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

[Charlie Monroe]

> 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.

I see what Janis meant in the first email. It's not that the planet would be identified by the mass or radius. It could very much be 

case Mercury = 1 where (mass: 3, radius: 2),

- Mercury's rawValue would be 1.

The issue here is that sometimes you want additional information with the enum. There are many cases where you extend the enum with a variable:

enum Error {
	case NoError
	case FileNotFound
	...

	var isFatal: Bool {
		/// swtich over all values of self goes here.
	}

	var isNetworkError: Bool {
		/// swtich over all values of self goes here.
	}

	var isIOError: Bool {
		/// swtich over all values of self goes here.
	}
}

What the propsal suggests is to simplify this to the following:

enum Error {
	var isFatal: Bool

	case NoError where (isFatal: false, isNetworkError: false, isIOError: false)
	case FileNotFound  where (isFatal: true, isNetworkError: false, isIOError: true)
	...

}

So that you assign the additional information to the enum value itself.

Charlie

> 
> 
>> On 26 May 2016, at 1:47 PM, David Sweeris via swift-evolution <swift-evolution at swift.org <mailto: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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