compound-types.md

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Sometimes it is necessary to express that the type of an object is a subtype of several other types.

In Scala this can be expressed with the help of intersection types, (or compound types in Scala 2) which are types that behave like any part of the intersection.

Suppose we have two traits Cloneable and Resetable:

{% tabs compound-types_1 class=tabs-scala-version %} {% tab 'Scala 2' for=compound-types_1 %}

trait Cloneable extends java.lang.Cloneable {
  override def clone(): Cloneable = { // makes clone public
    super.clone().asInstanceOf[Cloneable]
  }
}
trait Resetable {
  def reset: Unit
}

{% endtab %} {% tab 'Scala 3' for=compound-types_1 %}

trait Cloneable extends java.lang.Cloneable:
  override def clone(): Cloneable =  // makes clone public
    super.clone().asInstanceOf[Cloneable]
trait Resetable:
  def reset: Unit

{% endtab %} {% endtabs %}

Now suppose we want to write a function cloneAndReset which takes an object, clones it and resets the original object:

{% tabs compound-types_2 class=tabs-scala-version %} {% tab 'Scala 2' for=compound-types_2 %}

def cloneAndReset(obj: ?): Cloneable = {
  val cloned = obj.clone()
  obj.reset
  cloned
}

{% endtab %} {% tab 'Scala 3' for=compound-types_2 %}

def cloneAndReset(obj: ?): Cloneable =
  val cloned = obj.clone()
  obj.reset
  cloned

{% endtab %} {% endtabs %}

The question arises what the type of the parameter obj is. If it's Cloneable then the object can be cloned, but not reset; if it's Resetable we can reset it, but there is no clone operation. To avoid type casts in such a situation, we can specify the type of obj to be both Cloneable and Resetable. {% tabs compound-types_3 class=tabs-scala-version %} {% tab 'Scala 2' for=compound-types_3 %} This compound type is written in Scala as Cloneable with Resetable.

Here's the updated function:

def cloneAndReset(obj: Cloneable with Resetable): Cloneable = {
  //...
}

Note that you can have more than two types: A with B with C with .... This means the same as thing as (...(A with B) with C) with ... ) {% endtab %} {% tab 'Scala 3' for=compound-types_3 %} This intersection type is written in Scala as Cloneable & Resetable.

Here's the updated function:

def cloneAndReset(obj: Cloneable & Resetable): Cloneable = {
  //...
}

Note that you can have more than two types: A & B & C & .... And & is associative, so parentheses can be added around any part without changing the meaning. {% endtab %} {% endtabs %}