cityBikes.js

const API_BASE = "https://api.citybik.es";

const fetchP = require("node-fetch");
const {
  Async,
  pipeK,
  propOr,
} = require("crocks");

const fetch = Async.fromPromise(fetchP);
const json = response =>
  Async((rej, res) =>
    response
      .json()
      .then(res)
      .catch(res)
  );
const fetchJson = pipeK(fetch, json);

const fetchNetworks = () => fetchJson(API_BASE + "/v2/networks");
const fetchNetworkById = id =>
  fetchJson(API_BASE + "/v2/networks/" + id).map(propOr({}, "network"));

module.exports = {
  fetchNetworks,
  fetchNetworkById
}

safer-programming-ex-4.js

const { fetchNetworks, fetchNetworkById } = require("./cityBikes");
const {
  Async,
  Pair,
  Pred,
  Sum,
  assign,
  bimap,
  concat,
  fanout,
  filter,
  isString,
  map,
  mapProps,
  mapReduce,
  merge,
  pick,
  pipe,
  propOr,
  propPathOr,
  traverse,
  valueOf
} = require("crocks");

/**
 * Using the CityBikes API (http://api.citybik.es/v2/):
 * 1. Get all networks and filter down to just those in GB
 * 2. Get all stations for each of the GB networks
 * 3. Sum the number of free_bikes for each network
 * 4. Output a list sorted by number of free bikes
 */

const isGb = Pred(isString)
  .concat(Pred(x => x === "GB"))
  .contramap(propPathOr(null, ["location", "country"]));

const calculateFreeBikes = pipe(
  fanout(
    pick(["name", "location"]),
    pipe(
      propOr([], "stations"),
      mapReduce(
        s => Pair(Sum(s.free_bikes), Sum(s.empty_slots)),
        concat,
        Pair(Sum(0), Sum(0))
      ),
      bimap(valueOf, valueOf),
      p => ({ freeBikes: p.fst(), emptySlots: p.snd() })
    )
  ),
  merge(assign),
  mapProps({
    location: propOr("", "city")
  })
);

const getNetworks = pipe(
  propOr([], "networks"),
  filter(isGb),
  traverse(
    Async,
    pipe(propOr("", "id"), fetchNetworkById, map(calculateFreeBikes))
  )
);

const getFreeBikes = propOr(0, "freeBikes");
const sortByFreeBikes = n =>
  n.sort((a, b) => getFreeBikes(b) - getFreeBikes(a));

fetchNetworks()
  .chain(getNetworks)
  .map(sortByFreeBikes)
  .fork(console.error, console.log);