Yet another attempt of creating Anyval Option

Question

I'm trying to implement an Option which doesn't consume extra memory for a wrapper. I create a class. Null stands for None, non-null value stands for 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")
}

But when I try to run it I get an NPE:

try to create Maybe.none

start initializing none

1

2

Initialized

Exception in thread "main" java.lang.NullPointerException at com.MaybeTest$.delayedEndpoint$com$MaybeTest$1(Maybe.scala:34) at com.MaybeTest$delayedInit$body.apply(Maybe.scala:30) at scala.Function0$class.apply$mcV$sp(Function0.scala:34) at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12) at scala.App$$anonfun$main$1.apply(App.scala:76) at scala.App$$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) at scala.App$class.main(App.scala:76) at com.MaybeTest$.main(Maybe.scala:30) at com.MaybeTest.main(Maybe.scala)

If I remove 'extends AnyVal' everything works fine. Could anyone explain such behavior?

So... you want to introduce a null... which is more of a runtime entity and can not be distinguished at compile time into your "supposedly null-free" ecosystem. And then you are even working hard to do it. Well Done. — sarveshseri
Scala represent a Maybe[Int] at runtime as an Int. Scala's type system mandates that Nothing is inhabited by zero values, not even null. Yet it will find a null at runtime to represent your Maybe[Nothing]. Strange things will happen. — Jasper-M
@Jasper-M make sense, thanks. Does a better implementation exist? — simpadjo

Alexey Romanov Alexey Romanov · Accepted Answer · 2017-04-27T12:55:56

@Jasper-M's comment explains why your solution can't work, but this is close:

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])
}

But it isn't really "an Option which doesn't consume extra memory for a wrapper": it's just calling null by another name. E.g. Option[Option[A]] works as expected, but Maybe[Maybe[A]] doesn't (and the compiler won't tell you!).

Yet another attempt of creating Anyval Option

1 Answers