/// A protocol that describes how to evolve the current state of an application to the next state,
/// given an action, and describes what ``Effect``s should be executed later by the store, if any.
///
/// Types that conform to this protocol represent the domain, logic and behavior for a feature.
/// Rather than defining a conformance directly, it is more common to use the ``Reducer()`` macro:
///
/// ```swift
/// @Reducer
/// struct Feature {
/// }
/// ```
///
/// The domain of a feature is specified by the "state" and "actions", which can be nested types
/// inside the reducer:
///
/// ```swift
/// @Reducer
/// struct Feature {
///   struct State {
///     var count = 0
///   }
///   enum Action {
///     case decrementButtonTapped
///     case incrementButtonTapped
///   }
///
///   // ...
/// }
/// ```
///
/// The logic of your feature is implemented by mutating the feature's current state when an action
/// comes into the system. This is most easily done by constructing a ``Reduce`` inside the
/// ``body-8lumc`` of your reducer:
///
/// ```swift
/// @Reducer
/// struct Feature {
///   // ...
///
///   var body: some ReducerOf<Self> {
///     Reduce { state, action in
///       switch action {
///       case .decrementButtonTapped:
///         state.count -= 1
///         return .none
///
///       case .incrementButtonTapped:
///         state.count += 1
///         return .none
///       }
///     }
///   }
/// }
/// ```
///
/// The ``Reduce`` reducer's first responsibility is to mutate the feature's current state given an
/// action. Its second responsibility is to return effects that will be executed asynchronously
/// and feed their data back into the system. Currently `Feature` does not need to run any effects,
/// and so ``Effect/none`` is returned.
///
/// If the feature does need to do effectful work, then more would need to be done. For example,
/// suppose the feature has the ability to start and stop a timer, and with each tick of the timer
/// the `count` will be incremented. That could be done like so:
///
/// ```swift
/// @Reducer
/// struct Feature {
///   struct State {
///     var count = 0
///   }
///   enum Action {
///     case decrementButtonTapped
///     case incrementButtonTapped
///     case startTimerButtonTapped
///     case stopTimerButtonTapped
///     case timerTick
///   }
///   enum CancelID { case timer }
///
///   var body: some ReducerOf<Self> {
///     Reduce { state, action in
///       switch action {
///       case .decrementButtonTapped:
///         state.count -= 1
///         return .none
///
///       case .incrementButtonTapped:
///         state.count += 1
///         return .none
///
///       case .startTimerButtonTapped:
///         return .run { send in
///           while true {
///             try await Task.sleep(for: .seconds(1))
///             await send(.timerTick)
///           }
///         }
///         .cancellable(CancelID.timer)
///
///       case .stopTimerButtonTapped:
///         return .cancel(CancelID.timer)
///
///       case .timerTick:
///         state.count += 1
///         return .none
///       }
///     }
///   }
/// }
/// ```
///
/// > Note: This sample emulates a timer by performing an infinite loop with a `Task.sleep`
/// inside. This is simple to do, but is also inaccurate since small imprecisions can accumulate.
/// It would be better to inject a clock into the feature so that you could use its `timer`
/// method. Read the <doc:DependencyManagement> and <doc:Testing> articles for more
/// information.
///
/// That is the basics of implementing a feature as a conformance to ``Reducer``. There is actually
/// an alternative way to define a reducer: you can implement the ``reduce(into:action:)-1t2ri``
/// method directly, which, like ``Reduce``, is given direct mutable access to application ``State``
/// whenever an ``Action`` is fed into the system, and returns an ``Effect`` that can communicate
/// with the outside world and feed additional ``Action``s back into the system.
///
/// At most one of these requirements should be implemented. If a conformance implements both
/// ``body-8lumc`` and ``reduce(into:action:)-1t2ri``, only ``reduce(into:action:)-1t2ri`` will be
/// called by the ``Store``. If your reducer assembles a body from other reducers _and_ has
/// additional business logic it needs to layer onto the feature, introduce this logic into the body
/// instead, either with ``Reduce``:
///
/// ```swift
/// var body: some Reducer<State, Action> {
///   Reduce { state, action in
///     // extra logic
///   }
///   Activity()
///   Profile()
///   Settings()
/// }
/// ```
///
/// …or moving the extra logic to a method that is wrapped in ``Reduce``:
///
/// ```swift
/// var body: some Reducer<State, Action> {
///   Reduce(self.core)
///   Activity()
///   Profile()
///   Settings()
/// }
///
/// func core(state: inout State, action: Action) -> Effect<Action> {
///   // extra logic
/// }
/// ```
///
/// If you are implementing a custom reducer operator that transforms an existing reducer, _always_
/// invoke the ``reduce(into:action:)-1t2ri`` method, never the ``body-swift.property``. For
/// example, this operator that logs all actions sent to the reducer:
///
/// ```swift
/// extension Reducer {
///   func logActions() -> some Reducer<State, Action> {
///     Reduce { state, action in
///       print("Received action: \(action)")
///       return self.reduce(into: &state, action: action)
///     }
///   }
/// }
/// ```
///
public protocol Reducer<State, Action> {
  /// A type that holds the current state of the reducer.
  associatedtype State

  /// A type that holds all possible actions that cause the ``State`` of the reducer to change
  /// and/or kick off a side ``Effect`` that can communicate with the outside world.
  associatedtype Action

  // NB: For Xcode to favor autocompleting `var body: Body` over `var body: Never` we must use a
  //     type alias. We compile it out of release because this workaround is incompatible with
  //     library evolution.
  #if DEBUG
    associatedtype _Body

    /// A type representing the body of this reducer.
    ///
    /// When you create a custom reducer by implementing the ``body-swift.property``, Swift infers
    /// this type from the value returned.
    ///
    /// If you create a custom reducer by implementing the ``reduce(into:action:)-1t2ri``, Swift
    /// infers this type to be `Never`.
    typealias Body = _Body
  #else
    /// A type representing the body of this reducer.
    ///
    /// When you create a custom reducer by implementing the ``body-swift.property``, Swift infers
    /// this type from the value returned.
    ///
    /// If you create a custom reducer by implementing the ``reduce(into:action:)-1t2ri``, Swift
    /// infers this type to be `Never`.
    associatedtype Body
  #endif

  /// Evolves the current state of the reducer to the next state.
  ///
  /// Implement this requirement for "primitive" reducers, or reducers that work on leaf node
  /// features. To define a reducer by combining the logic of other reducers together, implement the
  /// ``body-swift.property`` requirement instead.
  ///
  /// - Parameters:
  ///   - state: The current state of the reducer.
  ///   - action: An action that can cause the state of the reducer to change, and/or kick off a
  ///     side effect that can communicate with the outside world.
  /// - Returns: An effect that can communicate with the outside world and feed actions back into
  ///   the system.
  func reduce(into state: inout State, action: Action) -> Effect<Action>

  /// The content and behavior of a reducer that is composed from other reducers.
  ///
  /// Implement this requirement when you want to incorporate the behavior of other reducers
  /// together.
  ///
  /// Do not invoke this property directly.
  ///
  /// > Important: if your reducer implements the ``reduce(into:action:)-1t2ri`` method, it will
  /// > take precedence over this property, and only ``reduce(into:action:)-1t2ri`` will be called
  /// > by the ``Store``. If your reducer assembles a body from other reducers and has additional
  /// > business logic it needs to layer into the system, introduce this logic into the body
  /// > instead, either with ``Reduce``, or with a separate, dedicated conformance.
  @ReducerBuilder<State, Action>
  var body: Body { get }
}

extension Reducer where Body == Never {
  /// A non-existent body.
  ///
  /// > Warning: Do not invoke this property directly. It will trigger a fatal error at runtime.
  @_transparent
  public var body: Body {
    fatalError(
      """
      '\(Self.self)' has no body. …

      Do not access a reducer's 'body' property directly, as it may not exist. To run a reducer, \
      call 'Reducer.reduce(into:action:)', instead.
      """
    )
  }
}

extension Reducer where Body: Reducer, Body.State == State, Body.Action == Action {
  /// Invokes the ``Body-40qdd``'s implementation of ``reduce(into:action:)-1t2ri``.
  @inlinable
  public func reduce(
    into state: inout Body.State, action: Body.Action
  ) -> Effect<Body.Action> {
    self.body.reduce(into: &state, action: action)
  }
}

/// A convenience for constraining a ``Reducer`` conformance.
///
/// This allows you to specify the `body` of a ``Reducer`` conformance like so:
///
/// ```swift
/// var body: some ReducerOf<Self> {
///   // ...
/// }
/// ```
///
/// …instead of the more verbose:
///
/// ```swift
/// var body: some Reducer<State, Action> {
///   // ...
/// }
/// ```
public typealias ReducerOf<R: Reducer> = Reducer<R.State, R.Action>