创建Anyval选项的又一次尝试

Question

我试图实现一个不为包装器消耗额外内存的选项。我创建一个类。Null表示无，非Null值表示有。

class Maybe[+T](private val nullable: T) extends AnyVal {
  @inline final def isDefined: Boolean = {
    println("isDefined called")
    val res = nullable != null
    println(s"result = $res")
    res
  }
  @inline final def get: T = if (isDefined) nullable else throw new NoSuchElementException
  @inline final def getOrElse[B >: T](default: => B): B = if (isDefined) this.get else default
  @inline final def map[B](f: T => B) : Maybe[B] = if (isDefined) Maybe.some(f(this.get)) else Maybe.none
  @inline final def flatMap[B](f: T => Maybe[B]): Maybe[B] = if (!isDefined) Maybe.none else f(get)
  def toOption : Option[T] = if (isDefined) Some(get) else None
}

object Maybe {
  def some[T](value:T) : Maybe[T] = new Maybe(value)

  final val none : Maybe[Nothing] = {
    println("start initializing none")
    val v = new Maybe(null)
    println("1")
    val res = v.asInstanceOf[Maybe[Nothing]]
    println("2")
    res
  }
}

object MaybeTest extends App {
  println("try to create Maybe.none")
  val myNone = Maybe.none
  println("Initialized")
  println(myNone.isDefined)
  println("Accessed")
}

2

已初始化

在com.maybetest$.delayedendpoint$com$maybetest$1(Maybe.scala:34)在com.maybetest$delayedinit$body.apply(maybe.scala:30)在scala.function0$class.apply$mcv$sp(function0.scala:34)在scala.runtime.abstractfunction0.apply$mcv$sp(abstractfunction0.12)在scala.app$$anonfunt$main$1.apply(app.scala:76)(list.scala:392)在scala.collection.generic.traversableForwarder$class.foreach（traversableForwarder.scala:35)在scala.app$class.main（app.scala:76)在com.maybetest$.main（maybe.scala:30)在com.maybetest.main（maybe.scala）

如果我删除“扩展anyval”，一切都很好。有人能解释这样的行为吗？

Answer 1

@Jasper-M的评论解释了为什么你的解决方案不能工作，但这是接近的：

class Maybe[+T] private (private val nullable: Object) extends AnyVal {
  @inline private final def get0 = nullable.asInstanceOf[T]
  @inline final def isDefined: Boolean = nullable != null
  @inline final def get: T = if (isDefined) get0 else throw new NoSuchElementException
  @inline final def getOrElse[B >: T](default: => B): B = if (isDefined) get0 else default
  @inline final def map[B](f: T => B) : Maybe[B] = if (isDefined) Maybe.some(f(get0)) else Maybe.none
  @inline final def flatMap[B](f: T => Maybe[B]): Maybe[B] = if (!isDefined) Maybe.none else f(get0)
  @inline final def toOption: Option[T] = if (isDefined) Some(get0) else None
}

object Maybe {
  def some[A](value: A) : Maybe[A] = {
    val value1 = value.asInstanceOf[Object]
    assert(value1 != null)
    new Maybe[A](value1)
  }

  private val _none = new Maybe[Null](null)
  def none[A]: Maybe[A] = _none.asInstanceOf[Maybe[A]]

  def apply[A](value: A) = new Maybe[A](value.asInstanceOf[Object])
}

但它实际上并不是“一个选项，不会为包装器消耗额外的内存”：它只是用另一个名称调用null。例如。选项[option[A]]按预期工作，但可能[maybe[A]]不工作（编译器不会告诉您！）。