A Tour of Scala: 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 compound types, which are intersections of object types.

Suppose we have two traits Cloneable and Resetable:

trait Cloneable extends java.lang.Cloneable {
  override def clone(): Cloneable = { super.clone(); this }
}
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
}

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. This compound type is written like this in Scala:Cloneable with Resetable.

Here's the updated function:

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

Compound types can consist of several object types and they may have a single refinement which can be used to narrow the signature of existing object members.

The general form is: A with B with C ... { refinement }

An example for the use of refinements is given on the page about abstract types.