java - 斯卡拉 2.12 : What is the equivalent of Java 8 method reference for universally quantified SAM trait?

Question

我的目标是通过新的 Scala 2.12 支持 SAM(单一抽象方法)特征来实现代数数据类型(教会编码)的单一值。

在 Java 中，以下程序返回 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".
  }
}

我尝试将此编码移植到 scala 2.12，但它无法编译:

@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"???
  }
}

我得到的错误是: missing parameter type for expanded function ((x$1: <error>, x$2: <error>) => empty0(x$1, x$2))

我的目标是让 scalac 触发 Javac 完成的相同优化，使 java 程序打印“true”。只要不使用 asInstanceOf，我愿意接受满足 scalac 所需的任何东西。也不Nothing/方差注释。

编辑:由于目前不支持，我打开了一个 feature request on the scala issue tracker为此(请投票!;-)。

Answer 1

很遗憾，根据 the specification 不允许这样做:

It follows that:

• if class C defines a constructor, it must be accessible and must define exactly one, empty, argument list;

• m cannot be polymorphic;

• it must be possible to derive a fully-defined type U from S by inferring any unknown type parameters of C.