In Scala, why doesn't Unit extend the Product trait just like the Tuple* classes and case classes (including the "empty" ones, like in case class Empty(), and case objects)?
Unit (the unit value () to be more specific) definitely stands as the empty product and tuple. It is used this way in shapeless, for example.
Unit is not analogous to to an empty collection, so it doesn't make sense to make it a product of arity zero. Even though a Product0 wouldn't contain any data, it would still define behaviour via the productElement and productIterator methods.
The goal of Unit is to represent the absence of any behaviour or data beyond the bare minimum defined in Object. In fact, the getClass() function returns null to indicate that it has no type, no by definition it should have no metadata associated with it.
If you were to make Unit extend Product, then it would violate the concept of "no type".
().getClass() does not return null in 2.11.2. () does have a type - Unit e.g. val c: Class[Unit] = ().getClass().
().getClass() == null and ().getClass eq null are both false, so that void you're seeing is different from null. As the comment above class Unit says, () "is not represented by any object in the underlying runtime system." It's treated specially.The current Scala Language Specification seems to requires this in conforming implementations of Scala, according to section 12.2.3:
Class
Unit... implements only the three methods equals,hashCode, andtoStringfrom classAny.
To extend Product and implement its methods, this part of the specification would have to change.
Unitshould beTuple0, maybe its just a mistake of scala.