How to compose promises with Ramda

ramda-promises-compose.js

// Custom promise-based compose
const composeWithPromise = (...args) =>
  R.composeWith((f, val) => {
    if (val && val.then) {
      return val.then(f);
    }
    if (Array.isArray(val) && val.length && val[0] && val[0].then) {
      return Promise.all(val).then(f);
    }
    return f(val);
  })(args);

/**************************/
/******** EXAMPLE *********/
/**************************/

const getRandomLimit = (min, max) => Math.random() * (max - min) + min;

const fetchPokemonList = limit => {
  return fetch(`https://pokeapi.co/api/v2/pokemon?limit=${limit}`).then(r =>
    r.json()
  );
};

const fetchPokemon = name => {
  return fetch(`https://pokeapi.co/api/v2/pokemon/${name}/`).then(r => {
    return r.json();
    // Uncomment the line below to see error handling
    // throw new Error("Simulated error");
  });
};

const cleanupPokemonData = data => ({
  id: data.id,
  name: data.name,
  weight: data.weight
});

////////////////////////////////////
// Built-in promise-based compose //
////////////////////////////////////
const getPokemons = R.composeP(
  R.map(cleanupPokemonData),
  Promise.all.bind(Promise),
  R.map(fetchPokemon),
  R.map(x => x.name),
  R.prop("results"),
  fetchPokemonList
  // getRandomLimit --> this doesn't work because it doesn't return Promise
);

getPokemons(3, 10)
  .then(r => console.log("Here are your pokemons:", r))
  .catch(err => console.log("Catched an error:", err));

////////////////////
// Custom compose //
////////////////////

const getPokemonsCustom = composeWithPromise(
  R.map(cleanupPokemonData),
  Promise.all.bind(Promise),
  R.map(fetchPokemon),
  R.map(x => x.name),
  R.prop("results"),
  fetchPokemonList,
  getRandomLimit
);

getPokemonsCustom(3, 10)
  .then(r => console.log("[custom] Here are your pokemons:", r))
  .catch(err => console.log("[custom] Catched an error:", err));

Great stuff thanks!

I have added some types to your function as well as specific handling for the last call. For the scenario where the return value is an array of promises. In that case, the function does a Promise.all() before returning, which changes the return type from e.g.:

Promise<number>[]

to

Promise<number[]>

It's a choice, but since the same choice is made for intermediate calls, it makes it cleaner.

here's the code

You can also find it in the package npm swiss-army-knifey