Adding store scoping for optional state keypath and cached value

[sdkdimon]

Hello, i am a using TCACoordinators for navigation in my projects, and i am noticed that in new version of TCA 1.19.0 were done "Store internals rewrite".
TCACoordinators using custom store scoping here a part of scoping code.

//Router store
  @Perception.Bindable private var store: Store<[Route<Screen>], RouterAction<ID, Screen, ScreenAction>>

//Router scoping function
  func scopedStore(index: Int, screen: Screen) -> Store<Screen, ScreenAction> {
    var screen = screen
    let id = identifier(screen, index)
    return store.scope(
      id: store.id(state: \.[index], action: \.[id: id]),
      state: ToState {
        screen = $0[safe: index]?.screen ?? screen
        return screen
      },
      action: {
        .routeAction(id: id, action: $0)
      },
      isInvalid: { !$0.indices.contains(index) }
    )
  }

This scoping were based on function that now is missing starting from v1.19.0

@_spi(Internals)
  public func scope<ChildState, ChildAction>(
    id: ScopeID<State, Action>?,
    state: ToState<State, ChildState>,
    action fromChildAction: @escaping (ChildAction) -> Action,
    isInvalid: ((State) -> Bool)?
  ) -> Store<ChildState, ChildAction> 

So i propose to add function with support of optional state keypath and cached value.
It adds the ability to use future versions of TCA in TCACoordinators
Can we add it? (Of course the naming may be different)

@_spi(Internals)
extension Store {
  public func scope<ChildState, ChildAction>(
    cached: ChildState,
    keypath: KeyPath<State, ChildState?>,
    action: CaseKeyPath<Action, ChildAction>
  ) -> Store<ChildState, ChildAction> {
    let id = self.id(state: keypath, action: action)
    @MainActor
    func open(
      _ core: some Core<State, Action>
    ) -> any Core<ChildState, ChildAction> {
      IfLetCore(
        base: core,
        cachedState: cached,
        stateKeyPath:keypath,
        actionKeyPath: action
      )
    }
    return self.scope(id: id, childCore: open(self.core))
  }
}

[mbrandonw]

Hi @sdkdimon, as of 1.19.1 we have @_spi'd a bit of the internals: #3657. Does that help?

[sdkdimon]

@mbrandonw it helps me to understand how to solve TCACoordinators scoping problem for new TCA version, and I created this topic. Also my function proposal is similar that added in #3657 but it's more generic and not constrained to StackState and StackAction.
Because TCACoordinators is not using StackState and StackAction (it using swift Array to hold navigation stack) i can't use scope function added in #3657.
I suppose that my proposal can be used in both scenarios for TCACoordinators and #3657 case.