CODEWITHSUNDEEP
scalainheritancebottom-type
Kvass
Kvass

Reputation: 8434

Scala: Placing type at bottom of inheritance tree

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?

Upvotes: 3

Views: 169

Answers (3)

Owen
Owen

Reputation: 39356

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

Upvotes: 0

wheaties
wheaties

Reputation: 35970

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].

Upvotes: 1

Ben Kovitz
Ben Kovitz

Reputation: 5020

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.

Upvotes: 0

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