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

[Brent Royal-Gordon]

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

My opinions so far:

- Abusing rawValue is just that: an abuse.

- Using `where` just doesn't match the use of `where` elsewhere in the language; everywhere else, it's some kind of condition.

- Dictionaries are the most straightforward way to handle this with the current language, but their lack of exhaustiveness checking is a problem.

What I would do is borrow the "accessors" concept from the property behaviors proposal and extend it so that it supported both functions and variables. Then I would let you write this:

	enum Planet {
	   accessor var mass: Float
	   accessor var 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
	   }
	}

You would also be able to declare methods like this; each implementation would just look like `methodName { code }`. And you could provide default implementations too:

	enum Planet {
	   accessor var mass: Float
	   accessor var radius: Float
	   accessor var habitable: Bool = false
	   
	   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
	      habitable = true
	   }
	   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
	   }
	}

-- 
Brent Royal-Gordon
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