Professor Frisby's functional programming exercises (Part 5: applicative functors)

fp.exercises_5.js

require('./support')
const expect = require('expect')
const Task = require('data.task')
const { add, curry, reduce } = require('ramda')

// TEST HELPERS
// =====================

const getPost = i =>
  new Task((rej, res) =>
    setTimeout(() => res({ id: i, title: 'Love them futures' }), 300)
  )

const getComments = i =>
  new Task((rej, res) =>
    setTimeout(
      () =>
        res(['This book should be illegal', 'Monads are like space burritos']),
      300
    )
  )

// fib browser for test
const localStorage = {}

// Exercise 1
// ==========
// Write a function that adds two possibly null numbers together using Maybe and ap().

//  ex1 :: Number -> Number -> Maybe Number
const ex1 = (x, y) =>
  Maybe.of(add)
    .ap(Maybe.of(x))
    .ap(Maybe.of(y))
expect(ex1(2, 3)).toEqual(Maybe.of(5))
expect(ex1(null, 3)).toEqual(Maybe.of(null))

// Exercise 2
// ==========
// Now write a function that takes 2 Maybe's and adds them. Use liftA2 instead of ap().

//  ex2 :: Maybe Number -> Maybe Number -> Maybe Number
const ex2 = liftA2(add)
expect(ex2(Maybe.of(2), Maybe.of(3))).toEqual(Maybe.of(5))
expect(ex2(Maybe.of(null), Maybe.of(3))).toEqual(Maybe.of(null))

// Exercise 3
// ==========
// Run both getPost(n) and getComments(n) then render the page with both. (The n arg is arbitrary.)
const makeComments = reduce((acc, c) => `${acc}<li>${c}</li>`, '')
const render = curry(
  ({ title }, cs) => `<div>${title}</div>${makeComments(cs)}`
)

//  ex3 :: Task Error HTML
const ex3 = liftA2(render, getPost(13), getComments(13))
ex3.fork(console.log, html =>
  expect(html).toBe(
    '<div>Love them futures</div><li>This book should be illegal</li><li>Monads are like space burritos</li>'
  )
)

// Exercise 4
// ==========
// Write an IO that gets both player1 and player2 from the cache and starts the game.
localStorage.player1 = 'toby'
localStorage.player2 = 'sally'

const getCache = x => new IO(() => localStorage[x])
const game = curry((p1, p2) => `${p1} vs ${p2}`)

//  ex4 :: IO String
const ex4 = liftA2(game, getCache('player1'), getCache('player2'))
expect(ex4.unsafePerformIO()).toBe('toby vs sally')

console.info('You are now an APPLICATIVE FUNCTOR genius!!! 🤓 👏🏼 👏🏼 👏🏼')