declarative-fetch.js

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

function* fetchThing(action) {
   try {
      const thing = yield call(fetch, `https://api.example.com/things/${action.payload.thingId}`);
      yield put({ type: "THING_RECEIVED", thing });
   } catch(err) {
      yield put({ type: "THING_GETTING_FAILED", message: err.message});
   }
}

export function* thingRequestSaga() {
  yield* takeLatest("THING_GET_REQUESTED", fetchThing);
}