Tweet
Tweet

Slide 1

Slide 1 text

Practical Functional Programming Jeremy Fairbank @elpapapollo

Slide 2

Slide 2 text

@testdouble helps improves how the world build software. testdouble.com/agency

Slide 3

Slide 3 text

In beta now! bit.ly/programming-elm

Slide 4

Slide 4 text

Functional programming:

Slide 5

Slide 5 text

Pure Functional programming:

Slide 6

Slide 6 text

Pure Total Functional programming:

Slide 7

Slide 7 text

Pure Total Idempotent Functional programming:

Slide 8

Slide 8 text

Pure Total Idempotent Monad Functional programming:

Slide 9

Slide 9 text

Pure Total Idempotent Monad Functor Functional programming:

Slide 10

Slide 10 text

Easier-to-reason-about- oriented programming

Slide 11

Slide 11 text

Pure Total Idempotent Monad Functor

Slide 12

Slide 12 text

No content

Slide 13

Slide 13 text

6 Become a rockstar programmer with these tricks!

Slide 14

Slide 14 text

Hard to test

Slide 15

Slide 15 text

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) } }

Slide 16

Slide 16 text

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) } }

Slide 17

Slide 17 text

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) } }

Slide 18

Slide 18 text

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) } }

Slide 19

Slide 19 text

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) } }

Slide 20

Slide 20 text

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 })

Slide 21

Slide 21 text

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 })

Slide 22

Slide 22 text

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 })

Slide 23

Slide 23 text

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 })

Slide 24

Slide 24 text

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')) }) })

Slide 25

Slide 25 text

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')) }) })

Slide 26

Slide 26 text

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')) }) })

Slide 27

Slide 27 text

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')) }) })

Slide 28

Slide 28 text

Side Effects

Slide 29

Slide 29 text

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) } }

Slide 30

Slide 30 text

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) } }

Slide 31

Slide 31 text

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) } }

Slide 32

Slide 32 text

Less Predictable

Slide 33

Slide 33 text

Harder to Test

Slide 34

Slide 34 text

Functions

Slide 35

Slide 35 text

const add = (x, y) => x + y add(2, 3) === 5 add(2, 3) === 5 add(2, 3) === 5

Slide 36

Slide 36 text

No content

Slide 37

Slide 37 text

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 }

Slide 38

Slide 38 text

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 }

Slide 39

Slide 39 text

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 }

Slide 40

Slide 40 text

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 }

Slide 41

Slide 41 text

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 }

Slide 42

Slide 42 text

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 }

Slide 43

Slide 43 text

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 }

Slide 44

Slide 44 text

it('uses the correct url', () => { expect(url([1, 2, 3])).toEqual('/orders?ids=1,2,3') })

Slide 45

Slide 45 text

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', ]) })

Slide 46

Slide 46 text

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]) } }

Slide 47

Slide 47 text

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]) } }

Slide 48

Slide 48 text

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]) } }

Slide 49

Slide 49 text

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]) } }

Slide 50

Slide 50 text

youtu.be/yTkzNHF6rMs

Slide 51

Slide 51 text

Hard to test Hard to follow

Slide 52

Slide 52 text

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 }

Slide 53

Slide 53 text

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 }

Slide 54

Slide 54 text

Imperative

Slide 55

Slide 55 text

Declarative

Slide 56

Slide 56 text

const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

Slide 57

Slide 57 text

const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

Slide 58

Slide 58 text

{ id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }

Slide 59

Slide 59 text

{ 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

Slide 60

Slide 60 text

[ '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

Slide 61

Slide 61 text

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' }

Slide 62

Slide 62 text

Hard to test Hard to follow Code breaks unexpectedly

Slide 63

Slide 63 text

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}` } }

Slide 64

Slide 64 text

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}` } }

Slide 65

Slide 65 text

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}` } }

Slide 66

Slide 66 text

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}` } }

Slide 67

Slide 67 text

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

Slide 68

Slide 68 text

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

Slide 69

Slide 69 text

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

Slide 70

Slide 70 text

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

Slide 71

Slide 71 text

let hobbies = this.hobbies.splice(0, 2).join(' and ')

Slide 72

Slide 72 text

Mutable Data

Slide 73

Slide 73 text

Immutable Data Safety and consistency

Slide 74

Slide 74 text

class Person { constructor(name, hobbies) { this.name = name this.hobbies = Object.freeze(hobbies) } // ... }

Slide 75

Slide 75 text

class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } }

Slide 76

Slide 76 text

class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } } TypeError: Cannot add/remove sealed array elements

Slide 77

Slide 77 text

class Person { // ... describe() { // ... let hobbies = this.hobbies.slice(0, 2).join(' and ') // ... } }

Slide 78

Slide 78 text

Create New State

Slide 79

Slide 79 text

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}` }

Slide 80

Slide 80 text

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}` }

Slide 81

Slide 81 text

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}` }

Slide 82

Slide 82 text

const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

Slide 83

Slide 83 text

const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

Slide 84

Slide 84 text

const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

Slide 85

Slide 85 text

const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

Slide 86

Slide 86 text

const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

Slide 87

Slide 87 text

let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

Slide 88

Slide 88 text

let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

Slide 89

Slide 89 text

let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

Slide 90

Slide 90 text

let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

Slide 91

Slide 91 text

let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

Slide 92

Slide 92 text

Hard to test Hard to follow Code breaks unexpectedly Too much code

Slide 93

Slide 93 text

const greetInEnglish = name => `Hi, ${name}` const greetInSpanish = name => `Hola, ${name}`

Slide 94

Slide 94 text

const greet = (greeting, name) => `${greeting}, ${name}` // .. const greetInEnglish = name => greet('Hi', name) const greetInSpanish = name => greet('Hola', name)

Slide 95

Slide 95 text

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

Slide 96

Slide 96 text

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

Slide 97

Slide 97 text

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

Slide 98

Slide 98 text

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

Slide 99

Slide 99 text

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

Slide 100

Slide 100 text

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

Slide 101

Slide 101 text

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

Slide 102

Slide 102 text

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

Slide 103

Slide 103 text

const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') Curried Function Partial Application

Slide 104

Slide 104 text

greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

Slide 105

Slide 105 text

greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

Slide 106

Slide 106 text

greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

Slide 107

Slide 107 text

greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

Slide 108

Slide 108 text

greetInEnglish "Lambda Squared" -- Hi, Lambda Squared greetInSpanish "Lambda Squared" -- Hola, Lambda Squared

Slide 109

Slide 109 text

No content

Slide 110

Slide 110 text

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 } }

Slide 111

Slide 111 text

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 } }

Slide 112

Slide 112 text

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 } }

Slide 113

Slide 113 text

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 } }

Slide 114

Slide 114 text

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 } }

Slide 115

Slide 115 text

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 } }

Slide 116

Slide 116 text

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 } }

Slide 117

Slide 117 text

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 } }

Slide 118

Slide 118 text

const excitedGreeting = name => // ...

Slide 119

Slide 119 text

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!

Slide 120

Slide 120 text

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!

Slide 121

Slide 121 text

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!

Slide 122

Slide 122 text

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!

Slide 123

Slide 123 text

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!

Slide 124

Slide 124 text

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!

Slide 125

Slide 125 text

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!

Slide 126

Slide 126 text

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!

Slide 127

Slide 127 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 128

Slide 128 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 129

Slide 129 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 130

Slide 130 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 131

Slide 131 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, // LAMBDA SQUARED greet('Hi'), exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 132

Slide 132 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), // Hi, LAMBDA SQUARED exclaim, )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 133

Slide 133 text

const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, // Hi, LAMBDA SQUARED! )(name) excitedGreeting('Lambda Squared') // Hi, LAMBDA SQUARED!

Slide 134

Slide 134 text

Function Composition

Slide 135

Slide 135 text

greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!

Slide 136

Slide 136 text

greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!

Slide 137

Slide 137 text

greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!

Slide 138

Slide 138 text

greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!

Slide 139

Slide 139 text

greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper -- LAMBDA SQUARED |> greet "Hi" |> exclaim excitedGreeting "Lambda Squared" -- Hi, LAMBDA SQUARED!

Slide 140

Slide 140 text

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!

Slide 141

Slide 141 text

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!

Slide 142

Slide 142 text

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 } }

Slide 143

Slide 143 text

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 } }

Slide 144

Slide 144 text

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 } }

Slide 145

Slide 145 text

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 } }

Slide 146

Slide 146 text

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 } }

Slide 147

Slide 147 text

Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor

Slide 148

Slide 148 text

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 } }

Slide 149

Slide 149 text

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 } }

Slide 150

Slide 150 text

createOrder(myShoppingCart, 'Tucker')

Slide 151

Slide 151 text

Strong Static Types

Slide 152

Slide 152 text

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 }

Slide 153

Slide 153 text

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 }

Slide 154

Slide 154 text

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 }

Slide 155

Slide 155 text

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 }

Slide 156

Slide 156 text

-- 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

Slide 157

Slide 157 text

-- 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

Slide 158

Slide 158 text

-- 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

Slide 159

Slide 159 text

conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount

Slide 160

Slide 160 text

conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount

Slide 161

Slide 161 text

conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ toString amount

Slide 162

Slide 162 text

Custom Types

Slide 163

Slide 163 text

type alias Item = { price : Float }

Slide 164

Slide 164 text

type alias Item = { price : Float }

Slide 165

Slide 165 text

type alias Item = { price : Float }

Slide 166

Slide 166 text

type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }

Slide 167

Slide 167 text

type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }

Slide 168

Slide 168 text

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 }

Slide 169

Slide 169 text

createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...

Slide 170

Slide 170 text

createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...

Slide 171

Slide 171 text

type alias Order = { status : String , customer : String , total : Float }

Slide 172

Slide 172 text

type alias Order = { status : String , customer : String , total : Float }

Slide 173

Slide 173 text

type Status = New | Processing | Shipped

Slide 174

Slide 174 text

type Status = New | Processing | Shipped

Slide 175

Slide 175 text

type Status = New | Processing | Shipped

Slide 176

Slide 176 text

type Status = New | Processing | Shipped type alias Order = { status : Status , customer : String , total : Float }

Slide 177

Slide 177 text

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 }

Slide 178

Slide 178 text

Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null

Slide 179

Slide 179 text

function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

Slide 180

Slide 180 text

function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

Slide 181

Slide 181 text

function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

Slide 182

Slide 182 text

function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

Slide 183

Slide 183 text

getBestFriendLocation(42, users) Uncaught TypeError: Cannot read property 'bestFriendId' of null

Slide 184

Slide 184 text

function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location } null

Slide 185

Slide 185 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } }

Slide 186

Slide 186 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } } null

Slide 187

Slide 187 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } }

Slide 188

Slide 188 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } } null

Slide 189

Slide 189 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }

Slide 190

Slide 190 text

No content

Slide 191

Slide 191 text

if () {} if () {} if () {}

Slide 192

Slide 192 text

type Maybe a = Nothing | Just a

Slide 193

Slide 193 text

type Maybe a = Nothing | Just a

Slide 194

Slide 194 text

type Maybe a = Nothing | Just a

Slide 195

Slide 195 text

type Maybe a = Nothing | Just a

Slide 196

Slide 196 text

42 Just 42 "Hi" Just "Hi"

Slide 197

Slide 197 text

Just 42 [42] Nothing [ ]

Slide 198

Slide 198 text

getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

Slide 199

Slide 199 text

getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

Slide 200

Slide 200 text

getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

Slide 201

Slide 201 text

getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

Slide 202

Slide 202 text

getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users Maybe User

Slide 203

Slide 203 text

getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

Slide 204

Slide 204 text

getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

Slide 205

Slide 205 text

getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name user

Slide 206

Slide 206 text

getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

Slide 207

Slide 207 text

-- 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!

Slide 208

Slide 208 text

getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name Nothing -> ""

Slide 209

Slide 209 text

function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }

Slide 210

Slide 210 text

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

Slide 211

Slide 211 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 212

Slide 212 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 213

Slide 213 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 214

Slide 214 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 215

Slide 215 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 216

Slide 216 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 217

Slide 217 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 218

Slide 218 text

getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\id -> Dict.get id users) |> Maybe.andThen (\friend -> friend.location)

Slide 219

Slide 219 text

P.S. You kinda, sorta just learned monads

Slide 220

Slide 220 text

Hard to test Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null

Slide 221

Slide 221 text

Hard to test Pure Functions Hard to follow Code breaks unexpectedly Too much code Scary to refactor Oh, null

Slide 222

Slide 222 text

Hard to test Pure Functions Hard to follow Declarative Code Code breaks unexpectedly Too much code Scary to refactor Oh, null

Slide 223

Slide 223 text

Code breaks unexpectedly Immutable Data Too much code Scary to refactor Oh, null Hard to test Pure Functions Hard to follow Declarative Code

Slide 224

Slide 224 text

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

Slide 225

Slide 225 text

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

Slide 226

Slide 226 text

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

Slide 227

Slide 227 text

Thanks! bit.ly/lambda-squared-practical-fp Jeremy Fairbank @elpapapollo