Consider the set of natural numbers^[12].

sealed trait Natural
final case object Zero extends Natural
final case class Succ(s: Natural) extends Natural
            

We can now write a function to add two natural numbers.

def add(x: Natural, y: Natural): Natural = x match {
  case Zero => y
  case Succ(t) => add(t, Succ(y))
}

scala> val k = add(Succ(Succ(Zero)), Succ(Zero)) // add 2 1
k: Natural = Succ(Succ(Succ(Zero)))
            

We destructure algebraic data types using pattern matching — the case and match keywords.

We could write many functions over this algebraic data type.