[swift-evolution] [Proposal] Enum subsets

[T.J. Usiyan]

Since this seems to have some interest, I've made a gist.

https://gist.github.com/griotspeak/963bc87a0c244c120264b11fb022d78c
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
    …
}
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 ‘minimal’ 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 ‘actual’ 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 at 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 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)
>>
>> ## 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.
>>
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>
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