What is the correct way to use var body: some ReducerProtocol in Swift 5.6?

[ylorn]

Following code does not compile with Xcode 13.4 which embeds Swift 5.6.1:

struct Demo: ReducerProtocol {

    struct State {
        var value: String
    }

    enum Action {
        case task
    }

    @ReducerBuilder<State, Action>
    var body: some ReducerProtocol {

        Reduce { state, action in
            switch action {
                case .task:
                    return .none
            }
        }
    }
}

It requires func reduce(into state: inout State, action: Action) -> EffectTask<Action> to be implemented even if var body property presents.

[mbrandonw]

Hi @ylorn, since Swift 5.6.1 doesn't support primary associated types you must structure the body like this:

struct Demo: ReducerProtocol {
  struct State {
    var value: String
  }
  enum Action {
    case task
  }

  var body: Reduce<State, Action> {
    Reduce { state, action in
      switch action {
      case .task:
        return .none
      }
    }
  }
}

We have more information in our ReducerProtocol migration guide, in particular the section on Swift 5.6.

[mbrandonw]

Quick note that the way this works is that secretly under the hood the result builders will do type erasure, and this is why if you try to compile this style of code in Swift 5.7+ you will get a deprecation warning letting you know that you should upgrade to the some ReducerProtocol<State, Action> style.

[ylorn]

I wasn't aware of the migration notes, Thanks!