maybe.js

const R = require('ramda'),
  prop = R.prop,
  path = R.path,
  curry = R.curry;

// Test Data
let user1 = {
  id: 1,
  login: 'andrew',
  details: {
    address: {
      city: 'toronto'
    }
  }
}

let user2 = {
  id: 1,
  login: 'andrew',
  details: {
    address: null
  }
}

/**
 * Maybe monad
 *
 * @class Maybe
 */
class Maybe {
  constructor(value) {
    this.value = value
  }

  static of(value) {
    return new Maybe(value);
  }

  get isNothing() {
    return (this.value === null || this.value === undefined);
  }

  map(f) {
    return this.isNothing ? Maybe.of(null) : Maybe.of(f(this.value));
  }

  join() {
    return this.value;
  }

  chain(f) {
    return this.map(f).join();
  }

  orElse(defaultValue) {
    return this.isNothing ? Maybe.of(defaultValue) : this;
  }

  ap(someOtherMaybe) {
    return someOtherMaybe.map(this.value);
  }
}

// ---------------------
// Tests

Maybe.of(34).map(R.identity).chain(R.identity) /*?*/

var a = Maybe.of(user2)
  .map(prop('details'))
  .map(prop('address'))
  .map(prop('city'))
  .chain(R.identity) /*?*/
  .join()

Maybe.of(user2)
  .map(path(['details', 'address', 'city']))
  .chain(R.identity) /*?*/
  // .join() /*?*/

// Maybe.of(user1)
//   .map(path(['details', 'address', 'city']))
//   .chain(Maybe.of(R.identity)) /*?*/
//   .orElse(1) /*?*/

// Maybe.of(user2)
//   .map(prop('details'))
//   .map(prop('address'))
//   .map(prop('city'))
//   .orElse(1) /*?*/

// const add = curry((a, b) => a + b)

// add(10, 2) /*?*/

// Maybe.of(10).map(add) /*?*/
// Maybe.of(10).map(add).ap(Maybe.of(10)) /*?*/