Scala 2.12: What is the equivalent of a Java 8 method reference for a universally quantified SAM characteristic?

Question

My goal is to implement the singleton value of algebraic data types (church-encoded) using the new scala 2.12 support for the SAM feature (single abstract method).

In Java, the following program returns true:

import java.util.function.Function;
import java.util.function.Supplier;

@FunctionalInterface
public interface Maybe<A> {

  <X> X fold(Supplier<X> empty, Function<A, X> just);

  static <A, X> X empty0(Supplier<X> empty, Function<A, X> just) {
    return empty.get();
  }

  static <A> Maybe<A> empty() {
      return Maybe::empty0;
  }

  static void main(String[] args) {
    Maybe<?> emptyString = Maybe.<String>empty();
    Maybe<?> emptyInt = Maybe.<Integer>empty();

    System.out.println(emptyString == emptyInt); // print "true".
  }
}

I tried porting this encoding to scala 2.12, but it does not compile:

@FunctionalInterface
trait Maybe[A] {
  def fold[X](empty: => X, just: A => X): X
}

object Maybe {
  def empty0[A, X](empty: => X, just: A => X): X = empty

  def empty[A]: Maybe[A] = empty0(_ ,_) // does not compile

  def main(args: Array[String]): Unit = {
    val emptyString: Maybe[String] = Maybe.empty
    val emptyInt: Maybe[Integer] = Maybe.empty

    print(emptyString eq emptyInt) // how to make this print "true"???
  }
}

The error I am getting is: missing parameter type for expanded function ((x$1: <error>, x$2: <error>) => empty0(x$1, x$2))

My goal is to get a scalar run of the same optimization performed by Javac, which makes the java print "true" program. I am open to what is needed to meet scalac requirements if it does not use annotation asInstanceOfand Nothing/ variance.

EDIT. , scala ( !).

Answer 1

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