combineReducersが辛い ref: http://qiita.com/ryo33/items/4f50ca9dd38ad47dc3ea

file0.js

// State
{
  todos: {
    todoA: [],
    todoB: [],
    todoC: []
  },
  currentTodoKey: "todoA"
}

// Reducer
const reducer = (state = { todos: {}, currentTodoKey: "default" }, action) => {
  const addTodo = (state, key, todo) {
    return Object.assign({}, state, {
      todos: Object.assign({}, state.todos, {
        [key]: (state.todos[key] || []).concat(todo)
      })
    })
  }

  switch (action.type) {
    case 'ADD_TODO':
      var { key, todo } = action.payload
      return addTodo(state, key, todo)
    case 'ADD_TODO_TO_CURRENT_KEY':
      var { todo } = action.payload
      return addTodo(state, state.currentTodoKey, todo)
    case 'UPDATE_CURRENT_KEY':
      var { newKey } = action.payload
      return Object.assign({}, state, {
        currentTodoKey: newKey
      })
    default:
      return state
  }
}

file1.js

const todos = (state = {}, action, entireState) => {
  switch (action.type) {
    case 'ADD_TODO':
      var { key, todo } = action.payload
      return Object.assign({}, state, {
        [key]: (state[key] || []).concat(todo)
      })
    case 'ADD_TODO_TO_CURRENT_KEY':
      var { todo } = action.payload
      var key = entireState.currentTodoKey
      return Object.assign({}, state, {
        [key]: (state[key] || []).concat(todo)
      })
    default:
      return state
  }
}

const currentTodoKey = (state = "default", action) => {
  switch (action.type) {
    case 'UPDATE_CURRENT_KEY':
      var { newKey } = action.payload
      return newKey
    default:
      return state
  }
}

const reducer = (state, action) => ({
  todos: todos(state.todos, action, state),
  currentTodoKey: currentTodoKey(state.currentTodoKey, action)
})

file2.js

const reducer = (state, action) => ({
  todos: todos(state.todos, action, state),
  currentTodoKey: currentTodoKey(state.currentTodoKey, action)
})

file3.js

import combineSectionReducers from 'combine-section-reducers'

const reducer = combineSectionReducers({
  todo, currentTodoKey
})