[2.11] :kind command outputs A for Any and Nothing

[Eugene Yokota]

Hi,

2.11 adds [:kind][1] command to the REPL, which currently outputs Any and Nothing's kind to be A.

scala> :kind -v Nothing

scala.Nothing's kind is A

*

This is a proper type.

scala> :kind -v Any

java.lang.Object's kind is A

*

This is a proper type.

Inspired by Jason saying "Any and nothing are kind polymorphic in scala." @xuwei_k blogged that

the above :kind outputs may be inaccurate since they can be used in place of a type constructor:

class A[X]

class B[X[_]]

class C[X[_[_]]]

new A[Any]

new B[Any]

new C[Any]

new A[Nothing]

new B[Nothing]

new C[Nothing]

This is backed by SLS 3.5.2 "For every type constructor T (with any number of type parameters), scala.Nothing <: T <: scala.Any."

What should the correct :kind output be?

I am thinking about something like "scala.Nothing is a bottom type for all kinds" and "scala.Any is a top type for all kinds."

-eugene

  [1]: https://github.com/scala/scala/pull/2340