Question

我正在尝试实现一个不会为包装器消耗额外内存的Option。我创建了一个类。 Null代表None，非null值代表Some。

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")
}

但是当我尝试运行它时，我得到了一个NPE：

尝试创建Maybe.none

开始初始化无

1

2

初始化

线程“main”中的异常java.lang.NullPointerException at   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）at   scala.runtime.AbstractFunction0.apply $ MCV $ SP（AbstractFunction0.scala：12）   在scala.App $$ anonfun $ main $ 1.apply（App.scala：76）at   scala.App $$ anonfun $ main $ 1.apply（App.scala：76）at   scala.collection.immutable.List.foreach（List.scala：392）at   scala.collection.generic.TraversableForwarder $ class.foreach（TraversableForwarder.scala：35）   在scala.App $ class.main（App.scala：76）at   com.MaybeTest $ .main（Maybe.scala：30）at   com.MaybeTest.main（Maybe.scala）

如果我删除'extends AnyVal'一切正常。任何人都可以解释这种行为吗？

Answer 1

@Jasper-M的评论解释了为什么你的解决方案不起作用，但这很接近：

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])
}

但它并不是真正的“Option，它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。 Option[Option[A]]按预期工作，但Maybe[Maybe[A]]没有（并且编译器不会告诉你！）。

另一种创建Anyval Option的尝试

1 个答案: