Usages of Null / Nothing / Unit in Scala

Question

I've just read: http://oldfashionedsoftware.com/2008/08/20/a-post-about-nothing/

As far as I understand, Null is a trait and its only instance is null.

When a method takes a Null argument, then we can only pass it a Null reference or null directly, but not any other reference, even if it is null (nullString: String = null for example).

I just wonder in which cases using this Null trait could be useful. There is also the Nothing trait for which I don't really see any more examples.

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I don't really understand either what is the difference between using Nothing and Unit as a return type, since both doesn't return any result, how to know which one to use when I have a method that performs logging for example?

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Do you have usages of Unit / Null / Nothing as something else than a return type?

Answer 1

You only use Nothing if the method never returns (meaning it cannot complete normally by returning, it could throw an exception). Nothing is never instantiated and is there for the benefit of the type system (to quote James Iry: "The reason Scala has a bottom type is tied to its ability to express variance in type parameters."). From the article you linked to:

One other use of Nothing is as a return type for methods that never return. It makes sense if you think about it. If a method’s return type is Nothing, and there exists absolutely no instance of Nothing, then such a method must never return.

Your logging method would return Unit. There is a value Unit so it can actually be returned. From the API docs:

Unit is a subtype of scala.AnyVal. There is only one value of type Unit, (), and it is not represented by any object in the underlying runtime system. A method with return type Unit is analogous to a Java method which is declared void.