All Things Open 2018: 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. You will 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, you will leave this talk with a deeper understanding and appreciation for functional programming so you can start exploring it yourself.

Transcript

1. Practical Functional Programming Jeremy Fairbank @elpapapollo
2. @testdouble helps improves how the world build software. testdouble.com
3. 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. 5 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 ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
38. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
39. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
40. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
41. const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)
42. { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }
43. { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } [ 'Order 1 on 12/1/17 by Tucker', ] map orderDetails
44. [ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', ] orderDetails { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } map
45. orderDetails [ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ] map { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }
46. it('uses the correct url', () => { expect(url([1, 2, 3])).toEqual('/orders?ids=1,2,3') })
47. it('creates details for orders', () => { let orders = [ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ] expect(detailsForOrders(orders)).toEqual([ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ]) })
48. async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) details.forEach(orderDetails => { console.log(orderDetails) }) }
49. Hard to test Code breaks unexpectedly
50. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
51. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
52. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
53. class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }
54. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
55. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
56. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
57. let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music
58. let hobbies = this.hobbies.splice(0, 2).join(' and ')
59. Mutable Data
60. Immutable Data Safety and consistency
61. class Person { constructor(name, hobbies) { this.name = name this.hobbies = Object.freeze(hobbies) } // ... }
62. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } }
63. class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } } TypeError: Cannot add/remove sealed array elements
64. class Person { // ... describe() { // ... let hobbies = this.hobbies.slice(0, 2).join(' and ') // ... } }
65. Create New State
66. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
67. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }
68. const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${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}` }
69. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
70. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
71. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
72. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
73. const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })
74. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
75. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
76. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
77. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
78. let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading
79. Hard to test Code breaks unexpectedly Too much code
80. const greetInEnglish = name => `Hi, ${name}` const greetInSpanish = name => `Hola, ${name}`
81. const greet = (greeting, name) => `${greeting}, ${name}` // .. const greetInEnglish = name => greet('Hi', name) const greetInSpanish = name => greet('Hola', name)
82. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
83. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
84. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
85. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
86. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
87. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
88. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
89. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open
90. const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') Curried Function Partial Application
91. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
92. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
93. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
94. greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"
95. greetInEnglish "All Things Open" -- Hi, All Things Open greetInSpanish "All Things Open" -- Hola, All Things Open
97. 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 } }
98. 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 } }
99. 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 } }
100. 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 } }
101. 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 } }
102. 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 } }
103. 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 } }
104. 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 } }
105. const excitedGreeting = name => // ...
106. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
107. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
108. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
109. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
110. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
111. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
112. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
113. const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
114. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
115. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
116. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
117. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
118. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, // ALL THINGS OPEN greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
119. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), // Hi, ALL THINGS OPEN exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
120. const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, // Hi, ALL THINGS OPEN! )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!
121. Function Composition
122. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
123. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
124. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
125. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
126. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper -- ALL THINGS OPEN |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
127. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" -- Hi, ALL THINGS OPEN |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
128. greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim -- Hi, ALL THINGS OPEN! excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!
129. 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 } }
130. 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 } }
131. 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 } }
132. 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 } }
133. 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 } }
134. Hard to test Code breaks unexpectedly Too much code Scary to refactor
135. 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 } }
136. 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 } }
137. createOrder(myShoppingCart, 'Tucker')
138. Strong Static Types
139. 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 }
140. 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 }
141. 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 }
142. 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 }
143. -- TYPE MISMATCH ------------------------- Responsibilities.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 }
144. -- TYPE MISMATCH ------------------------- Responsibilities.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 }
145. -- TYPE MISMATCH ------------------------- Responsibilities.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 }
146. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount
147. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount
148. conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount
149. Custom Types
150. type alias Item = { price : Float }
151. type alias Item = { price : Float }
152. type alias Item = { price : Float }
153. type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }
154. type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }
155. 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 }
156. createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...
157. createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...
158. type alias Order = { status : String , customer : String , total : Float }
159. type alias Order = { status : String , customer : String , total : Float }
160. type Status = New | Processing | Shipped
161. type Status = New | Processing | Shipped
162. type Status = New | Processing | Shipped
163. type Status = New | Processing | Shipped type alias Order = { status : Status , customer : String , total : Float }
164. 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 }
165. Hard to test Code breaks unexpectedly Too much code Scary to refactor Oh, null
166. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
167. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
168. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
169. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }
170. getBestFriendLocation(42, users) Uncaught TypeError: Cannot read property 'bestFriendId' of null
171. function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location } null
172. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } }
173. function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } } null
174. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } }
175. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } } null
176. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
178. if () {} if () {} if () {}
179. type Maybe a = Nothing | Just a
180. type Maybe a = Nothing | Just a
181. type Maybe a = Nothing | Just a
182. type Maybe a = Nothing | Just a
183. 42 Just 42 "Hi" Just "Hi"
184. Just 42 [42] Nothing [ ]
185. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
186. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
187. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
188. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users
189. getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users Maybe User
190. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
191. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
192. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name user
193. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name
194. -- MISSING PATTERNS ------------------------------ App.elm This `case` does not have branches for all possibilities: 31|> case Dict.get id users of 32|> Just user -> 33|> user.name Missing possibilities include: Nothing I would have to crash if I saw one of those. Add branches for them!
195. getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name Nothing -> "<anonymous>"
196. function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }
197. 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
198. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
199. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
200. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
201. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
202. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
203. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
204. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
205. getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)
206. P.S. You kinda, sorta just learned monads
207. Hard to test Code breaks unexpectedly Too much code Scary to refactor Oh, null
208. Hard to test Pure Functions Code breaks unexpectedly Too much code Scary to refactor Oh, null
209. Code breaks unexpectedly Immutable Data Too much code Scary to refactor Oh, null Hard to test Pure Functions
210. Too much code Curried, Composable Functions Scary to refactor Oh, null Code breaks unexpectedly Immutable Data Hard to test Pure Functions
211. Scary to refactor Strong, Static Types Oh, null Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions
212. 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
213. Thanks! bit.ly/ato-practical-fp Jeremy Fairbank @elpapapollo