CODEWITHSUNDEEP
scala
Dave Cahill
Dave Cahill

Reputation: 448

Why can I assign null to an Option?

The following are valid statements in Scala:

scala> var x: Option[Int] = Some(3)
x: Option[Int] = Some(3)

scala> var x: Option[Int] = None
x: Option[Int] = None

The following is invalid:

scala> var x: Option[Int] = 3
<console>:7: error: type mismatch;
 found   : Int(3)
 required: Option[Int]
       var x: Option[Int] = 3

So far these examples make sense to me; a value of type Option[T] can be either of type Some[T] or None, so the compiler prevents you from assigning a value which is of neither type.

However, the Scala compiler appears to accept this:

scala> val x: Option[Int] = null
x: Option[Int] = null

If I then try to do a pattern match on the option (e.g. as below), I'll get failures I didn't expect - why doesn't the compiler protect me from this by rejecting the assignment of null?

x match {
  case Some(y) => println("Number: ", y)
  case None => println("No number")
}

Upvotes: 0

Views: 1499

Answers (2)

Mark Lister
Mark Lister

Reputation: 1143

The answer to the question you probably intended to ask goes something like this:

Idiomatic Scala will never use null. Null references exist purely for interoperability with Java and other JVM languages. If you see null in Scala code it should immediately attract scrutiny. You would never use null by mistake so a compiler warning is unnecessary. As noted in Daniel Sobral's answer and the comments the compiler cannot provide protection against null.

Upvotes: 0

Daniel C. Sobral
Daniel C. Sobral

Reputation: 297195

If you look at the Scala class hierarchy, you'll see that all classes deriving from AnyRef are super classes of Null, and any such super class can be assigned the value null. Since Option is one such class, you can assign Null to it.

Note that both Some and None.type (that is, the singleton type of the None object) are super classes of Null, so null is a valid value for either one.

You cannot assign 3 to Option because 3 is not a value of a sub class of Option (evidently).

Upvotes: 7

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