CODEWITHSUNDEEP
scala
Pooya
Pooya

Reputation: 4481

What is the meaning of >: Null <: in Scala?

I searched a lot, but I didn't find anything, What is the meaning of this type of type declaration in Scala? 

type Ident >: Null <: AnyRef

Upvotes: 9

Views: 1885

Answers (3)

4lex1v
4lex1v

Reputation: 21557

In your case Null is a subtype of Ident and AnyRef is its supertype. In without this boundaries common subtype of all type is Nothing, but you specify it to be Null, with is a subtype of all object when Nothing is a subtype of everything (including Int, Long, etc..)

Things like Null and Nothing are used in Type System, basically in type (Contra/Co)variance. Example:

sealed trait Container[+A >: Null <: AnyRef]
case class Full[A >: Null <: AnyRef](value: A) extends Container[A]
case object Empty extends Container[Null]

This compiles fine:

val c: Container[String] = Full("String")
val e: Container[String] = Empty

But this fails, because our lower bound is Null and upper is AnyRef:

val ff: Container[Int] = Full(10)
val f: Container[Int] = Empty

We can't put Int here beacuse this type violates out contraints

Upvotes: 1

senia
senia

Reputation: 38045

Keyword type is for type alias declaration, just like val and def are for value and method declaration. In this case it's an abstract type alias with constraints, so it's a type member of some trait or class - type alias in local scope can't be abstract and can't have constraints.

Type Ident is a subtype of AnyRef and supertype of Null.

AnyRef

AnyRef is an ancestor of all reference types, all types except Int, Long, Char and so on (Java primitives).

Null

Null is subtype of all "nullable" types. In fact it's a subtype of all reference types.

Since all AnyRef are nullable the only additional constraint from >: Null is that Ident is not Nothing.

See Scala’s type hierarchy:

Scala’s type hierarchy

Upvotes: 16

Alexey Romanov
Alexey Romanov

Reputation: 170765

To add to @senia's answer: this isn't a type declaration, this is an abstract type member declaration, found in context like

trait Foo { // or perhaps class
  type Ident >: Null <: AnyRef
}

and means that any concrete implementation of Foo must define some type to be Ident, and this type has to satisfy constraints described in @senia's answer.

Upvotes: 7

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