Using the new Scala final case class

Question

In chapter 22 of Scala Programming, the class ::(cons) is defined as

final case class ::[T](hd: T, tl: List[T]) extends List[T] {
  //...
}

The method ::in the class Listis defined as follows:

def ::[U >: T](x: U): List[U] = new scala.::(x, this)

Why newis it necessary to create an instance ? Is that pure for meanings?final case class ::

Answer 1

In class classes, you automatically get a companion object, the method applycalls the constructor, just as you can do this with a regular class:

class Foo(val value: Int) 
object Foo { def apply(value: Int) = new Foo(value) }

val x = new Foo(42)  //
val y = Foo(42)      // both work the same

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Answer 2

; new . List#:: :

def ::[U >: T](x: U): List[U] = scala.::(x, this)

(, :

type :: = collection.immutable.::
val  :: = collection.immutable.::

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def ::[U >: T](x: U) = ::(x, this)

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