Add ReducerReader for building reducers out of state and action

Sources/ComposableArchitecture/Documentation.docc/Extensions/ReducerProtocol.md

@@ -24,6 +24,7 @@
 - ``Reduce``
 - ``CombineReducers``
 - ``EmptyReducer``
+- ``ReducerReader``
 - ``BindingReducer``
 
 ### Reducer modifiers

Sources/ComposableArchitecture/Reducer/Reducers/ReducerReader.swift

@@ -0,0 +1,24 @@
+/// A reducer that builds a reducer from the current state and action.
+public struct ReducerReader<State, Action, Reader: ReducerProtocol>: ReducerProtocol
+where Reader.State == State, Reader.Action == Action {
+  @usableFromInline
+  let reader: (State, Action) -> Reader
+
+  /// Initializes a reducer that builds a reducer from the current state and action.
+  ///
+  /// - Parameter reader: A reducer builder that has access to the current state and action.
+  @inlinable
+  public init(@ReducerBuilder<State, Action> _ reader: @escaping (State, Action) -> Reader) {
+    self.init(internal: reader)
+  }
+
+  @usableFromInline
+  init(internal reader: @escaping (State, Action) -> Reader) {
+    self.reader = reader
+  }
+
+  @inlinable
+  public func reduce(into state: inout State, action: Action) -> EffectTask<Action> {
+    self.reader(state, action).reduce(into: &state, action: action)
+  }
+}

Tests/ComposableArchitectureTests/ReducerBuilderTests.swift

@@ -191,20 +191,32 @@ private struct Root: ReducerProtocol {
 
     #if swift(>=5.7)
       var body: some ReducerProtocol<State, Action> {
-        Scope(state: /State.featureA, action: /Action.featureA) {
-          Feature()
-        }
-        Scope(state: /State.featureB, action: /Action.featureB) {
-          Feature()
+        ReducerReader { state, _ in
+          switch state {
+          case .featureA:
+            Scope(state: /State.featureA, action: /Action.featureA) {
+              Feature()
+            }
+          case .featureB:
+            Scope(state: /State.featureB, action: /Action.featureB) {
+              Feature()
+            }
+          }
         }
       }
     #else
       var body: Reduce<State, Action> {
-        Scope(state: /State.featureA, action: /Action.featureA) {
-          Feature()
-        }
-        Scope(state: /State.featureB, action: /Action.featureB) {
-          Feature()
+        ReducerReader { state, _ in
+          switch state {
+          case .featureA:
+            Scope(state: /State.featureA, action: /Action.featureA) {
+              Feature()
+            }
+          case .featureB:
+            Scope(state: /State.featureB, action: /Action.featureB) {
+              Feature()
+            }
+          }
         }
       }
     #endif

Tests/ComposableArchitectureTests/ReducerReaderTests.swift

@@ -0,0 +1,36 @@
+import ComposableArchitecture
+import XCTest
+
+@MainActor
+final class StateReaderTests: XCTestCase {
+  func testDependenciesPropagate() async {
+    struct Feature: ReducerProtocol {
+      struct State: Equatable {}
+
+      enum Action: Equatable {
+        case tap
+      }
+
+      @Dependency(\.date.now) var now
+
+      #if swift(>=5.7)
+        var body: some ReducerProtocol<State, Action> {
+          ReducerReader { _, _ in
+            let _ = self.now
+          }
+        }
+      #else
+        var body: Reduce<State, Action> {
+          ReducerReader { _, _ in
+            let _ = self.now
+          }
+        }
+      #endif
+    }
+
+    let store = TestStore(initialState: Feature.State(), reducer: Feature()) {
+      $0.date.now = Date(timeIntervalSince1970: 1234567890)
+    }
+    await store.send(.tap)
+  }
+}