Improve type inference of case reducers

Previously, the TypeScript compiler would reject case reducer maps
with different incompatible PayloadAction types. The case reducers
map and createReducer() / createSlice() types have now been
restructured to allow for better type inference.

Fixes #131

Author: denisw

src/createReducer.ts

@@ -1,6 +1,11 @@
 import createNextState, { Draft } from 'immer'
 import { AnyAction, Action, Reducer } from 'redux'
 
+/**
+ * Defines a mapping from action types to corresponding action object shapes.
+ */
+export type Actions<T extends keyof any = string> = Record<T, Action>
+
 /**
  * An *case reducer* is a reducer function for a speficic action type. Case
  * reducers can be composed to full reducers using `createReducer()`.
@@ -23,8 +28,8 @@ export type CaseReducer<S = any, A extends Action = AnyAction> = (
 /**
  * A mapping from action types to case reducers for `createReducer()`.
  */
-export interface CaseReducersMapObject<S = any, A extends Action = AnyAction> {
-  [actionType: string]: CaseReducer<S, A>
+export type CaseReducers<S, AS extends Actions> = {
+  [T in keyof AS]: AS[T] extends Action ? CaseReducer<S, AS[T]> : void
 }
 
 /**
@@ -43,17 +48,17 @@ export interface CaseReducersMapObject<S = any, A extends Action = AnyAction> {
  * @param actionsMap A mapping from action types to action-type-specific
  *   case redeucers.
  */
-export function createReducer<S = any, A extends Action = AnyAction>(
-  initialState: S,
-  actionsMap: CaseReducersMapObject<S, A>
-): Reducer<S> {
+export function createReducer<
+  S,
+  CR extends CaseReducers<S, any> = CaseReducers<S, any>
+>(initialState: S, actionsMap: CR): Reducer<S> {
   return function(state = initialState, action): S {
     // @ts-ignore createNextState() produces an Immutable<Draft<S>> rather
     // than an Immutable<S>, and TypeScript cannot find out how to reconcile
     // these two types.
     return createNextState(state, (draft: Draft<S>) => {
       const caseReducer = actionsMap[action.type]
-      return caseReducer ? caseReducer(draft, action as A) : undefined
+      return caseReducer ? caseReducer(draft, action) : undefined
     })
   }
 }

src/createSlice.test.ts

@@ -1,12 +1,13 @@
 import { createSlice } from './createSlice'
-import { createAction } from './createAction'
+import { createAction, PayloadAction } from './createAction'
 
 describe('createSlice', () => {
   describe('when slice is empty', () => {
     const { actions, reducer, selectors } = createSlice({
       reducers: {
         increment: state => state + 1,
-        multiply: (state, action) => state * action.payload
+        multiply: (state, action: PayloadAction<number>) =>
+          state * action.payload
       },
       initialState: 0
     })

src/createSlice.ts

@@ -1,16 +1,17 @@
-import { Action, AnyAction, Reducer } from 'redux'
+import { Reducer } from 'redux'
 import { createAction, PayloadAction } from './createAction'
-import { createReducer, CaseReducersMapObject } from './createReducer'
+import { createReducer, CaseReducers } from './createReducer'
 import { createSliceSelector, createSelectorName } from './sliceSelector'
 
 /**
  * An action creator atttached to a slice.
  */
-export type SliceActionCreator<P> = (payload: P) => PayloadAction<P>
+export type SliceActionCreator<P> = P extends void
+  ? () => PayloadAction<void>
+  : (payload: P) => PayloadAction<P>
 
 export interface Slice<
   S = any,
-  A extends Action = AnyAction,
   AP extends { [key: string]: any } = { [key: string]: any }
 > {
   /**
@@ -21,7 +22,7 @@ export interface Slice<
   /**
    * The slice's reducer.
    */
-  reducer: Reducer<S, A>
+  reducer: Reducer<S>
 
   /**
    * Action creators for the types of actions that are handled by the slice
@@ -43,9 +44,7 @@ export interface Slice<
  */
 export interface CreateSliceOptions<
   S = any,
-  A extends Action = AnyAction,
-  CR extends CaseReducersMapObject<S, A> = CaseReducersMapObject<S, A>,
-  CR2 extends CaseReducersMapObject<S, A> = CaseReducersMapObject<S, A>
+  CR extends CaseReducers<S, any> = CaseReducers<S, any>
 > {
   /**
    * The slice's name. Used to namespace the generated action types and to
@@ -70,19 +69,15 @@ export interface CreateSliceOptions<
    * functions. These reducers should have existing action types used
    * as the keys, and action creators will _not_ be generated.
    */
-  extraReducers?: CR2
+  extraReducers?: CaseReducers<S, any>
 }
 
-type ExtractPayloads<
-  S,
-  A extends PayloadAction,
-  CR extends CaseReducersMapObject<S, A>
-> = {
-  [type in keyof CR]: CR[type] extends (state: S) => any
+type CaseReducerActionPayloads<CR extends CaseReducers<any, any>> = {
+  [T in keyof CR]: CR[T] extends (state: any) => any
     ? void
-    : (CR[type] extends (state: S, action: PayloadAction<infer P>) => any
+    : (CR[T] extends (state: any, action: PayloadAction<infer P>) => any
         ? P
-        : never)
+        : void)
 }
 
 function getType(slice: string, actionKey: string): string {
@@ -97,13 +92,9 @@ function getType(slice: string, actionKey: string): string {
  *
  * The `reducer` argument is passed to `createReducer()`.
  */
-export function createSlice<
-  S = any,
-  A extends PayloadAction = PayloadAction<any>,
-  CR extends CaseReducersMapObject<S, A> = CaseReducersMapObject<S, A>
->(
-  options: CreateSliceOptions<S, A, CR>
-): Slice<S, A, ExtractPayloads<S, A, CR>> {
+export function createSlice<S, CR extends CaseReducers<S, any>>(
+  options: CreateSliceOptions<S, CR>
+): Slice<S, CaseReducerActionPayloads<CR>> {
   const { slice = '', initialState } = options
   const reducers = options.reducers || {}
   const extraReducers = options.extraReducers || {}

type-tests/files/createReducer.typetest.ts

@@ -23,7 +23,7 @@ import { AnyAction, createReducer, Reducer } from 'redux-starter-kit'
 }
 
 /**
- * Test: createReducer() type parameters can be specified expliclity.
+ * Test: createReducer() state type can be specified expliclity.
  */
 {
   type CounterAction =
@@ -36,19 +36,13 @@ import { AnyAction, createReducer, Reducer } from 'redux-starter-kit'
   const decrementHandler = (state: number, action: CounterAction) =>
     state - action.payload
 
-  createReducer<number, CounterAction>(0, {
+  createReducer<number>(0, {
     increment: incrementHandler,
     decrement: decrementHandler
   })
 
   // typings:expect-error
-  createReducer<string, CounterAction>(0, {
-    increment: incrementHandler,
-    decrement: decrementHandler
-  })
-
-  // typings:expect-error
-  createReducer<number, AnyAction>(0, {
+  createReducer<string>(0, {
     increment: incrementHandler,
     decrement: decrementHandler
   })