chiefGui

const todos = (state = INITIAL_STATE, action) => reswitch(
  ADD_TODO,
    [...state.todos, action.todo],

  REMOVE_TODO,
    () => state.todos.filter(todo => todo.id !== action.todo.id)
)(state, action.type)

chiefGui

import reswitch from 'reswitch'
import {USERS_GET, USERS_GET__SUCCESS, USERS_GET__FAILURE} from 'consts/users'

const INITIAL_STATE = {areLoading: false, hasError: false, users: null}

const users(state = INITIAL_STATE, action) => reswitch(
  USERS_GET,
    {...defaultState, areLoading: true},

  USERS_GET__SUCCESS,