Professor Frisby's functional programming exercises (Part 4: monads)

fp.exercises_4.js

require('./support')
const path = require('path')
const expect = require('expect')
const Task = require('data.task')
const {
  chain,
  curry,
  compose,
  identity,
  prop,
  split,
  last,
  map,
} = require('ramda')

// Exercise 1
// ==========
// Use safeProp and map/join or chain to safely get the street name when given
// a user.

const safeProp = curry((x, o) => Maybe.of(o[x]))

const user = {
  id: 2,
  name: 'albert',
  address: {
    street: {
      number: 22,
      name: 'Walnut St',
    },
  },
}

const ex1 = compose(
  chain(safeProp('name')),
  chain(safeProp('street')),
  safeProp('address')
)

expect(ex1(user)).toEqual(Maybe.of('Walnut St'))

// Exercise 2
// ==========
// Use getFile to get the filename, remove the directory so it's just the file,
// then purely log it.

const getFile = () => new IO(() => __filename)

const pureLog = x => new IO(() => (console.log(x), `logged ${x}`))

const ex2 = compose(chain(compose(pureLog, last, split(path.sep))), getFile)

expect(ex2().unsafePerformIO()).toBe('logged monad.js')

// Exercise 3
// ==========
// Use getPost() then pass the post's id to getComments().
//
const getPost = i =>
  new Task((rej, res) =>
    setTimeout(() => res({ id: i, title: 'Love them tasks' }), 300)
  )

const getComments = i =>
  new Task((rej, res) =>
    setTimeout(
      () =>
        res([
          { post_id: i, body: 'This book should be illegal' },
          { post_id: i, body: 'Monads are like smelly shallots' },
        ]),
      300
    )
  )

const ex3 = compose(chain(compose(getComments, prop('id'))), getPost)

ex3(13).fork(console.log, res => {
  expect(res.map(prop('post_id'))).toEqual([13, 13])
  expect(res.map(prop('body'))).toEqual([
    'This book should be illegal',
    'Monads are like smelly shallots',
  ])
})

// Exercise 4
// ==========
// Use validateEmail, addToMailingList, and emailBlast to implement ex4's type
// signature.

//  addToMailingList :: Email -> IO([Email])
const addToMailingList = (list => email =>
  new IO(() => (list.push(email), list)))([])

const emailBlast = list => new IO(() => `emailed: ${list.join(',')}`)

const validateEmail = x =>
  x.match(/\S+@\S+\.\S+/) ? new Right(x) : new Left('invalid email')

//  ex4 :: Email -> Either String (IO String)
const ex4 = compose(map(compose(emailBlast, addToMailingList)), validateEmail)

const getResult = either(identity, unsafePerformIO)
expect(getResult(ex4('notanemail'))).toBe('invalid email')
expect(getResult(ex4('[email protected]'))).toBe(
  'emailed: [email protected]'
)

console.info('You are now a MONAD genius!!! 🤓 👏🏼 👏🏼 👏🏼')