What AnyRefs are not nullable?

[Haoyi Li]

This:

abstract class Reference[T <: AnyRef](var referent: T){

  def get: T = referent

  def clear: Unit = referent = null

}

Fails with

[error]  type mismatch;

[error]  found   : Null(null)

[error]  required: T

[error]   def clear: Unit = referent = null

This works though:

abstract class Reference[T >: Null <: AnyRef](var referent: T){

  def get: T = referent

  def clear: Unit = referent = null

}

Can someone explain what's going on here? If I have already bound T to be an AnyRef, how could it be non-nullable? Furthermore, why must I bound T to AnyRef in the first place? Aren't generics erased unless i @specialize them? 

Source: I'm trying to translate this code to ScalaJS from Java, where everything works fine without any weird type bound trickery.

[Som Snytt]

Nothing is not Null, and AnyVal is not AnyRef.

Is that the question? Kind of late here.

Maybe you're thinking everything is boxed. But that's under the hood, or under the carpet.

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[Jason Zaugg]

On Fri, Jan 24, 2014 at 9:03 AM, Haoyi Li <haoy...@gmail.com> wrote:

Can someone explain what's going on here? If I have already bound T to be an AnyRef, how could it be non-nullable? Furthermore, why must I bound T to AnyRef in the first place? Aren't generics erased unless i @specialize them? 

Source: I'm trying to translate this code to ScalaJS from Java, where everything works fine without any weird type bound trickery.

If A is an abstract type, `expr : A` will only typecheck if `expr` conforms to the lower bound of `A`. The default lower bound is `Nothing`. `Null` is not a subtype of `Nothing`.

If we allowed that:

scala> def foo[A <: AnyRef]: A = null.asInstanceOf[A]

foo: [A <: AnyRef]=> A

scala> {foo[Nothing]; ()}

-jason

[martin odersky]

Only classes and traits that extend AnyRef are considered nullable. Abstract types and type parameters are not; they need an explicit >: Null bound. 

That's so that we can at some point introduce non-null types which might still inherit from AnyRef. 

Hope this helps

 - Martin

On Fri, Jan 24, 2014 at 9:03 AM, Haoyi Li <haoy...@gmail.com> wrote:

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[Haoyi Li]

> That's so that we can at some point introduce non-null types which might still inherit from AnyRef. 

Ok, that makes sense. Thanks =)