Slightly different variation of https://gist.github.com/minedeljkovic/7347c0b528110889aa50

app.js

import {createStore, applyMiddleware} from 'redux';
import {createEffectCapableStore} from 'redux-side-effects';
import createLogger from 'redux-logger';

import userRegistration, {START_COUNTRY_SELECTION} from './userRegistration';
import {CHANGE_SELECTION, CONFIRM_SELECTION} from './countrySelection';

const storeFactory = createEffectCapableStore(applyMiddleware(createLogger())(createStore));
const  store = storeFactory(userRegistration);

// scenario that should select a country for a user
store.dispatch({ type: START_COUNTRY_SELECTION });
store.dispatch({ type: CHANGE_SELECTION, payload: 1 });
store.dispatch({ type: CONFIRM_SELECTION });
// expected state:
// {
//   "user": {
//     "countryId": 1
//   },
//   "countrySelection": null
// }

/*
// scenario that should raise a warning based on state in countrySelection slice
store.dispatch({ type: START_COUNTRY_SELECTION });
store.dispatch({ type: CONFIRM_SELECTION });
// expected state:
// {
//   "user": {     
//   },
//   "countrySelection": {
//     "selectedCountryId": null,
//     "showNoSelectionWarning": true
//   }
// }
*/

countrySelection.js

export const CHANGE_SELECTION = 'country-selection/CHANGE_SELECTION';
export const CONFIRM_SELECTION = 'country-selection/CONFIRM_SELECTION';

export default function *countrySelection(state = { selectedCountryId: null, showNoSelectionWarning: false }, action = {}, props = {}) {
  switch (action.type) {
  case CHANGE_SELECTION:
    return {
      ...state,
      selectedCountryId: action.payload
    };
  case CONFIRM_SELECTION:
    if (state.selectedCountryId == null) return {...state, showNoSelectionWarning: true}; 
    
    yield dispatch => dispatch({type: props.selectionSuccessActionType, payload: state.selectedCountryId});
    return state;
  default:
    return state;
  }
}

userRegistration.js

import countrySelection from './countrySelection';

export const COUNTRY_SELECTION_SUCCESS = 'user-registration/COUNTRY_SELECTION_SUCCESS';
export const START_COUNTRY_SELECTION = 'user-registration/START_COUNTRY_SELECTION';

export default function *userRegistration(state = { user: {}, countrySelection: null }, action = {}) {
  switch (action.type) {
  case START_COUNTRY_SELECTION:
    return {
      ...state,
      countrySelection: yield* countrySelection(undefined, action, {selectionSuccessActionType: COUNTRY_SELECTION_SUCCESS})
    };
  case COUNTRY_SELECTION_SUCCESS:
    return {
      ...state,
      countrySelection: null,
      user: {
        ...state.user,
        countryId: action.payload
      }
    };
  default:
    return {
      ...state,
      countrySelection: state.countrySelection && (yield* countrySelection(state.countrySelection, action, {selectionSuccessActionType: COUNTRY_SELECTION_SUCCESS}))
    };
  }
}