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Synthesized Comparable conformance for enum types

Introduction

SE-185 introduced synthesized, opt-in Equatable and Hashable conformances for eligible types. Their sibling protocol Comparable was left out at the time, since it was less obvious what types ought to be eligible for a synthesized Comparable conformance and where a comparison order might be derived from. This proposal seeks to allow users to opt-in to synthesized Comparable conformances for enum types without raw values or associated values not themselves conforming to Comparable, a class of types which I believe make excellent candidates for this feature. The synthesized comparison order would be based on the declaration order of the enum cases, and then the lexicographic comparison order of the associated values for an enum case tie.

Motivation

Oftentimes, you want to define an enum where the cases have an obvious semantic ordering:

enum Membership {
    case premium    // <
    case preferred  // <
    case general
}
enum Brightness {
   case low         // <
   case medium      // <
   case high
}

However, implementing it requires a lot of boilerplate code which is error-prone to write and difficult to maintain. Some commonly used workarounds include:

  • Declaring a raw enum, with an Int backing, and implementing the comparison using self.rawValue. This has the downside of associating and exposing a meaningless numeric value on your enum API, as well as requiring a copy-and-paste < implementation. Such an enum would also receive the built-in init(rawValue:) initializer, which may be unwanted.
enum Membership: Int, Comparable {
    case premium
    case preferred
    case general

    static func < (lhs: Self, rhs: Self) -> Bool {
        return lhs.rawValue < rhs.rawValue
    }
}
  • Manually implementing the < operator with a private minimum(_:_:) helper function. This is the “proper” implementation, but is fairly verbose and error-prone to write, and does not scale well with more enumeration cases.
enum Brightness: Comparable {
    case low
    case medium
    case high

    private static func minimum(_ lhs: Self, _ rhs: Self) -> Self {
        switch (lhs, rhs) {
        case (.low,    _), (_, .low   ):
            return .low
        case (.medium, _), (_, .medium):
            return .medium
        case (.high,   _), (_, .high  ):
            return .high
        }
    }

    static func < (lhs: Self, rhs: Self) -> Bool {
        return (lhs != rhs) && (lhs == Self.minimum(lhs, rhs))
    }
}

As the second workaround is non-obvious to many, users also often attempt to implement “private” integer values for enumeration cases by manually numbering them. Needless to say, this approach scales very poorly, and incurs a high code maintenance cost as simple tasks like adding a new enumeration case require manually re-numbering all the other cases. Workarounds for the workaround, such as numbering by tens (to “make room” for future cases) or using Double as the key type (to allow numbering “on halves”) reflect poorly on the language.

enum Membership: Comparable {
    case premium
    case preferred
    case general

    private var comparisonValue: Int {
        switch self {
        case .premium:
            return 0
        case .preferred:
            return 1
        case .general:
            return 2
        }
    }

    static func < (lhs: Self, rhs: Self) -> Bool {
        return lhs.comparisonValue < rhs.comparisonValue
    }
}

Proposed solution

Enumeration types which opt-in to a synthesized Comparable conformance would compare according to case declaration order, with later cases comparing greater than earlier cases. Only enum types with no associated values and enum types with only Comparable associated values would be eligible for synthesized conformances. The latter kind of enums will compare by case declaration order first, and then lexicographically by payload values. No enum types with raw values would qualify.

While basing behavior off of declaration order is unusual for Swift, as we generally hew to the “all fields are reorderable by the compiler” principle, it is not a foreign concept to enums. For example, reordering cases in a numeric-backed raw enum already changes its runtime behavior, since the case declaration order is taken to be meaningful in that context. I also believe that enum cases and struct/class fields are sufficiently distinct concepts that making enumeration case order meaningful would not make the language incoherent.

Later cases will compare greater than earlier cases, as Swift generally views sort orders to be “ascending” by default. It also harmonizes with the traditional C/C++ paradigm where a sequence of enumeration cases is merely a sequence of incremented integer values.

Detailed design

Synthesized Comparable conformances for eligible types will work exactly the same as synthesized Equatable, Hashable, and Codable conformances today. A conformance will not be synthesized if a type is ineligible (has raw values or non recursively-conforming associated values) or already provides an explicit < implementation.

enum Membership: Comparable {
    case premium(Int)
    case preferred
    case general
}

([.preferred, .premium(1), .general, .premium(0)] as [Membership]).sorted()
// [Membership.premium(0), Membership.premium(1), Membership.preferred, Membership.general]

Source compatibility

This feature is strictly additive.

ABI compatibility

This feature does not affect the ABI.

API stability

This feature does not affect the standard library.

Alternatives considered

  • Basing comparison order off of raw values or RawRepresentable. This alternative is inapplicable, as enumerations with “raw” representations don’t always have an obvious sort order anyway. Raw String backings are also commonly (ab)used for debugging and logging purposes making them a poor source of intent for a comparison-order definition.
enum Month: String, Comparable {
    case january
    case february
    case march
    case april
    ...
}
// do we compare alphabetically or by declaration order?