Scala：在继承树的底部放置类型

Question

Scala defines a type called Nothing that is a sub-type of all types. Is it possible to define our own "bottom types" that should be at the bottom of an inheritance tree that extends from a given super-type?

For example, suppose I have some type Foo , and I want to say Xyzzy should be at the bottom of Foo 's inheritance tree - in other words, it should inherit from anything that inherits from Foo. If we introduce a new type Bar , and Bar <: Foo , then Xyzzy <: Bar . I'm basically looking for a way to say class Xyzzy isBottom Foo { ... } . Is there any way to achieve this directly, or indirectly achieve this effect?

Answer 1

It is possible to come close to the idea you're looking to do. This is not a general "bottom" type for all types in an inheritance chain. Instead, it's a bottom type specific to traits that use generics. You need to reuse Nothing in the type definition within the inheritance structure. Take a look at List[_] to see how they've defined Nil :

case class ::[A](value: A, next: List[A]) extends List[A]
case object Nil extends List[Nothing]

Hence, Nil is a List[A] for all A because the type parameter of list is defined as List[+A] .

Answer 2

This doesn't work:

class Foo
abstract final class Xyzzy extends Foo
class Bar extends Foo

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> typeOf[Xyzzy] <:< typeOf[Foo]
res0: Boolean = true

scala> typeOf[Xyzzy] <:< typeOf[Bar]
res1: Boolean = false

It appears that Nothing is handled specially by the compiler.

Answer 3

I mean you could do this

trait Foo
abstract final class Xyzzy extends Foo
implicit def XyzzyIsBottom[A <: Foo]: Xyzzy <:< A = null

// Example usage
class X extends Foo
def xyzzyToX(x: Xyzzy): X = x