On Jun 3, 2016, at 3:35 PM, T.J. Usiyan via swift-evolution <swift-evolution@swift.org> wrote:
Since this seems to have some interest, I've made a gist.
Enum Subsets ¡ GitHub
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 <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160530/020025.html>
Motivation
Consider a situation where we have an enum Color 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 .
enum Color {
case red, orange, yellow, green, blue, indigo, violet
âŚ
}
enum LCDColor {
case red, green, blue
}
The cases in LCDColor in our scenario do not require different behavior from their similarly named cases in Color. 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 LCDColor
- Completely manually
- Protocols with âminimalâ manual duplication
2. Avoid duplication by allowing conversion to Color.
Neither of these solutions make the subset relationship between Color and LCDColor clear or strict.
Proposed solution
Add syntax to describe a restricted set of cases from an enum.
typealias LCDColor = Color.(red|green|blue)
LCDColor has all of the type and instance methods of Color.
Barring any technical reason, the âactualâ name of the type, in this example, is Color.(red|green|blue) This makes the relationship to Colorsyntactically clear. If a typealias is not desired, Color.(red|green|blue) should refer to the same type as LCDColor
Switching over Color.(red|green|blue) should only need to be exhaustive for the three cases .red, .green, and .blue.
Two initializers should be implicitly created
Color to LCDColor?
returns nil for all cases not in LCDColor
LCDColor to Color
Obvious and trivial implementation mapping cases from LCDColor to Color
Casting should be allowed
from superset to subset only using as? or as! syntax.
from subset to superset using as
Creating subsets of subsets is not allowed but reasonable conversions among subsets should be allowed if technically feasible such that:
Given subsets of C A and B, where A is a superset of B, the casting relationship between A and B should be similar to that between C and either of the other two named subsets.
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.
If a mangled name approach is taken, the ordering of cases should be sorted to ensure stability.
Alternatives considered
Do nothing. This feature is not strictly necessary but does allow for expressivity not currently available in the language.
implicitly create properties which convert to superset type.
Impact on existing code
This is an additive change which should have no breaking change to existing code.
On Fri, Jun 3, 2016 at 4:57 PM, Austin Zheng <austinzheng@gmail.com <mailto:austinzheng@gmail.com>> wrote:
I really like the idea behind this proposal.
Some questions:
- Would the enum 'slice' be a distinct type relative to the base enum?
- On a related note, would shared cases between the sliced enum and the base enum be implicitly convertible?
- If they aren't implicitly convertible between each other, would there be an affordance to perform conversions (e.g. a "parentEnumType" property and an "init?(parentEnumType:)" initializer)/
- Would you be able to further slice a sliced enum? If so, would they share the same parent, or would the 'parent' of the sliced sliced enum be the sliced enum?
- If the parent enum has members that switch on 'self', would those members be available to the child automatically?
- What happens if you have two (or more) slices with disjoint members? (e.g. 'LCDColors' and 'TrafficLightColors') Would they be considered completely separate 'sub-types'?
Best,
Austin
On Fri, Jun 3, 2016 at 6:22 AM, T.J. Usiyan via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@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\)
## 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` .
enum Colors {
case red, orange, yellow, green, blue, indigo, violet
âŚ
}
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.
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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