1

Just going through the "Adopting a Protocol Using a Synthesized Implementation" section of the the swift programming language.

When the following enum code is run, the following error is displayed in Xcode (project, playground or repl/CLI):

enum SkillLevel: Comparable {
    case beginner
    case intermediate
    case expert(stars: Int)
}

Type 'SkillLevel' does not conform to protocol 'Comparable'

According to the text above this code it should conform automatically. Although intuitively, since SkillLevel is a custom type, it is not obvious how the compiler would know how the cases compare.

What am I missing here?

2

It was added in Swift 5.3 (SE-0266 Synthesized Comparable conformance for enum types), so it's not yet available.

3

Ok. :cool:. So, how does it compare without something obvious to compare?

4

You mean the (upcoming) synthesised comparator? From the linked proposal:

[enums with associated values] will compare by case declaration order first, and then lexicographically by payload values.

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5

Thank you, Lantua. I've just read the proposal and found the bit I was after. Very :cool:.

Comparable conformance would compare according to case declaration order, with later cases comparing greater than earlier cases.

6

Does this conformance require all associated values to also conform to Comparable? Otherwise, how would it perform the secondary comparison for payload after the initial comparison by case?

7

The proposal states the following regarding associated values.

Only enum types with no associated values and enum types with only Comparable associated values would be eligible for synthesized conformances.

Note: The SkillLevel enum works fine in Xcode 12 beta 6 :nerd_face:

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