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Synthesized Comparable conformance for enums

Available from Swift 5.3

Paul Hudson      @twostraws

SE-0266 lets us opt in to Comparable conformance for enums that either have no associated values or have associated values that are themselves Comparable. This allows us to compare two cases from the same enum using <, >, and similar.

For example, if we had an enum that describes clothing sizes we could ask Swift to synthesize Comparable conformance like this:

enum Size: Comparable {
    case small
    case medium
    case large
    case extraLarge

We can now create two instances of that enum and compare them using <, like this:

let shirtSize = Size.small
let personSize = Size.large

if shirtSize < personSize {
    print("That shirt is too small")

This synthesized conformance works great with associated values that are Comparable. For example, if we had an enum that described the football World Cup wins for a team, we might write this:

enum WorldCupResult: Comparable {
    case neverWon
    case winner(stars: Int)

We could then create several instances of that enum with varying values, and have Swift sort them:

let americanMen = WorldCupResult.neverWon
let americanWomen = WorldCupResult.winner(stars: 4)
let japaneseMen = WorldCupResult.neverWon
let japaneseWomen = WorldCupResult.winner(stars: 1)

let teams = [americanMen, americanWomen, japaneseMen, japaneseWomen]
let sortedByWins = teams.sorted()

That will sort the array so that the two teams who haven’t won the World Cup come first, then the Japanese women’s team, then the American women’s team – it considers the two winner cases to be higher than the two neverWon cases, and considers winner(stars: 4) to be higher than winner(stars: 1).

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