Intersection Types, aka Compound Types

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:

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

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

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

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

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

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.

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

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