Question

The scala type Nothing represents (as I understand it) the bottom of the type hierarchy, also denoted by the symbol ⊥. That is, Nothing is a sub-type of any given type. The requirement for a Nothing type is explained well by James Iry for those of us without a theoretical background in type theory!

So my question is, if Nothing is a subtype of every type, why can I not call any type s methods on Nothing? Obviously, I cannot instantiate Nothing but why doesn t the following compile?

var n: Nothing = _

def main(args: Array[String]) {
  println(n.length) //compile error: value length is not a member of Nothing
}

Surely as Nothing is a subtype of String this should be OK? Note that the following compiles just fine!

var n: Nothing = _

def foo(s: String) : Int =  s.length

def main(args: Array[String]) {
  println(foo(n))
}

as does:

def main(args: Array[String]) {
  println(n.asInstanceOf[String].length) 
}

Answer 1

While Nothing is a subtype of everything, it does not inherit any method except for those in Any. This is because Nothing is more geared toward the functional end of the language. It s necessary for things like Option and List, but only as a type, not as a class.

The distinction here is a bit weird for those coming from an object-oriented background, but the fact is that subtyping as a concept is very distinct from OOP. Granted, object-oriented really implies subtyping in some form, but the reverse is not true. Benjamin Pierce s Types and Programming Languages does a good job of presenting the language F_< (pronounced "F sub"), which serves as a minimal example of a language with subtyping (but not OO).

Now, with all that said, I do agree that the fact that Nothing is immune from the normal inheritance rules does seem a bit inconsistent. However, from a theoretical standpoint, it makes perfect sense.

Answer 2

I suppose Nothing could accept any method, and perform a standard operation on all of them (throwing an exception). That wouldn t be very useful, though.

By presenting a compile error, the compiler is warning the programmer that a type he most likely didn t want, Nothing, got inferred somehow at a certain point in the code.

Answer 3

You can call toString on Nothing variable because of it s defintion:
final trait Nothing extends Any
And toString is member of Any. I think scala compiler treat s Nothing in type bounds only and treats it like any other trait on all over cases. Letting invoke any method on variable with type of Nothing will be very strange I think.

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