Comparing enum cases while ignoring associated values

sjavora · September 6, 2018, 9:06pm

Multiple times I have now wanted to compare two values of an enum while ignoring their respective associated values.

enum Foo {
    case string(String)
    case number(Int)
}

let a = Foo.string("str")

a == Foo.string // Binary operator '==' cannot be applied to operands of type 'Foo' and '(String) -> Foo'

I understand why this doesn't work, but... I really want it to! Looking around the web for workarounds has led me to Erica Sadun's article, an interesting gist from the comments of said article, and even a post from Swift 1.2 era asking basically the same question.

All the solutions I've seen so far either require me to manually write type-specific code, or are plain ugly. Even if there are workarounds, though, I'd love for the following to work as well:

foos.contains(.number)

...which could only work if the cases without their associated values were somehow Equatable.

I suppose I'm posting this here out of frustration and hoping that perhaps someone from the Swift team will chime in, because this feels like it should work and just doesn't...

Naxum · September 6, 2018, 10:11pm

My only use of Sourcery is to auto-generate a subtype on my associated value enums so I can do simple == and != checks for their case names. I would love to see this feature exist in Swift.

It should be easy to do things like the following without resorting to weird quirks or a bunch of extra code

enum State {
  case ready
  case loading(reasons: [String])
}

let stateA = State.ready
let stateB = State.loading(reasons: ["ReasonA", "ReasonB"])

print(stateA.case == stateB.case) //false

if stateA.case != .loading { 
  // do something
}

There are a bunch of hiccups when writing if case let something = .enumCase (including the inability to use != with case let).
I too would love to see it simplified!

mpangburn · September 6, 2018, 10:37pm

Relevant discussion regarding enum ergonomics: Automatically derive properties for enum cases

lancep · September 6, 2018, 10:51pm

You can always do

if case .string = a { 
  print("found a string!") 
}

Dante-Broggi · September 7, 2018, 12:21am

I think that, similar to SE-0194, the best way to provide this feature would be to add a CaseDiscriminatable (to be bike-shedded) protocol, for which, like CaseEnumerable a compiler-provided default implementation would be created for same-file extensions.

e.g:

protocol CaseDiscriminatable {
  associatedtype Discriminator: Hashable
  var case: Discriminator
}