Lambda Squared 2018: Practical Functional Programming

Transcript

1. Practical Functional Programming Jeremy Fairbank @elpapapollo
2. @testdouble helps improves how the world build software. testdouble.com/agency
3. In beta now! bit.ly/programming-elm
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- oriented 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 ${customer}` console.log(details) } }
32. Less Predictable
33. Harder to Test
34. Functions
35. const add = (x, y) => x + y add(2, 3) === 5 add(2, 3) === 5 add(2, 3) === 5
37. const url = ids => `/orders?ids=${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 ${hobbies}` } }
67. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
68. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
69. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
70. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
71. let hobbies = this.hobbies.splice(0, 2).join(' and ')
72. Mutable Data
73. Immutable Data Safety and consistency
74. class Person { constructor(name, hobbies) { this.name = name this.hobbies = Object.freeze(hobbies) } // ... }
75. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } }
76. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } } TypeError: Cannot add/remove sealed array elements
77. class Person { // ... describe() { // ... let hobbies = this.hobbies.slice(0, 2).join(' and ') // ... } }
78. Create New State
79. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
80. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
81. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${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 addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
83. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
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. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
88. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
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. Hard to test Hard to follow Code breaks unexpectedly Too much code
93. const greetInEnglish = name => `Hi, ${name}` const greetInSpanish = name => `Hola, ${name}`
94. const greet = (greeting, name) => `${greeting}, ${name}` // .. const greetInEnglish = name => greet('Hi', name) const greetInSpanish = name => greet('Hola', name)
95. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
96. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
97. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
98. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
99. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
100. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
101. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
102. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('Lambda Squared') // Hi, Lambda Squared greetInSpanish('Lambda Squared') // Hola, Lambda Squared
103. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') Curried Function Partial Application
104. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
105. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
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. greetInEnglish "Lambda Squared" -- Hi, Lambda Squared greetInSpanish "Lambda Squared" -- Hola, Lambda Squared
110. 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 } }
111. 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 } }
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. const excitedGreeting = name => // ...
119. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
120. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
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('Lambda Squared') // Hi, LAMBDA SQUARED!
127. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
128. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
129. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
130. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
131. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, // LAMBDA SQUARED greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
132. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), // Hi, LAMBDA SQUARED exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
133. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, // Hi, LAMBDA SQUARED! )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!
134. Function Composition
135. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
136. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
137. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
138. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
139. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper -- LAMBDA SQUARED |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
140. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" -- Hi, LAMBDA SQUARED |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
141. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim -- Hi, LAMBDA SQUARED! excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!
142. 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 } }
143. 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 } }
144. 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 } }
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. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor
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. 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 } }
150. createOrder(myShoppingCart, 'Tucker')
151. Strong Static Types
152. 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 }
153. 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 }
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 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 }
156. -- TYPE MISMATCH -------------------------------------- App.elm The 2nd argument to function `createOrder` is causing a mismatch. 25| createOrder myShoppingCart "Tucker" ^^^^^^^^ Function `createOrder` is expecting the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float } But it is: String
157. -- TYPE MISMATCH -------------------------------------- App.elm The 2nd argument to function `createOrder` is causing a mismatch. 25| createOrder myShoppingCart "Tucker" ^^^^^^^^ Function `createOrder` is expecting the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float } But it is: String
158. -- TYPE MISMATCH -------------------------------------- App.elm The 2nd argument to function `createOrder` is causing a mismatch. 25| createOrder myShoppingCart "Tucker" ^^^^^^^^ Function `createOrder` is expecting the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float } But it is: String
159. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount
160. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount
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. Custom Types
163. type alias Item = { price : Float }
164. type alias Item = { price : Float }
165. type alias Item = { price : Float }
166. type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }
167. type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }
168. type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float } type alias Order = { status : String , customer : String , total : Float }
169. createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...
170. createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...
171. type alias Order = { status : String , customer : String , total : Float }
172. type alias Order = { status : String , customer : String , total : Float }
173. type Status = New | Processing | Shipped
174. type Status = New | Processing | Shipped
175. type Status = New | Processing | Shipped
176. type Status = New | Processing | Shipped type alias Order = { status : Status , customer : String , total : Float }
177. createOrder : ShoppingCart -> String -> Order 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 }
178. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null
179. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
180. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
181. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
182. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
183. getBestFriendLocation(42, users) Uncaught TypeError: Cannot read property 'bestFriendId' of null
184. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location } null
185. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } }
186. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } } null
187. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } }
188. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } } null
189. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
191. if () {} if () {} if () {}
192. type Maybe a = Nothing | Just a
193. type Maybe a = Nothing | Just a
194. type Maybe a = Nothing | Just a
195. type Maybe a = Nothing | Just a
196. 42 Just 42 "Hi" Just "Hi"
197. Just 42 [42] Nothing [ ]
198. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
199. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
200. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
201. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
202. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users Maybe User
203. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
204. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
205. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name user
206. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
207. -- MISSING PATTERNS ------------------------------ App.elm This `case` does not have branches for all possibilities. 30|> case Dict.get id users of 31|> Just user -> 32|> user.name You need to account for the following values: Maybe.Nothing Add a branch to cover this pattern!
208. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name Nothing -> "<anonymous>"
209. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
210. getBestFriendLocation : Id -> Dict Id User -> Maybe Location getBestFriendLocation id users = case Dict.get id users of Just user -> case user.bestFriendId of Just bestFriendId -> case Dict.get bestFriendId users of Just friend -> friend.location Nothing -> Nothing Nothing -> Nothing Nothing -> Nothing
211. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
212. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
213. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
214. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
215. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
216. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
217. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
218. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)
219. P.S. You kinda, sorta just learned monads
220. Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null
221. Hard to test Pure Functions Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null
222. Hard to test Pure Functions Hard to follow Declarative Code Code breaks unexpectedly Too much code Scary to refactor Oh, null
223. Code breaks unexpectedly Immutable Data Too much code Scary to refactor Oh, null Hard to test Pure Functions Hard to follow Declarative Code
224. Too much code Curried, Composable Functions Scary to refactor Oh, null Code breaks unexpectedly Immutable Data Hard to test Pure Functions Hard to follow Declarative Code
225. Scary to refactor Strong, Static Types Oh, null Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions Hard to follow Declarative Code
226. Oh, null 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
227. Thanks! bit.ly/lambda-squared-practical-fp Jeremy Fairbank @elpapapollo