redux: Use type inference in reducers.

Cutting these explicit type annotations makes our reducer modules a
lot shorter and simpler to read, which I'm happy about.

A crucial ingredient in making this work is that Flow now understands
this code, following our recent fixes to reducer and action types:

  7ea0b89 redux: Fix thoroughly-bogus type signatures on most reducers.
  ea2b119 actions: Make EventAction complete.
  e6868d8 actions: Clarify to Flow that these constants are type-tags.
  ... and some neighboring commits.

In particular, this change means zero loss of type coverage.
Both before and after, `yarn test:flow-coverage` shows:

    Percent    Total    Covered    Uncovered
      94 %     28871     27173        1698

---

To perform this refactor automatically, I wrote the following commands:

    perl -i -0pe '
        s{^ const \s+ \w+ \s* = \s* \(
          \K \s* state (: \s* \w+)? ,
             \s* action (: \s* [^)]+)? ,?
             \s* \)(:
             \s* \w+State)? }
         {state, action)}xmsg;
        s{^ ( import\ type\ .*? )
            (?: ,? \s+ \w+Action )+ \b
            ( [^\}]* \}\ from\ .\. ) }
         {$1$2}xmsg
        ' \
      src/*/*Reducer*.js

    tools/fmt

Author: gnprice

src/account/accountsReducers.js

@@ -9,23 +9,12 @@ import {
   ACCOUNT_REMOVE,
 } from '../actionConstants';
 
-import type {
-  AccountsState,
-  Identity,
-  Action,
-  RealmAddAction,
-  AccountSwitchAction,
-  AccountRemoveAction,
-  AckPushTokenAction,
-  UnackPushTokenAction,
-  LoginSuccessAction,
-  LogoutAction,
-} from '../types';
+import type { AccountsState, Identity, Action } from '../types';
 import { NULL_ARRAY } from '../nullObjects';
 
 const initialState = NULL_ARRAY;
 
-const realmAdd = (state: AccountsState, action: RealmAddAction): AccountsState => {
+const realmAdd = (state, action) => {
   const accountIndex = state.findIndex(account => account.realm === action.realm);
 
   if (accountIndex !== -1) {
@@ -43,7 +32,7 @@ const realmAdd = (state: AccountsState, action: RealmAddAction): AccountsState =
   ];
 };
 
-const accountSwitch = (state: AccountsState, action: AccountSwitchAction): AccountsState => {
+const accountSwitch = (state, action) => {
   if (action.index === 0) {
     return state;
   }
@@ -58,7 +47,7 @@ const findAccount = (state: AccountsState, identity: Identity): number => {
   );
 };
 
-const loginSuccess = (state: AccountsState, action: LoginSuccessAction): AccountsState => {
+const loginSuccess = (state, action) => {
   const { realm, email, apiKey } = action;
   const accountIndex = findAccount(state, { realm, email });
   if (accountIndex === -1) {
@@ -71,7 +60,7 @@ const loginSuccess = (state: AccountsState, action: LoginSuccessAction): Account
   ];
 };
 
-const ackPushToken = (state: AccountsState, action: AckPushTokenAction): AccountsState => {
+const ackPushToken = (state, action) => {
   const { pushToken: ackedPushToken, identity } = action;
   const accountIndex = findAccount(state, identity);
   if (accountIndex === -1) {
@@ -84,7 +73,7 @@ const ackPushToken = (state: AccountsState, action: AckPushTokenAction): Account
   ];
 };
 
-const unackPushToken = (state: AccountsState, action: UnackPushTokenAction): AccountsState => {
+const unackPushToken = (state, action) => {
   const { identity } = action;
   const accountIndex = findAccount(state, identity);
   if (accountIndex === -1) {
@@ -97,12 +86,9 @@ const unackPushToken = (state: AccountsState, action: UnackPushTokenAction): Acc
   ];
 };
 
-const logout = (state: AccountsState, action: LogoutAction): AccountsState => [
-  { ...state[0], apiKey: '' },
-  ...state.slice(1),
-];
+const logout = (state, action) => [{ ...state[0], apiKey: '' }, ...state.slice(1)];
 
-const accountRemove = (state: AccountsState, action: AccountRemoveAction): AccountsState => {
+const accountRemove = (state, action) => {
   const newState = state.slice();
   newState.splice(action.index, 1);
   return newState;

src/alertWords/alertWordsReducer.js

@@ -1,15 +1,13 @@
 /* @flow strict-local */
-import type { AlertWordsState, Action, RealmInitAction, EventAlertWordsAction } from '../types';
+import type { AlertWordsState, Action } from '../types';
 import { REALM_INIT, INIT_ALERT_WORDS } from '../actionConstants';
 import { NULL_ARRAY } from '../nullObjects';
 
 const initialState = NULL_ARRAY;
 
-const realmInit = (state: AlertWordsState, action: RealmInitAction): AlertWordsState =>
-  action.data.alert_words || initialState;
+const realmInit = (state, action) => action.data.alert_words || initialState;
 
-const initAlertWords = (state: AlertWordsState, action: EventAlertWordsAction): AlertWordsState =>
-  action.alertWords || initialState;
+const initAlertWords = (state, action) => action.alertWords || initialState;
 
 export default (state: AlertWordsState = initialState, action: Action): AlertWordsState => {
   switch (action.type) {

src/caughtup/caughtUpReducers.js

@@ -1,5 +1,5 @@
 /* @flow strict-local */
-import type { CaughtUpState, Action, MessageFetchCompleteAction } from '../types';
+import type { CaughtUpState, Action } from '../types';
 import {
   DEAD_QUEUE,
   LOGOUT,
@@ -12,10 +12,7 @@ import { NULL_CAUGHTUP, NULL_OBJECT } from '../nullObjects';
 
 const initialState: CaughtUpState = NULL_OBJECT;
 
-const messageFetchComplete = (
-  state: CaughtUpState,
-  action: MessageFetchCompleteAction,
-): CaughtUpState => {
+const messageFetchComplete = (state, action) => {
   const key = JSON.stringify(action.narrow);
 
   if (action.anchor === LAST_MESSAGE_ANCHOR) {

src/chat/fetchingReducers.js

@@ -1,10 +1,5 @@
 /* @flow strict-local */
-import type {
-  FetchingState,
-  Action,
-  MessageFetchStartAction,
-  MessageFetchCompleteAction,
-} from '../types';
+import type { FetchingState, Action } from '../types';
 import {
   DEAD_QUEUE,
   LOGOUT,
@@ -18,10 +13,7 @@ import { NULL_FETCHING, NULL_OBJECT } from '../nullObjects';
 
 const initialState: FetchingState = NULL_OBJECT;
 
-const messageFetchStart = (
-  state: FetchingState,
-  action: MessageFetchStartAction,
-): FetchingState => {
+const messageFetchStart = (state, action) => {
   const key = JSON.stringify(action.narrow);
   const currentValue = state[key] || NULL_FETCHING;
 
@@ -34,10 +26,7 @@ const messageFetchStart = (
   };
 };
 
-const messageFetchComplete = (
-  state: FetchingState,
-  action: MessageFetchCompleteAction,
-): FetchingState => {
+const messageFetchComplete = (state, action) => {
   const key = JSON.stringify(action.narrow);
   const currentValue = state[key] || NULL_FETCHING;
 

src/chat/flagsReducers.js

@@ -1,13 +1,5 @@
 /* @flow strict-local */
-import type {
-  Action,
-  FlagsState,
-  Message,
-  MessageFetchCompleteAction,
-  EventNewMessageAction,
-  EventUpdateMessageFlagsAction,
-  MarkMessagesReadAction,
-} from '../types';
+import type { Action, FlagsState, Message } from '../types';
 import {
   DEAD_QUEUE,
   MESSAGE_FETCH_COMPLETE,
@@ -88,16 +80,12 @@ const processFlagsForMessages = (state: FlagsState, messages: Message[]): FlagsS
   return stateChanged ? deeperMerge(state, newState) : state;
 };
 
-const messageFetchComplete = (state: FlagsState, action: MessageFetchCompleteAction): FlagsState =>
-  processFlagsForMessages(state, action.messages);
+const messageFetchComplete = (state, action) => processFlagsForMessages(state, action.messages);
 
-const eventNewMessage = (state: FlagsState, action: EventNewMessageAction): FlagsState =>
+const eventNewMessage = (state, action) =>
   addFlagsForMessages(state, [action.message.id], action.message.flags);
 
-const eventUpdateMessageFlags = (
-  state: FlagsState,
-  action: EventUpdateMessageFlagsAction,
-): FlagsState => {
+const eventUpdateMessageFlags = (state, action) => {
   if (action.all) {
     return addFlagsForMessages(initialState, Object.keys(action.allMessages).map(Number), ['read']);
   }
@@ -113,8 +101,7 @@ const eventUpdateMessageFlags = (
   return state;
 };
 
-const markMessagesRead = (state: FlagsState, action: MarkMessagesReadAction): FlagsState =>
-  addFlagsForMessages(state, action.messageIds, ['read']);
+const markMessagesRead = (state, action) => addFlagsForMessages(state, action.messageIds, ['read']);
 
 export default (state: FlagsState = initialState, action: Action): FlagsState => {
   switch (action.type) {

src/chat/narrowsReducers.js

@@ -1,14 +1,7 @@
 /* @flow strict-local */
 import union from 'lodash.union';
 
-import type {
-  NarrowsState,
-  Action,
-  MessageFetchCompleteAction,
-  EventNewMessageAction,
-  EventMessageDeleteAction,
-  EventUpdateMessageFlagsAction,
-} from '../types';
+import type { NarrowsState, Action } from '../types';
 import {
   DEAD_QUEUE,
   LOGOUT,
@@ -25,10 +18,7 @@ import { NULL_OBJECT } from '../nullObjects';
 
 const initialState: NarrowsState = NULL_OBJECT;
 
-const messageFetchComplete = (
-  state: NarrowsState,
-  action: MessageFetchCompleteAction,
-): NarrowsState => {
+const messageFetchComplete = (state, action) => {
   const key = JSON.stringify(action.narrow);
   const fetchedMessageIds = action.messages.map(message => message.id);
   const replaceExisting =
@@ -41,7 +31,7 @@ const messageFetchComplete = (
   };
 };
 
-const eventNewMessage = (state: NarrowsState, action: EventNewMessageAction): NarrowsState => {
+const eventNewMessage = (state, action) => {
   let stateChange = false;
   const newState = Object.keys(state).reduce((msg, key) => {
     const isInNarrow = isMessageInNarrow(action.message, JSON.parse(key), action.ownEmail);
@@ -59,10 +49,7 @@ const eventNewMessage = (state: NarrowsState, action: EventNewMessageAction): Na
   return stateChange ? newState : state;
 };
 
-const eventMessageDelete = (
-  state: NarrowsState,
-  action: EventMessageDeleteAction,
-): NarrowsState => {
+const eventMessageDelete = (state, action) => {
   let stateChange = false;
   const newState = Object.keys(state).reduce((updatedState, key) => {
     updatedState[key] = state[key].filter(id => id !== action.messageId);
@@ -72,10 +59,7 @@ const eventMessageDelete = (
   return stateChange ? newState : state;
 };
 
-const eventUpdateMessageFlags = (
-  state: NarrowsState,
-  action: EventUpdateMessageFlagsAction,
-): NarrowsState => {
+const eventUpdateMessageFlags = (state, action) => {
   const { messages, flag, operation } = action;
   const messagesSet = new Set(messages);
   const updates = [];

src/drafts/draftsReducers.js

@@ -1,11 +1,11 @@
 /* @flow strict-local */
-import type { DraftState, Action, DraftUpdateAction } from '../types';
+import type { DraftState, Action } from '../types';
 import { DRAFT_UPDATE, LOGOUT } from '../actionConstants';
 import { NULL_OBJECT } from '../nullObjects';
 
 const initialState = NULL_OBJECT;
 
-const draftUpdate = (state: DraftState, action: DraftUpdateAction): DraftState => {
+const draftUpdate = (state, action) => {
   const narrowStr = JSON.stringify(action.narrow);
 
   if (action.content.trim().length === 0) {

src/loading/loadingReducers.js

@@ -1,12 +1,5 @@
 /* @flow strict-local */
-import type {
-  LoadingState,
-  Action,
-  InitialFetchStartAction,
-  InitialFetchCompleteAction,
-  InitStreamsAction,
-  InitSubscriptionsAction,
-} from '../types';
+import type { LoadingState, Action } from '../types';
 import {
   DEAD_QUEUE,
   ACCOUNT_SWITCH,
@@ -24,31 +17,28 @@ const initialState: LoadingState = {
   users: false,
 };
 
-const initialFetchStart = (state: LoadingState, action: InitialFetchStartAction): LoadingState => ({
+const initialFetchStart = (state, action) => ({
   ...state,
   presence: true,
   subscriptions: true,
   unread: true,
   users: true,
 });
 
-const initialFetchComplete = (
-  state: LoadingState,
-  action: InitialFetchCompleteAction,
-): LoadingState => ({
+const initialFetchComplete = (state, action) => ({
   ...state,
   presence: false,
   subscriptions: false,
   unread: false,
   users: false,
 });
 
-const initStreams = (state: LoadingState, action: InitStreamsAction): LoadingState => ({
+const initStreams = (state, action) => ({
   ...state,
   streams: false,
 });
 
-const initSubscriptions = (state: LoadingState, action: InitSubscriptionsAction): LoadingState => ({
+const initSubscriptions = (state, action) => ({
   ...state,
   subscriptions: false,
 });