[Pitch] Enum Quality of Life Improvements

Redux-style systems are perfect for SwiftUI, but the ability to store both the data for a given screen and its view-route as associated enum cases causes view refreshes on every update, even if the outermost view observing the case is only interested in its route.

E.g. with a view route that looks like this:

enum Navigation: Hashable {
    case firstScreen(FirstScreenData)
    case secondScreen(SecondScreenData)
}

And a navigation router that looks like this:

struct NavigationRouter: View {
    @StateBinding var navigation = GlobalState.shared.lensing(\.navigation)

    var body: some View {
        switch navigation {
        case .firstScreen:
            FirstScreen()
        case .secondScreen:
            SecondView()
        }
    }
}

NavigationRouter will refresh even if the case hasn't changed.

It is of course possible to store the navigation route as an enum without associated cases and to store the data for each screen in a separate location, but that leaves us with a system that needs to be kept manually in sync, which can be a security concern if any of your screens contain sensitive data that is required to be forgotten once the screen has been navigated away from.

What I'm proposing is very similar to CaseIterable in nature. For the sake of reference, let's give it the preliminary name of UnassociatedCases.

An example of what applying UnassociatedCases would do:

extension Navigation: UnassociatedCases {
    // Compiler generated
    enum Unassociated: Hashable, CaseIterable {
        case firstScreen
        case secondScreen
    }
    
    // Compiler generated
    var unassociatedCase: Unassociated {
        switch self {
        case .firstScreen:
            return .firstScreen
        case .secondScreen:
            return .secondScreen
        }
    }

    // Compiler generated
    func isCase(_ unassociatedCase: Unassociated) -> Bool {
        switch (self, unassociatedCase) {
        case (.firstScreen, .firstScreen):
            return true
        case (.secondScreen, .secondScreen):
            return true
        default:
            return false
        }
    }
}

With these improvements to enums, we could then use GlobalState.shared.lensing(\.navigation. unassociatedCase) and views would no longer refresh unless the route itself changed.

I'm sure there's many other places such improvements would be useful, I'm just speaking to my personal headaches here, which required me to write a workaround to achieve this functionality using Sourcery.

Problems considered:

In the event that two cases share the same name, a differentiating element would need to be added. The most favourable option would be to include the names of the associated values.
E.g.
case firstScreen_data_counter

3 Likes

In my own code I do a similar thing, but would love it if Swift supported this as a sort of special identity inquiry. I refer to it in my own stuff as “celf” (case-self) and mock variables to handle it much how you do with an inner enum stripped down without values, but would love if something like this was feasible without boilerplate

enum CelfRepresenting {
  case a(Int)
  case b(Double)
  case c(String)

  enum CaseSelf {
     case a
     case b
     case c
  }
  
  var celf: CaseSelf {
     switch self {
       case .a: return .a
       case .b: return .b
       case .c: return .c
     }
  }
}

let x: CelfRepresenting = .a(42)
Print(x.celf == .a) // true

Aside from convenience, it cuts down on unwrapping enums in cases where I don’t care about the value and (personally) find the brevity a huge help as I’m usually doing that in conditionals where it clutters legibility, IMO.

Also allows use of it in ternary operators, which I favor, and would be great for things like hardcoding fast paths in more involved enums

let x = Set<MyEnum.celf>(.a, .b, .c, —- .z)
…
let y = MyEnum.f(ComplexObject)
x.contains(y.celf) ? easyCalc(y) : slowCalc(y)
3 Likes

This would definitely be nice to have. Cf. Pitch: Auto-synthesize cases for enums which I think is the last time this was raised.

(Also maybe of interest, a suggestion for the companion property for the associated value Extract Payload for enum cases having associated value)

1 Like

There's a very clever library by Point-Free which introduces the concept of CasePaths. This can certainly help with case matching.

Nevertheless, I'm not against the principle of this pitch. There's still plenty to explore in this particular area and I'm very strongly in favor of these mechanisms being baked into the language itself.

7 Likes

I raised something very similar a little while ago too:
https://forums.swift.org/t/idea-enum-intcase-stringcase

I think the conversation got sidetracked on the Codable stuff, but I was essentially just proposing auto-synthesised cases for enums with associated types.

I think IntCase and StringCase provide much more flexibility. It's nice to know the index/name of the enum case when handling custom Codable implementations.

We wrote the library because Swift doesn't come with case paths / enum key paths, but we would be more than happy to retire it if someone gets an implementation working! We'd even be happy to help write the pitch and work out some of the design details.

This is exactly what case paths was born out of. We use them heavily in swift-composable-architecture. They're useful for far more, though. Even "vanilla" SwiftUI benefits, since it gives you the ability to derive bindings of enum cases from bindings of enums.

11 Likes

Hi. Your task can be solved without language changes, if I understand it correctly.

// Firstly, create this struct: 
@propertyWrapper
public struct EquatableExcluded<T>: Equatable {
  public var wrappedValue: T
  
  public init(wrappedValue: T) {
    self.wrappedValue = wrappedValue
  }
  
  public init(_ wrappedValue: T) {
    self.init(wrappedValue: wrappedValue)
  }
  
  public static func == (lhs: Self, rhs: Self) -> Bool { true }
}

extension EquatableExcluded: Hashable {
  /// Empty Implementation
  public func hash(into hasher: inout Hasher) {}
}

// Then wrap your associated values

enum Navigation: Hashable {
    case firstScreen(EquatableExcluded<FirstScreenData>)
    case secondScreen(EquatableExcluded<SecondScreenData>)
}

There are also some other similar pithes: Comparing enum cases while ignoring associated values - #8 by Dmitriy_Ignatyev

The goal is to have it be equatable at the case level but not at the associated values level, so that solution doesn't work.

If there's other pitches then I'd say it's got traction as something the developers want in the language, or at least those of us willing to pitch it, so perhaps we should see if we can move it along into a real pitch?

You can implement caseName property generically in several lines of code, and then conform Equatable protocol where caseName is compared.

caseName is the least important part of this proposal, I've implemented it myself many times.

IIUC, this is yet another thing that could be implemented in a library if we had structural generic programming. /cc @shabalin

5 Likes

And in an even more general sense, a lot of these things could be unlocked if the reflection capabilities of the language were better. By bringing over more "knowledge" from the compiler onto the language, Swift would enable so many of these capabilities by third parties to flourish — and potentially be added onto the language officially, if uptake is positive.

IMHO reflection is the bottleneck that needs addressing.

2 Likes

Sure: structural generic programming capabilities are reflection. But it is not just any old reflection that's needed—it's reflection that can interact with the type system, which is what you need for synthesizing conformances and associated types. That's what the structural generic programming proposal provides.

4 Likes

The structural generic programming proposal is interesting, but I'd hesitate to say that it's the best way we can expose that information to the type system. Variadic generics will likely improve how these things can be represented - e.g. a struct's members can be represented as a variadic list of parameters, presumably there'd be ways to write extensions and protocol conformances if all parameters met some constraints, etc.

It might actually be happening... :crossed_fingers:

2 Likes

You can use reflection to reproduce that CasePath example. I use this, but it does not make me happy, because explicit typing is required.

In the current language, it probably is.

Variadic generics will likely improve how these things can be represented

Yes, I take it for granted that programming with the kinds of type structures in the structural generic programming proposal would be improved with variadic generics, and I look forward to the introduction of those features with antici… pation. The basic approach is still valid, and could subsume many one-off language extension proposals such as this one, and most ideas for general reflection.

3 Likes

CasePaths used to use reflection, but using a mirror is about 1,000x slower than manually writing an if case let. This may not be a big deal for a casual match/extraction, but can add up quickly in a hot code path. These days CasePaths uses the Swift runtime metadata, instead, and is only about 2–5x slower than an if case let.

As nice as it is to have runtime metadata around to implement this functionality ourselves in a performant-enough way, this really is a feature that should be built into the language, given that key paths are built into the language and are used all over the place these days, like all of the dynamic member lookups in SwiftUI.

17 Likes

I would love to see CasePaths introduced as a language feature (as well as Enum properties). The discrepancy in ergonomics between Structs and Enums is probably the area where I'd most frequently benefit from quality of life improvements, and in my experience a non-negligible source of friction to the more wider adoption of safer, more unambiguous patterns/architectures in Swift codebases.

10 Likes

For reference, I just posted a call to action to try and find a compiler engineer willing to help formalize a pitch for CasePaths.

4 Likes
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