Sunday, June 15, 2008

Scala and Enums

Scala doesn't have language level support for enumerations, but I think its fairly easy to argue that its a good thing. First, something isn't quite right about Java enums. Sometime soon I'll post more about that. Scala is such a nice language that you can do things cleanly without needing built in support for extra things like enum. Extra features in a language clutter it up.

As an example, I lovingly ripped off the Planets example from the Java tutorial itself, and implemented it in Scala. Here is the Scala code.

case object MERCURY extends Planet(3.303e+23, 2.4397e6)
case object VENUS extends Planet(4.869e+24, 6.0518e6)
case object EARTH extends Planet(5.976e+24, 6.37814e6)
case object MARS extends Planet(6.421e+23, 3.3972e6)
case object JUPITER extends Planet(1.9e+27, 7.1492e7)
case object SATURN extends Planet(5.688e+26, 6.0268e7)
case object URANUS extends Planet(8.686e+25, 2.5559e7)
case object NEPTUNE extends Planet(1.024e+26, 2.4746e7)
case object PLUTO extends Planet(1.27e+22, 1.137e6)

// mass in kilograms, radius in meters
sealed case class Planet( mass: double, radius: double ){
// universal gravitational constant (m3 kg-1 s-2)
val G = 6.67300E-11
def surfaceGravity = G * mass / (radius * radius)
def surfaceWeight(otherMass: double) = otherMass * surfaceGravity

And here is the original Java code.

public enum Planet {
MERCURY (3.303e+23, 2.4397e6),
VENUS (4.869e+24, 6.0518e6),
EARTH (5.976e+24, 6.37814e6),
MARS (6.421e+23, 3.3972e6),
JUPITER (1.9e+27, 7.1492e7),
SATURN (5.688e+26, 6.0268e7),
URANUS (8.686e+25, 2.5559e7),
NEPTUNE (1.024e+26, 2.4746e7),
PLUTO (1.27e+22, 1.137e6);

private final double mass; // in kilograms
private final double radius; // in meters
Planet(double mass, double radius) {
this.mass = mass;
this.radius = radius;
public double mass() { return mass; }
public double radius() { return radius; }

// universal gravitational constant (m3 kg-1 s-2)
public static final double G = 6.67300E-11;

public double surfaceGravity() {
return G * mass / (radius * radius);
public double surfaceWeight(double otherMass) {
return otherMass * surfaceGravity();

The Scala code is nicer, though its unfortunate that you have to say "extends Planet" for each Planet. Each planet is defined as a Scala "object" which is really nothing more than a singleton, which is what enum values are in Java.

I could and should go into more detail on all of this, but I'm mostly posting it for my own reference.