Parallel API Calls with `all` Effect in Redux-Saga

Illustrate how to perform multiple API calls in parallel by leveraging the `all` effect in Redux-Saga, effectively improving the efficiency of fetching data when multiple pieces of data can be requested concurrently.

import { call, put, all } from 'redux-saga/effects';

First, import the necessary functions from redux-saga. `call` is used to call the API, `put` to dispatch actions, and `all` to run calls in parallel.

function* fetchUserData(action) { 
  try { 
    const [user, posts] = yield all([ 
      call(api.fetchUser, action.userId), 
      call(api.fetchPosts, action.userId) 
    ]); 
    yield put({type: 'FETCH_USER_DATA_SUCCESS', user, posts}); 
  } catch (error) { 
    yield put({type: 'FETCH_USER_DATA_FAILURE', error}); 
  } 
}

This generator function (saga) fetches user data and posts in parallel by using the `all` effect. `call` invokes the API functions, and the results are destructured into `user` and `posts`. `put` dispatches the success or failure actions.