CODEWITHSUNDEEP
scalanull
user3103957
user3103957

Reputation: 848

Type of user defined objects in Scala

In Scala, Nil is actually an object which returns an empty List. When I type, Nil, it prints the following.

res24: scala.collection.immutable.Nil.type = List()

res24 is the variable whose type is scala.collection.immutable.Nil.type. I don't quite understand the last piece 'type'. Where this is located ?

Another question is: Nil returns an empty list. How can we write an object which returns something? I tried below; it does not return the integer value what I expected.

object Sample { 123 }
val x = Sample
x: Sample.type = Sample$@36fe7b03

Thanks in advance!

Upvotes: 2

Views: 179

Answers (2)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

I don't quite understand the last piece 'type'. Where this is located ?

This is what Scala refers to as the "Singleton Type". Nil and Sample share the common property of being objects in Scala which entails they are classes with a single instance (given that it is a top level declaration, and not nested inside a class, inside a given package scope). It part of the type definition, but it isn't visible in the declaration site. For more on the uses of the singleton type see What is a Singleton Type exactly?

How can we write an object which returns something?

Writing an object that returns something is similar to asking "Can I create a class that returns a value when instantiated?". object are singletons and can have methods on them that returns values:

object Bar {
  def x(): Int = 42
}

def main(args: Array[String]): Unit = print(Bar.x())

Which yields 42.

Edit

Still I am not clear how the singleton object Nil, returning an empty list

If we can simplify and mimic the List[A] definition, we'll see that:

sealed trait List[+A]
case class ::(head: A, tail: List[A]) extends List[A]
case object Nil extends List[Nothing]

What we have here is an Algebraic Data Type (ADT), more specifically a Sum Type, which is a type that's composed of multiple possible values. Here, a List[A] can be either a ::, or Nil, and in our case it represents the empty list case.

Upvotes: 4

pedrorijo91
pedrorijo91

Reputation: 7845

In Scala, Nil is actually an object which returns an empty List.

No, Nil is an object that represents the empty list (by extending from List[Nothing]). See https://alvinalexander.com/scala/what-is-difference-between-nil-empty-list-in-scala/

Upvotes: 1

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