[swift-evolution] [Proposal] Enums with static stored properties for each case

Jānis Kiršteins janis.kirsteins at gmail.com
Wed May 25 06:58:45 CDT 2016

Hello everyone,

Currently Swift only supports computed properties for each enum case.
If you want to somehow get static values with each case you would
probably do it like this:

enum Planet {
    case mercury
    case venus
    case earth
    case mars
    case jupiter
    case saturn
    case uranus
    case neptune

    var mass: Float {
        switch self {
        case .mercury: return 3.303e+23
        case .venus: return 4.869e+24
        case .earth: return 5.976e+24
        case .mars: return 6.421e+23
        case .jupiter: return 1.9e+27
        case .saturn: return 5.688e+26
        case .uranus: return 8.686e+25
        case .neptune: return 1.024e+26

    var radius: Float {
        switch self {
        case .mercury: return 2.4397e6
        case .venus: return 6.0518e6
        case .earth: return 6.37814e6
        case .mars: return 3.3972e6
        case .jupiter: return 7.1492e7
        case .saturn: return 6.0268e7
        case .uranus: return 2.5559e7
        case .neptune: return 2.4746e7

However I see two problems with this approach:

1. These value definitions are spread out and difficult to read and
maintain (especially if you have many computed properties for each
enum case);
2. These values are not static. They are computed each time property
is accessed. This can be a problem when value is expensive to create.

The proposed solution is to have single static initializer for each
enum case that initializes stored properties. For example,

enum Planet {
    var mass: Float
    var radius: Float

    static init(mass: Float, radius: Float) {
        self.mass = mass
        self.radius = radius

    case mercury where (mass: 3.303e+23, radius: 2.4397e6)
    case venus where (mass: 4.869e+24, radius: 6.0518e6)
    case earth where (mass: 5.976e+24, radius: 6.37814e6)
    case mars where (mass: 6.421e+23, radius: 3.3972e6)
    case jupiter where (mass: 1.9e+27, radius: 7.1492e7)
    case saturn where (mass: 5.688e+26, radius: 6.0268e7)
    case uranus where (mass: 8.686e+25, radius: 2.5559e7)
    case neptune where (mass: 1.024e+26, radius: 2.4746e7)

This approach do not affect enums that have raw or associated values,
or custom enum initializers:

case A = "A" where (id: 0)


case B(Int, Int, Int) where (id: 0)

1. Less verbosity
2. Improved readability
3. Related values are closer to each other
4. Static values are not recomputed

More information about the swift-evolution mailing list