創建Anyval選項的另一個嘗試

Question

我試圖實現一個不佔用額外內存的選項。 我創建了一個類。空值表示無，非空值表示某些。

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") 
} 

但是當我嘗試運行它，我得到一個NPE：

嘗試創建Maybe.none

開始初始化沒有

初始化

異常在線程 「主」 顯示java.lang.NullPointerException在 com.MaybeTest $ $ .delayedEndpoint COM $ MaybeTest $ 1（Maybe.scala：34） 在 com.MaybeTest $ $ delayedInit body.apply（也許。 scala：30） at scala.Function0 $ class.apply $ mcV $ sp（Function0.scala：34）at scala.runtime.AbstractFunction0.apply $ mcV $ sp（AbstractFunction0.scala：12） at scala.App $ $ $ anonfun主$ 1.適用（App.scala：76）在 scala.App $$ anonfun $ $主1.適用（App.scala：76）在 scala.collection.immutable.List.foreach（List.scala： 392）at scala.collection.generic.TraversableForwarder $ class.foreach（TraversableForwarder.scala：35） at sc ala.App $ class.main（App.scala：76）在 com.MaybeTest $。主要（Maybe.scala：30） com.MaybeTest.main（Maybe.scala）

如果我刪除'擴展AnyVal'一切正常。 任何人都可以解釋這種行爲嗎？

Answer 1

@碧玉-M的評論解釋了爲什麼你的解決方案不能正常工作，但這是接近：

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]) 
} 

但它是不是真的「的Option不消耗額外的內存的包裝」 ：它只是以另一個名字叫null。例如。 Option[Option[A]]按預期工作，但Maybe[Maybe[A]]沒有（並且編譯器不會告訴你！）。