CodeMash 2020: Practical Functional Programming

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

Transcript

1. Practical
   Functional
   Programming
   Jeremy Fairbank
   @elpapapollo
   

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2. @testdouble helps improves
   how the world build software.
   testdouble.com
   

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3. Available in
   print or e-book
   programming-elm.com
   

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4. Functional
   programming:
   

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5. Pure
   Functional
   programming:
   

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6. Pure
   Total
   Functional
   programming:
   

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7. Pure
   Total
   Idempotent
   Functional
   programming:
   

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8. Pure
   Total
   Idempotent
   Monad
   Functional
   programming:
   

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9. Pure
   Total
   Idempotent
   Monad
   Functor
   Functional
   programming:
   

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10. Easier-to-reason-about-
    programming
    

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11. Pure
    Total
    Idempotent
    Monad
    Functor
    

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12. View Slide

13. 6
    Become a rockstar
    programmer with these
    tricks!
    

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14. Hard to test
    
    
    
    

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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)
    }
    }
    

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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)
    }
    }
    

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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)
    }
    }
    

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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)
    }
    }
    

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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)
    }
    }
    

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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
    })
    

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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
    })
    

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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
    })
    

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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
    })
    

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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'))
    })
    })
    

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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'))
    })
    })
    

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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'))
    })
    })
    

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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'))
    })
    })
    

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28. Side Effects
    

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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)
    }
    }
    

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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)
    }
    }
    

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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)
    }
    }
    

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32. Less Predictable
    

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33. Harder to Test
    

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34. Functions
    

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

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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
    }
    

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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
    }
    

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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
    }
    

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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
    }
    

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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
    }
    

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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
    }
    

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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
    }
    

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

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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',
    ])
    })
    

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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])
    }
    }
    

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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])
    }
    }
    

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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])
    }
    }
    

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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])
    }
    }
    

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50. youtu.be/yTkzNHF6rMs
    

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51. Hard to test Hard to follow
    
    
    

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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
    }
    

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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
    }
    

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54. Imperative
    

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55. Declarative
    

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

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

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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' }
    

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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
    

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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
    

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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' }
    

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62. Hard to test Hard to follow
    Code breaks
    unexpectedly
    
    
    

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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}`
    }
    }
    

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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}`
    }
    }
    

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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}`
    }
    }
    

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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}`
    }
    }
    

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67. class Person {
    constructor(name, hobbies) {
    this.name = name
    this.hobbies = hobbies
    }
    describe() {
    if (this.hobbies.length === 0) {
    return `${this.name} doesn't do much`
    }
    let hobbies = this.hobbies.splice(0, 2).join(' and ')
    return `${this.name} likes to do things such as ${hobbies}`
    }
    }
    

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68. class Person {
    constructor(name, hobbies) {
    this.name = name
    this.hobbies = hobbies
    }
    describe() {
    if (this.hobbies.length === 0) {
    return `${this.name} doesn't do much`
    }
    let hobbies = this.hobbies.splice(0, 2).join(' and ')
    return `${this.name} likes to do things such as ${hobbies}`
    }
    }
    

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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
    

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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
    

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71. let person = new Person('Jeremy', [
    'programming',
    'reading',
    'playing music',
    ])
    person.describe()
    // Jeremy likes to do things such as programming and reading
    person.describe()
    // Jeremy likes to do things such as playing music
    

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72. let person = new Person('Jeremy', [
    'programming',
    'reading',
    'playing music',
    ])
    person.describe()
    // Jeremy likes to do things such as programming and reading
    person.describe()
    // Jeremy likes to do things such as playing music
    

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

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74. Mutable Data
    

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75. Immutable Data
    Safety and consistency
    

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

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

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

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

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80. Create New State
    

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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}`
    }
    

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82. const create = (name, hobbies) =>
    Object.freeze({ name, hobbies: Object.freeze(hobbies) })
    function describe(person) {
    if (person.hobbies.length === 0) {
    return `${person.name} doesn't do much`
    }
    let hobbies = person.hobbies.slice(0, 2).join(' and ')
    return `${person.name} likes to do things such as ${hobbies}`
    }
    

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83. const create = (name, hobbies) =>
    Object.freeze({ name, hobbies: Object.freeze(hobbies) })
    function describe(person) {
    if (person.hobbies.length === 0) {
    return `${person.name} doesn't do much`
    }
    let hobbies = person.hobbies.slice(0, 2).join(' and ')
    return `${person.name} likes to do things such as ${hobbies}`
    }
    

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

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

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

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

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

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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
    

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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
    

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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
    

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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110. greetInEnglish "CodeMash"
     -- Hi, CodeMash
     greetInSpanish "CodeMash"
     -- Hola, CodeMash
     

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111. View Slide

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 }
     }
     

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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 }
     }
     

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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 }
     }
     

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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 }
     }
     

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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 }
     }
     

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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 }
     }
     

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118. function createOrder(shoppingCart, customer) {
     let total = 0
     shoppingCart.items.forEach(item => {
     total += item.price
     })
     shoppingCart.discounts.forEach(discount => {
     total *= 1 - discount
     })
     total *= 1 + shoppingCart.tax
     total += shoppingCart.shippingCost
     total = Math.round(total * 100) / 100
     return { status: 'new', customer, total }
     }
     

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119. function createOrder(shoppingCart, customer) {
     let total = 0
     shoppingCart.items.forEach(item => {
     total += item.price
     })
     shoppingCart.discounts.forEach(discount => {
     total *= 1 - discount
     })
     total *= 1 + shoppingCart.tax
     total += shoppingCart.shippingCost
     total = Math.round(total * 100) / 100
     return { status: 'new', customer, total }
     }
     

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120. const excitedGreeting = name =>
     // ...
     

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121. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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122. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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123. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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124. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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125. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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126. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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127. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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128. const toUpper = string => string.toUpperCase()
     const greet = greeting => name => `${greeting}, ${name}`
     const exclaim = phrase => `${phrase}!`
     const excitedGreeting = name =>
     exclaim(greet('Hi')(toUpper(name)))
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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129. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'),
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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130. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'),
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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131. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'),
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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132. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'),
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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133. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper, // CODEMASH
     greet('Hi'),
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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134. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'), // Hi, CODEMASH
     exclaim,
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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135. const R = require('ramda')
     const excitedGreeting = name =>
     R.pipe(
     toUpper,
     greet('Hi'),
     exclaim, // Hi, CODEMASH!
     )(name)
     excitedGreeting('CodeMash')
     // Hi, CODEMASH!
     

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136. Function
     Composition
     

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137. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi"
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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138. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi"
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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139. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi"
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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140. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi"
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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141. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper -- CODEMASH
     |> greet "Hi"
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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142. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi" -- Hi, CODEMASH
     |> exclaim
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

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143. greet greeting name =
     greeting ++ ", " ++ name
     exclaim phrase =
     phrase ++ "!"
     excitedGreeting name =
     name
     |> String.toUpper
     |> greet "Hi"
     |> exclaim -- Hi, CODEMASH!
     excitedGreeting "CodeMash"
     -- Hi, CODEMASH!
     

     View Slide

144. function createOrder(shoppingCart, customer) {
     let total = 0
     shoppingCart.items.forEach(item => {
     total += item.price
     })
     shoppingCart.discounts.forEach(discount => {
     total *= 1 - discount
     })
     total *= 1 + shoppingCart.tax
     total += shoppingCart.shippingCost
     total = Math.round(total * 100) / 100
     return { status: 'new', customer, total }
     }
     

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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 }
     }
     

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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 }
     }
     

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147. function createOrder(shoppingCart, customer) {
     let total = R.pipe(
     addPrices(shoppingCart.items),
     addDiscounts(shoppingCart.discounts),
     addTax(shoppingCart.tax),
     add(shoppingCart.shippingCost),
     roundTotal,
     )(0)
     return { status: 'new', customer, total }
     }
     

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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 }
     }
     

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

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150. function createOrder(shoppingCart, customer) {
     let total = R.pipe(
     addPrices(shoppingCart.items),
     addDiscounts(shoppingCart.discounts),
     addTax(shoppingCart.tax),
     add(shoppingCart.shippingCost),
     roundTotal,
     )(0)
     return { status: 'new', customer, total }
     }
     

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151. const createOrder = customer => shoppingCart => {
     let total = R.pipe(
     addPrices(shoppingCart.items),
     addDiscounts(shoppingCart.discounts),
     addTax(shoppingCart.tax),
     add(shoppingCart.shippingCost),
     roundTotal,
     )(0)
     return { status: 'new', customer, total }
     }
     

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152. createOrder(myShoppingCart, 'Tucker')
     

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153. Strong Static
     Types
     

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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
     }
     

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155. createOrder shoppingCart customer =
     let
     total =
     addPrices shoppingCart.items 0
     |> addDiscounts shoppingCart.discounts
     |> addTax shoppingCart.tax
     |> add shoppingCart.shippingCost
     |> roundTotal
     in
     { status = "new"
     , customer = customer
     , total = total
     }
     

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156. createOrder shoppingCart customer =
     let
     total =
     addPrices shoppingCart.items 0
     |> addDiscounts shoppingCart.discounts
     |> addTax shoppingCart.tax
     |> add shoppingCart.shippingCost
     |> roundTotal
     in
     { status = "new"
     , customer = customer
     , total = total
     }
     

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157. createOrder customer shoppingCart =
     let
     total =
     addPrices shoppingCart.items 0
     |> addDiscounts shoppingCart.discounts
     |> addTax shoppingCart.tax
     |> add shoppingCart.shippingCost
     |> roundTotal
     in
     { status = "new"
     , customer = customer
     , total = total
     }
     

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158. -- TYPE MISMATCH ------------------------- App.elm
     The 2nd argument to `createOrder` is not what I expect:
     66| createOrder myShoppingCart "Tucker"
     ^^^^^^^^
     This argument is a string of type:
     String
     But `createOrder` needs the 2nd argument to be:
     { b
     | discounts : List Float
     , items : List { a | price : Float }
     , shippingCost : Float
     , tax : Float
     }
     

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159. -- TYPE MISMATCH ------------------------- App.elm
     The 2nd argument to `createOrder` is not what I expect:
     66| createOrder myShoppingCart "Tucker"
     ^^^^^^^^
     This argument is a string of type:
     String
     But `createOrder` needs the 2nd argument to be:
     { b
     | discounts : List Float
     , items : List { a | price : Float }
     , shippingCost : Float
     , tax : Float
     }
     

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160. -- TYPE MISMATCH ------------------------- App.elm
     The 2nd argument to `createOrder` is not what I expect:
     66| createOrder myShoppingCart "Tucker"
     ^^^^^^^^
     This argument is a string of type:
     String
     But `createOrder` needs the 2nd argument to be:
     { b
     | discounts : List Float
     , items : List { a | price : Float }
     , shippingCost : Float
     , tax : Float
     }
     

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

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

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

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164. ?
     

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165. View Slide

166. Custom
     Types
     

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167. type Item = {
     price: number,
     }
     

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168. type Item = {
     price: number,
     }
     

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169. type Item = {
     price: number,
     }
     

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170. type Item = {
     price: number,
     }
     type ShoppingCart = {
     items: Item[],
     discounts: number[],
     tax: number,
     shippingCost: number,
     }
     

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171. type Item = {
     price: number,
     }
     type ShoppingCart = {
     items: Item[],
     discounts: number[],
     tax: number,
     shippingCost: number,
     }
     

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172. type Item = {
     price: number,
     }
     type ShoppingCart = {
     items: Item[],
     discounts: number[],
     tax: number,
     shippingCost: number,
     }
     type Order = {
     status: string,
     customer: string,
     total: number,
     }
     

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173. function createOrder(
     shoppingCart: ShoppingCart,
     customer: string
     ): Order {
     ...
     }
     

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174. function createOrder(
     shoppingCart: ShoppingCart,
     customer: string
     ): Order {
     ...
     }
     

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175. type Order = {
     status: string,
     customer: string,
     total: number,
     }
     

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176. type Order = {
     status: string,
     customer: string,
     total: number,
     }
     

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177. enum Status {
     New,
     Processing,
     Shipped,
     }
     

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178. enum Status {
     New,
     Processing,
     Shipped,
     }
     

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179. enum Status {
     New,
     Processing,
     Shipped,
     }
     

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180. enum Status {
     New,
     Processing,
     Shipped,
     }
     type Order = {
     status: Status,
     customer: string,
     total: number,
     }
     

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181. function createOrder(
     shoppingCart: ShoppingCart,
     customer: string
     ): Order {
     let total = R.pipe(
     addPrices(shoppingCart.items),
     addDiscounts(shoppingCart.discounts),
     addTax(shoppingCart.tax),
     add(shoppingCart.shippingCost),
     roundTotal,
     )(0)
     return { status: Status.New, customer, total }
     }
     

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182. Hard to test Hard to follow
     Code breaks
     unexpectedly
     Too much code
     Scary to refactor
     Oh, null and
     undefined
     

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

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

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

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

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

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

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

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

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

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

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

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194. View Slide

195. if () {}
     if () {}
     if
     ()
     {}
     

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196. type Maybe a
     = Nothing
     | Just a
     

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197. type Maybe a
     = Nothing
     | Just a
     

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198. type Maybe a
     = Nothing
     | Just a
     

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199. type Maybe a
     = Nothing
     | Just a
     

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200. 42
     Just 42
     "Hi"
     Just "Hi"
     

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201. Just 42 [42]
     Nothing [ ]
     

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202. import { Maybe } from 'true-myth'
     Maybe.just(42) // Just 42
     Maybe.nothing() // Nothing
     Maybe.of(42) // Just 42
     Maybe.of(null) // Nothing
     Maybe.of(undefined) // Nothing
     

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203. function mapGet(key, map) {
     let result = map.get(key)
     return result == null
     ? Maybe.nothing()
     : Maybe.just(result)
     }
     

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204. let mapGet = Maybe.wrapReturn(
     (key, map) => map.get(key)
     )
     

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205. function getUserName(id, users) {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     

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206. function getUserName(id, users) {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     Maybe User
     

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207. function getUserName(id, users) {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     

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208. function getUserName(id, users) {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     user
     

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209. function getUserName(id, users) {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     Nothing?
     

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210. function getUserName(
     id: Id,
     users: Map
     ): string {
     let user = mapGet(id, users)
     if (user.isJust()) {
     return user.unsafelyUnwrap().name
     }
     }
     

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211. null.ts:27:4 - error TS2366:
     Function lacks ending return statement
     and return type does not include 'undefined'.
     27 ): string {
     ~~~~~~
     Found 1 error.
     $ tsc --lib es2015 --strictNullChecks null.ts
     

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212. function getUserName(
     id: Id,
     users: Map
     ): string {
     let user = mapGet(id, users)
     return user.isJust()
     ? user.unsafelyUnwrap().name
     : ''
     }
     

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

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214. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     let user = mapGet(id, users)
     if (user.isJust()) {
     let { bestFriendId } = user.unsafelyUnwrap()
     if (bestFriendId.isJust()) {
     let friend = mapGet(bestFriendId.unsafelyUnwrap(), users)
     if (friend.isJust()) {
     return friend.unsafelyUnwrap().location
     }
     return Maybe.nothing()
     }
     return Maybe.nothing()
     }
     return Maybe.nothing()
     }
     

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215. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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216. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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217. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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218. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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219. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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220. .chain(user => user.bestFriendId)
     

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221. .chain(user => user.bestFriendId)
     .chain
     Nothing Nothing
     

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222. .chain(user => user.bestFriendId)
     .chain
     Just user
     ?
     

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223. user
     .chain(user => user.bestFriendId)
     

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224. .chain(user => user.bestFriendId)
     Just
     or
     Nothing
     

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225. .chain(user => user.bestFriendId)
     .chain
     Just user
     Just
     or
     Nothing
     

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226. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users) // Maybe
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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227. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId) // Maybe
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location)
     }
     

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228. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users)) // Maybe
     .chain(friend => friend.location)
     }
     

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229. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location) // Maybe
     }
     

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230. function getBestFriendLocation(
     id: Id,
     users: Map
     ): Maybe {
     return mapGet(id, users)
     .chain(user => user.bestFriendId)
     .chain(bestFriendId => mapGet(bestFriendId, users))
     .chain(friend => friend.location) // Maybe
     }
     

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231. P.S.
     You kinda, sorta just learned monads
     

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232. Hard to test Hard to follow
     Code breaks
     unexpectedly
     Too much code
     Scary to refactor
     Oh, null and
     undefined
     

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

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

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

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236. Too much code
     Curried, Composable
     Functions
     Scary to refactor
     Code breaks unexpectedly
     Immutable Data
     Hard to test
     Pure Functions
     Hard to follow
     Declarative Code
     Oh, null and
     undefined
     

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237. Scary to refactor
     Strong, Static Types
     Too much code
     Curried, Composable
     Functions
     Code breaks unexpectedly
     Immutable Data
     Hard to test
     Pure Functions
     Hard to follow
     Declarative Code
     Oh, null and
     undefined
     

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238. Oh, null and undefined
     Maybe and Monads
     Scary to refactor
     Strong, Static Types
     Too much code
     Curried, Composable
     Functions
     Code breaks unexpectedly
     Immutable Data
     Hard to test
     Pure Functions
     Hard to follow
     Declarative Code
     

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239. Thanks!
     bit.ly/cm-practical-fp
     Jeremy Fairbank
     @elpapapollo
     

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