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

[Patrick Smith]

Raw values are for coverting to and from an external representation only. That’s why it must be a primitive value, as they can be checked for equality in `init?(rawValue:)`.

The planets here have fuzzy floating point values, and so must never be checked for equality. The only source of truth are their names.

If you want a list of planets, the best way is to do this:

struct Planet {
  var mass: Float
  var radius: Float
  
  static let mercury = Planet(mass: 3.303e+23, radius: 2.4397e6)
  static let venus = Planet(mass: 4.869e+24, radius: 6.0518e6)
  static let earth = Planet(mass: 5.976e+24, radius: 6.37814e6)
  static let mars = Planet(mass: 6.421e+23, radius: 3.3972e6)
  static let jupiter = Planet(mass: 1.9e+27, radius: 7.1492e7)
  static let saturn = Planet(mass: 5.688e+26, radius: 6.0268e7)
  static let uranus = Planet(mass: 8.686e+25, radius: 2.5559e7)
  static let neptune = Planet(mass: 1.024e+26, radius: 2.4746e7)
}


What this proposal is asking for is an easier way to have derived values from enum cases. Asking for more flexible RawValues means mass and radius are not derived, they are the source of truth. It goes against the whole point of RawRepresentable. You are not saying ‘Mercury is identified by the case .mercury’, you are saying ‘Mercury is identified by a mass of 3.303e+23’. It’s backwards.


> On 26 May 2016, at 1:47 PM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On May 25, 2016, at 10:27 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>> 
>> On Wed, May 25, 2016 at 8:15 PM, David Sweeris via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On May 25, 2016, at 7:37 AM, Leonardo Pessoa via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Hi,
>>> 
>>> Couldn't this be solved by using tuples? If not because the syntax is not allowed I think this would be more coherent to do it using current syntax.
>>> 
>>> enum Planet : (mass: Float, radius: Float) {
>>>     case mercury = (mass: 3.303e+23, radius: 2.4397e6)
>>>     case venus = (mass: 4.869e+24, radius: 6.0518e6)
>>>     case earth = (mass: 5.976e+24, radius: 6.37814e6)
>>>     case mars = (mass: 6.421e+23, radius: 3.3972e6)
>>>     case jupiter = (mass: 1.9e+27, radius: 7.1492e7)
>>>     case saturn = (mass: 5.688e+26, radius: 6.0268e7)
>>>     case uranus = (mass: 8.686e+25, radius: 2.5559e7)
>>>     case neptune = (mass: 1.024e+26, radius: 2.4746e7)
>>> }
>> 
>> 
>> This would be my preferred solution… AFAIK, the only reason we can’t do it now is that Swift currently requires RawValue be an integer, floating-point value, or string. I don’t know why the language has this restriction, so I can’t comment on how hard it would be to change.
>> 
>> - Dave Sweeris
>> 
>> Except you'd have to write Planet.mercury.rawValue.mass, rather than Planet.mercury.mass.
>> 
>> This could be one or two proposals: allow enums with tuple RawValues, and allow `TupleName.caseName.propertyName` to access a tuple element without going through .rawValue.
> 
> Good point… Has there been a thread on allowing raw-valued enums to be treated as constants of type `RawValue` yet? Either way, removing the restriction on what types can be a RawValue is still my preferred solution.
> 
> - Dave Sweeris
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