Sequence.js

/**
 * This method sequences an array of Monads, chaining each one into the next.
 * The result will be one Monad of a list of each unwrapped value.
 */
function sequenceM(Type) {
  return ms => {
    function doIt(mArr, restMs) {
      if(restMs.length === 0) {
        return mArr;
      } else {
        const [m, ...moreMs] = restMs;
        return mArr['fantasy-land/chain'](as => {
          return doIt(m['fantasy-land/map'](aPrime => [...as, aPrime]), moreMs);
        });
      }
    }
    return doIt(Type['fantasy-land/of']([]), ms);
  }
}

/**
 * Maybe from https://evilsoft.github.io/crocks
 */
const Maybe = require('crocks/src/Maybe');

console.log(sequenceM(Maybe)([
  Maybe.Just(5),
  Maybe.Just(3),
  Maybe.Just(6),
  Maybe.Just({foo: 'bar'}),
])) // Just [ 5, 3, 6, { foo: "bar" } ]

console.log(sequenceM(Maybe)([
  Maybe.Just(5),
  Maybe.Just(3),
  Maybe.Nothing(),
  Maybe.Just({foo: 'bar'}),
])) // Nothing


/**
 * Async from https://evilsoft.github.io/crocks
 */
const Async = require('crocks/src/Async');
const delay = ms => x => Async((rej, res) => setTimeout(() => res(x), ms));
const delaySecond = delay(1000);

const subject = 
  sequenceM(Async)([
    delaySecond('a'),
    delaySecond('b'),
    delaySecond('c'),
    delaySecond('d'),
    delaySecond('e'),
  ]).fork(() => {}, console.log) // logs [ 'a', 'b', 'c', 'd', 'e' ] after 5 seconds