke.SerializedLambda;
import scala.Function1;
import scala.Predef$;
import scala.collection.StringOps$;

public final class ScalaClass1
{
    private String in;
    
    public ScalaClass1(final String in) {
        this.in = in;
    }
    
    private String in() {
        return this.in;
    }
    
    private void in_$eq(final String x$0) {
        this.in = x$0;
    }
    
    public String getIn() {
        return this.in();
    }
    
    public void setIn(final String in1) {
        this.in_$eq("Hello");
    }
    
    public void printAllCharacters() {
        StringOps$.MODULE$.foreach$extension(Predef$.MODULE$.augmentString(this.in()), (Function1)(x -> Predef$.MODULE$.println(x)));
    }
    
    @Override
    public String toString() {
        return new StringBuilder(13).append("ScalaClass1(").append(this.in()).append(")").toString();
    }
}

[divyanshu-labs]

What would happen if instead of using constants like:

public void setIn(final String in1) {
        this.in_$eq("Hello");
}

we use the variable instead

public void setIn(final String in1) {
        this.in_$eq(in1);
}

or produce runtime exception/compile time exception when compiling java/scala code combinedly.

[som-snytt]

A Scalafix rule could lint features not supported in Java or known interop issues.

At a glance, I don't see that such a rule has been written at https://scalacenter.github.io/scalafix/docs/rules/community-rules.html

[divyanshu-labs]

This fix rule in indeed required to reduce programmer error.

Thank you