Why does map/filter … not work with an Array of Nothing?

Question

Isn\'t Nothing a subtype of all types?

scala> val array = new Array(5)
array: Array[Nothing] = Array(null, null, null, null, null)

scala> array.map(_          


        

Answer 1

I suspect Scala shouldn't let you do that kind of Array[Nothing] instantiation. There are by definition no instances of nothing around, yet your array looks like it's filled with Nothings that are null, but null is not a valid value for Nothing. This for instance fails with the error type mismatch; found : Null(null) required: Nothing

val n: Nothing = null

So I'd expect to run into trouble each time you can actually fool the system to believe you are finally getting hold of a much sought for instance of Nothing…

Here's another weird case. Run this:

object Main {

  class Parametrized[T] { var value: T = _ }

  def main(args: Array[String]) {
    val p = new Parametrized // typed as Parametrized[Nothing]
    val n = p.value  // n is now actually an instance of Nothing... isn't it?
    println(p.value) // prints null, but null is not an instance of Nothing
    println(n)       // throws NullPointerException...
  }

}