Iterating through an Enum in Swift 3.0

Question

I have a simple enum that I would like to iterate over. For this purpose, I've adopted Sequence and IteratorProtocol as shown in the code below. BTW, this can be copy/pasted to a Playground in Xcode 8.

import UIKit

enum Sections: Int {
  case Section0 = 0
  case Section1
  case Section2
}

extension Sections : Sequence {
  func makeIterator() -> SectionsGenerator {
    return SectionsGenerator()
  }

  struct SectionsGenerator: IteratorProtocol {
    var currentSection = 0

    mutating func next() -> Sections? {
      guard let item = Sections(rawValue:currentSection) else {
        return nil
      }
      currentSection += 1
      return item
    }
  }
}

for section in Sections {
  print(section)
}

But the for-in loop generates the error message "Type 'Sections.Type' does not conform to protocol 'Sequence'". The protocol conformance is in my extension; so, what is wrong with this code?

I know there are other ways of doing this but I'd like to understand what's wrong with this approach.

Thanks.

Answer 1

Note that Martin’s solution can be refactored as a protocol:

import Foundation

protocol EnumSequence
{
    associatedtype T: RawRepresentable where T.RawValue == Int
    static func all() -> AnySequence<T>
}
extension EnumSequence
{
    static func all() -> AnySequence<T> {
        return AnySequence { return EnumGenerator() }
    }
}

private struct EnumGenerator<T: RawRepresentable>: IteratorProtocol where T.RawValue == Int {
    var index = 0
    mutating func next() -> T? {
        guard let item = T(rawValue: index) else {
            return nil
        }
        index += 1
        return item
    }
}

Then, given an enum

enum Fruits: Int {
    case apple, orange, pear
}

you slap the protocol and a typealias:

enum Fruits: Int, EnumSequence {
    typealias T = Fruits
    case apple, orange, pear
}

Fruits.all().forEach({ print($0) }) // apple orange pear

Answer 2

Update: As of Swift 4.2, you can simply add protocol conformance to CaseIterable, see How to enumerate an enum with String type?.

---

You can iterate over a value of a type which conforms to the Sequence protocol. Therefore

for section in Sections.Section0 {
  print(section)
}

would compile and give the expected result. But of course that is not really what you want because the choice of the value is arbitrary and the value itself not needed in the sequence.

As far as I know, there is no way to iterate over a type itself, so that

for section in Sections {
  print(section)
}

compiles. That would require that the "metatype" Sections.Type conforms to Sequence. Perhaps someone proves me wrong.

What you can do is to define a type method which returns a sequence:

extension Sections {
    static func all() -> AnySequence<Sections> {
        return AnySequence {
            return SectionsGenerator()
        }
    }

    struct SectionsGenerator: IteratorProtocol {
        var currentSection = 0

        mutating func next() -> Sections? {
            guard let item = Sections(rawValue:currentSection) else {
                return nil
            }
            currentSection += 1
            return item
        }
    }

}

for section in Sections.all() {
    print(section)
}

Answer 3

Simply add to enum: static var allTypes: [Sections] = [.Section0, .Section1, .Section2]

And than you can:

Sections.allTypes.forEach { (section) in
            print("\(section)")
}