[swift-evolution] [Proposal] Enum subsets

[Christopher Kornher]

This could be useful in categorizing and grouping within large enums ErrorType enums come to mind. Would there be any problem with making the subset more explicit? e.g.



``` swift
enum Colors {
	case red, orange, yellow, green, blue, indigo, violet
	…
}

extension Colors {
	subset  LCDColors  red, green, blue
}
```

> On Jun 3, 2016, at 7:22 AM, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This is loosely related to but not meant to 'compete' with the ad hoc enum proposal.
> 
> ## Introduction
> 
> This proposal adds/creates syntax to allow ad hoc creation of enums whose members are strict subsets of explicitly defined enums.
> 
> Swift-evolution thread: [Discussion thread topic for that proposal](http://news.gmane.org/gmane.comp.lang.swift.evolution <http://news.gmane.org/gmane.comp.lang.swift.evolution>)
> 
> ## Motivation
> Consider a situation where we have an enum `Colors` which represents the entire set of colors relevant to your application with many salient methods and operations. We have also declared an enum `LCDColorModel` with only three colors, `red, blue, green` .
> 
> ``` swift
> enum Colors {
> 	case red, orange, yellow, green, blue, indigo, violet
> 	…
> }
> 
> enum LCDColors {
> 	case red, green, blue
> }
> ```
> 
> The cases in `LCDColors` in our scenario do not require different behavior from their similarly named cases in `Colors`. We would like, simply stated, to explicitly restrict the cases allowed within a specific portion of our software. There are, currently, a few approaches 
> 	1. Duplicate functionality in `LCDColors` 
> 		- Completely manually
> 		- Protocols with 'minimal' manual duplication
> 	2. Avoid duplication by allowing conversion to `Colors`. 
> 
> Neither of these solutions make the subset relationship between `Colors` and `LCDColors`  clear or strict.

> ## Proposed solution
> 
> Add syntax to describe a restricted set of cases from an enum. 
> 
> ```swift
> typealias LCDColors = Colors.(red|green|blue)
> ```
> 
> `LCDColors ` has all of the type and instance methods of `Colors`. Cases must appear in the same order as their original declaration. 
> 
> 
> ## Detailed design
> 
> While I am unsure of the entirety of the design, I propose that name mangling be used which, along with the declaration order restriction should mean that all possible subsets have a stable and predictable name which contains all of the information necessary to infer cases. 
> 
> ## Impact on existing code
> 
> This is an additive change which should have no breaking change to existing code.
> 
> 
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