Enum

Here is a hierarchy of classes.

trait Expr
object Expr {
  case class Var(s: String) extends Expr
  case class Sum(e1: Expr, e2: Expr) extends Expr
  case class Prod(e1: Expr, e2: Expr) extends Expr
}

Note that there are no methods in these definitions. All we've done is define some base trait and case classes that extend the base trait and thus defined in particular cases how data of the case trait can be constructed and pattern matched upon.

Pure data definitions like these are called algebraic data types or ADTs for short. ADTs are quite pervasive in functional programming.

Enum is the shorter expression for ADT.

enum Expr {
  case Var(s: String)
  case Sum(e1: Expr, e2: Expr)
  case Prod(e1: Expr, e2: Expr)
}

Pattern matching on ADT

Since enum contains case classes, it can be used in pattern matching.

def show(e: Expr): String = e match {
  case Expr.Var(x) => x
  case Expr.Sum(e1, e2) => s"${show(e1) + show(e2)}"
  case Expr.Prod(e1, e2) => s"${show(e1) * show(e2)}"
  _ => "Not an expression"
}

More fun with Enum

Enum has some predefined functions.

• value(index) returns a case class indicated by index.

• ordinal returns the index of a case class in enum.

enum Direction(val dx: Int, val dy: Int) {
  case Right extends Direction(1, 0)
  case Up extends Direction(0, 1)
  case Left extends Direction(-1, 0)
  case Down extends Direction(0, -1)

  def leftTurn = Direction.value((ordinal + 1) % 4)
}

val r = Direction.Right
val u = r.leftTurn // Up
val v = (u.dx, u.dy) // (0, 1)

Actually, we can transform enum into normal class.

class Direction(val dx: Int, val dy: Int) {
  // implement value() and ordinal
  // ...
  def leftTurn = Direction.value((ordinal + 1) % 4)
}
object Direction {
  val Right = new Direction(1, 0)
  val Up = new Direction(0, 1)
  val Left = new Direction(-1, 0)
  val Down = new Direction(0, -1)
}

Domain Modeling

enum can combines simple cases and parameterized cases.

enum PayMethod {
  case CreditCard(kind: Card, number: Int, expires: Date)
  case PayPal(email: String)
  case Cash
}

enum Card {
  case Visa, MasterCard, Amex
}