CodeMash 2020: Practical Functional Programming

Functional programming is more than just math and monads. Functional programming empowers developers to solve real problems with safe, predictable, and maintainable code. In this talk, discover the basics of functional programming and how to apply functional concepts in a practical manner. Learn how pure functions are easily testable, how to compose specialized functions to create more complex functions, how immutable data prevents bugs, how to prevent runtime errors with static types, how to safely model nulls with special types, and more! Most importantly, leave this talk with a deeper understanding and appreciation for functional programming to start exploring it yourself.

1. Practical Functional Programming Jeremy Fairbank @elpapapollo
2. @testdouble helps improves how the world build software. testdouble.com
3. Available in print or e-book programming-elm.com
4. Functional programming:
5. Pure Functional programming:
6. Pure Total Functional programming:
7. Pure Total Idempotent Functional programming:
8. Pure Total Idempotent Monad Functional programming:
9. Pure Total Idempotent Monad Functor Functional programming:
10. Easier-to-reason-about- programming
11. Pure Total Idempotent Monad Functor
13. 6 Become a rockstar programmer with these tricks!
14. Hard to test
15. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
16. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
17. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
18. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
19. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
20. const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })
21. const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })
22. const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })
23. const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })
24. it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })
25. it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })
26. it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })
27. it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })
28. Side Effects
29. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
30. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
31. async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }
32. Less Predictable
33. Harder to Test
34. Functions
35. const add = (x, y) => x + y add(2, 3) === 5 add(2, 3) === 5 add(2, 3) === 5 Pure
37. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
38. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
39. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
40. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
41. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
42. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
43. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
44. it('uses the correct url', () => { expect(url([1, 2, 3])).toEqual('/orders?ids=1,2,3') })
45. it('creates details for orders', () => { let orders = [ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ] expect(detailsForOrders(orders)).toEqual([ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ]) })
46. async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) for (let i = 0; i < details.length; i++) { console.log(details[i]) } }
47. async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) for (let i = 0; i < details.length; i++) { console.log(details[i]) } }
48. async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) for (let i = 0; i < details.length; i++) { console.log(details[i]) } }
49. async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) for (let i = 0; i < details.length; i++) { console.log(details[i]) } }
50. youtu.be/yTkzNHF6rMs
51. Hard to test Hard to follow
52. const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
53. const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` function detailsForOrders(orders) { let details = [] for (let i = 0; i < orders.length; i++) { details.push(orderDetails(orders[i])) } return details }
54. Imperative
55. Declarative
56. const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
57. const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
58. { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }
59. { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } [ 'Order 1 on 12/1/17 by Tucker', ] map orderDetails
60. [ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', ] orderDetails { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } map
61. orderDetails [ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ] map { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }
62. Hard to test Hard to follow Code breaks unexpectedly
63. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
64. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
65. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
66. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
67. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
68. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
69. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
70. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
71. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
72. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
73. let hobbies = this.hobbies.splice(0, 2).join(' and ')
74. Mutable Data
75. Immutable Data Safety and consistency
76. class Person { constructor(name, hobbies) { this.name = name this.hobbies = Object.freeze(hobbies) } // ... }
77. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } }
78. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } } TypeError
79. class Person { // ... describe() { // ... let hobbies = this.hobbies.slice(0, 2).join(' and ') // ... } }
80. Create New State
81. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
82. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
83. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
84. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
85. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
86. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
87. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
88. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
89. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
90. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
91. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
92. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
93. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
94. Hard to test Hard to follow Code breaks unexpectedly Too much code
95. const greetInEnglish = name => `Hi, ${name}` const greetInSpanish = name => `Hola, ${name}`
96. const greet = (greeting, name) => `${greeting}, ${name}` // .. const greetInEnglish = name => greet('Hi', name) const greetInSpanish = name => greet('Hola', name)
97. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
98. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
99. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
100. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
101. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
102. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
103. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
104. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('CodeMash') // Hi, CodeMash greetInSpanish('CodeMash') // Hola, CodeMash
105. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') Curried Function Partial Application
106. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
107. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
108. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
109. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
110. greetInEnglish "CodeMash" -- Hi, CodeMash greetInSpanish "CodeMash" -- Hola, CodeMash
112. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
113. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
114. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
115. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
116. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
117. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
118. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
119. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
120. const excitedGreeting = name => // ...
121. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
122. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
123. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
124. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
125. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
126. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
127. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
128. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('CodeMash') // Hi, CODEMASH!
129. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
130. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
131. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
132. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
133. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, // CODEMASH greet('Hi'), exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
134. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), // Hi, CODEMASH exclaim, )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
135. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, // Hi, CODEMASH! )(name) excitedGreeting('CodeMash') // Hi, CODEMASH!
136. Function Composition
137. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
138. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
139. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
140. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
141. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper -- CODEMASH |> greet "Hi" |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
142. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" -- Hi, CODEMASH |> exclaim excitedGreeting "CodeMash" -- Hi, CODEMASH!
143. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim -- Hi, CODEMASH! excitedGreeting "CodeMash" -- Hi, CODEMASH!
144. function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }
145. function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
146. function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
147. function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
148. function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
149. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor
150. function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
151. const createOrder = customer => shoppingCart => { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }
152. createOrder(myShoppingCart, 'Tucker')
153. Strong Static Types
154. createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }
155. createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }
156. createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }
157. createOrder customer shoppingCart = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }
158. -- TYPE MISMATCH ------------------------- App.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }
159. -- TYPE MISMATCH ------------------------- App.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }
160. -- TYPE MISMATCH ------------------------- App.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }
161. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount
162. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount
163. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount
164. ?
166. Custom Types
167. type Item = { price: number, }
168. type Item = { price: number, }
169. type Item = { price: number, }
170. type Item = { price: number, } type ShoppingCart = { items: Item[], discounts: number[], tax: number, shippingCost: number, }
171. type Item = { price: number, } type ShoppingCart = { items: Item[], discounts: number[], tax: number, shippingCost: number, }
172. type Item = { price: number, } type ShoppingCart = { items: Item[], discounts: number[], tax: number, shippingCost: number, } type Order = { status: string, customer: string, total: number, }
173. function createOrder( shoppingCart: ShoppingCart, customer: string ): Order { ... }
174. function createOrder( shoppingCart: ShoppingCart, customer: string ): Order { ... }
175. type Order = { status: string, customer: string, total: number, }
176. type Order = { status: string, customer: string, total: number, }
177. enum Status { New, Processing, Shipped, }
178. enum Status { New, Processing, Shipped, }
179. enum Status { New, Processing, Shipped, }
180. enum Status { New, Processing, Shipped, } type Order = { status: Status, customer: string, total: number, }
181. function createOrder( shoppingCart: ShoppingCart, customer: string ): Order { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: Status.New, customer, total } }
182. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null and undefined
183. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
184. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
185. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
186. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
187. getBestFriendLocation(42, users) Uncaught TypeError: Cannot read property 'bestFriendId' of null
188. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location } null
189. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } }
190. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } } null
191. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } }
192. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } } null
193. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
195. if () {} if () {} if () {}
196. type Maybe a = Nothing | Just a
197. type Maybe a = Nothing | Just a
198. type Maybe a = Nothing | Just a
199. type Maybe a = Nothing | Just a
200. 42 Just 42 "Hi" Just "Hi"
201. Just 42 [42] Nothing [ ]
202. import { Maybe } from 'true-myth' Maybe.just(42) // Just 42 Maybe.nothing() // Nothing Maybe.of(42) // Just 42 Maybe.of(null) // Nothing Maybe.of(undefined) // Nothing
203. function mapGet(key, map) { let result = map.get(key) return result == null ? Maybe.nothing() : Maybe.just(result) }
204. let mapGet = Maybe.wrapReturn( (key, map) => map.get(key) )
205. function getUserName(id, users) { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } }
206. function getUserName(id, users) { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } } Maybe User
207. function getUserName(id, users) { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } }
208. function getUserName(id, users) { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } } user
209. function getUserName(id, users) { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } } Nothing?
210. function getUserName( id: Id, users: Map<Id, User> ): string { let user = mapGet(id, users) if (user.isJust()) { return user.unsafelyUnwrap().name } }
211. null.ts:27:4 - error TS2366: Function lacks ending return statement and return type does not include 'undefined'. 27 ): string { ~~~~~~ Found 1 error. $ tsc --lib es2015 --strictNullChecks null.ts
212. function getUserName( id: Id, users: Map<Id, User> ): string { let user = mapGet(id, users) return user.isJust() ? user.unsafelyUnwrap().name : '<anonymous>' }
213. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
214. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { let user = mapGet(id, users) if (user.isJust()) { let { bestFriendId } = user.unsafelyUnwrap() if (bestFriendId.isJust()) { let friend = mapGet(bestFriendId.unsafelyUnwrap(), users) if (friend.isJust()) { return friend.unsafelyUnwrap().location } return Maybe.nothing() } return Maybe.nothing() } return Maybe.nothing() }
215. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
216. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
217. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
218. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
219. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
220. .chain(user => user.bestFriendId)
221. .chain(user => user.bestFriendId) .chain Nothing Nothing
222. .chain(user => user.bestFriendId) .chain Just user ?
223. user .chain(user => user.bestFriendId)
224. .chain(user => user.bestFriendId) Just <id> or Nothing
225. .chain(user => user.bestFriendId) .chain Just user Just <id> or Nothing
226. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) // Maybe<User> .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
227. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) // Maybe<Id> .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) }
228. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) // Maybe<User> .chain(friend => friend.location) }
229. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) // Maybe<Location> }
230. function getBestFriendLocation( id: Id, users: Map<Id, User> ): Maybe<Location> { return mapGet(id, users) .chain(user => user.bestFriendId) .chain(bestFriendId => mapGet(bestFriendId, users)) .chain(friend => friend.location) // Maybe<Location> }
231. P.S. You kinda, sorta just learned monads
232. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null and undefined
233. Hard to test Pure Functions Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null and undefined
234. Hard to test Pure Functions Hard to follow Declarative Code Code breaks unexpectedly Too much code Scary to refactor Oh, null and undefined
235. Code breaks unexpectedly Immutable Data Too much code Scary to refactor Hard to test Pure Functions Hard to follow Declarative Code Oh, null and undefined
236. Too much code Curried, Composable Functions Scary to refactor Code breaks unexpectedly Immutable Data Hard to test Pure Functions Hard to follow Declarative Code Oh, null and undefined
237. Scary to refactor Strong, Static Types Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions Hard to follow Declarative Code Oh, null and undefined
238. Oh, null and undefined Maybe and Monads Scary to refactor Strong, Static Types Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions Hard to follow Declarative Code
239. Thanks! bit.ly/cm-practical-fp Jeremy Fairbank @elpapapollo