Question

Scala’s Option class has an orNull method, whose signature is shown below.

orNull [A1 >: A](implicit ev : <:<[Null, A1]) : A1

I’m bewildered by the implicit thing. Would somebody please explain how it can be used, ideally with an example?

Answer 1

scala> Some(1).orNull
<console>:10: error: could not find implicit value for parameter ev: <:<[Null,Int]
       Some(1).orNull
               ^
scala> (None : Option[Int]).orNull
<console>:10: error: could not find implicit value for parameter ev: <:<[Null,Int]
       (None : Option[Int]).orNull

scala> Some("hi").orNull
res21: java.lang.String = hi

scala> Some(null : String).orNull
res22: String = null

scala> (None : Option[String]).orNull
res23: String = null

To explain the implicit thing: orNull is a way of getting back from the Some|None idiom to Java’s value|null idiom (which is, of course, bad). Now only AnyRef values (instances of classes) can accept a null value.

So what we would have liked is def orNull[A >: Null] = ..... But A is already set and we don’t want to restrict it in the definition of the trait. Therefore, orNull expects an evidence that A is a nullable type. This evidence is in the form of an implicit variable (hence the name ‘ev’)

<:<[Null, A1] can be written as Null <:< A1 seeing it like this, it is similar to ‘Null <: A1’. <:< is defined in Predef as well as the method that provides the implicit value named conforms.

I think the use of A1 is not strictly required here and is because orNull uses getOrElse (where the default given can be a super type of A)

scala> class Wrapper[A](option: Option[A]) {
     | def orNull(implicit ev: Null <:< A): A = if(option.isEmpty) null else option.get
     | }
defined class Wrapper

scala> new Wrapper(Some("hi")).orNull
res18: java.lang.String = hi