我想按照这个代码在redux-saga

export const getUser = (state, login) => state.entities.users[login]
export const getRepo = (state, fullName) => state.entities.repos[fullName]

然后像这样在传奇中使用它：

import { getUser } from '../reducers/selectors'

// load user unless it is cached
function* loadUser(login, requiredFields) {
  const user = yield select(getUser, login)
  if (!user || requiredFields.some(key => !user.hasOwnProperty(key))) {
    yield call(fetchUser, login)
  }
}

该getUser减速器（甚至是减速器）看起来与我通常期望的减速器看起来非常不同。

谁能解释选择器是什么，以及getUserreduce-reducer以及reducer如何适合redux-saga？