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Codestock 2017: Functional Programming Basics in ES6

Codestock 2017: Functional Programming Basics in ES6

Jeremy Fairbank

May 05, 2017
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  1. let age = 10; age = 11; const name =

    'Tucker'; name = 'Sally'; All good Syntax error
  2. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  3. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  4. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  5. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  6. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  7. const add = (x, y) => { return x +

    y; }; const identity = x => x;
  8. const langs = [ 'JavaScript', 'Elm', 'Haskell', ]; const [js,

    ...rest] = langs; js === 'JavaScript'; rest[0] === 'Elm'; rest[1] === 'Haskell';
  9. const langs = [ 'JavaScript', 'Elm', 'Haskell', ]; const [js,

    ...rest] = langs; js === 'JavaScript'; rest[0] === 'Elm'; rest[1] === 'Haskell';
  10. const langs = [ 'JavaScript', 'Elm', 'Haskell', ]; const [js,

    ...rest] = langs; js === 'JavaScript'; rest[0] === 'Elm'; rest[1] === 'Haskell';
  11. const langs = [ 'JavaScript', 'Elm', 'Haskell', ]; const [js,

    ...rest] = langs; js === 'JavaScript'; rest[0] === 'Elm'; rest[1] === 'Haskell';
  12. const greet = (name, greeting = 'Hi') => { console.log(greeting,

    name); }; greet('Codestock', 'Hello'); // Hello Codestock greet('Knoxville'); // Hi Knoxville
  13. const greet = (name, greeting = 'Hi') => { console.log(greeting,

    name); }; greet('Codestock', 'Hello'); // Hello Codestock greet('Knoxville'); // Hi Knoxville
  14. const greet = (name, greeting = 'Hi') => { console.log(greeting,

    name); }; greet('Codestock', 'Hello'); // Hello Codestock greet('Knoxville'); // Hi Knoxville
  15. const greet = (name, greeting = 'Hi') => { console.log(greeting,

    name); }; greet('Codestock', 'Hello'); // Hello Codestock greet('Knoxville'); // Hi Knoxville
  16. Object.assign( {}, { hello: 'Knoxville' }, { hi: 'Codestock' }

    ); // { // hello: 'Knoxville', // hi: 'Codestock' // }
  17. Object.assign( {}, { hello: 'Knoxville' }, { hi: 'Codestock' }

    ); // { // hello: 'Knoxville', // hi: 'Codestock' // }
  18. Object.assign( {}, { hello: 'Knoxville' }, { hi: 'Codestock' }

    ); // { // hello: 'Knoxville', // hi: 'Codestock' // }
  19. class Point { constructor(x, y) { this.x = x; this.y

    = y; } moveBy(dx, dy) { this.x += dx; this.y += dy; } } function Point(x, y) { this.x = x; this.y = y; } Point.prototype.moveBy = function(dx, dy) { this.x += dx; this.y += dy; };
  20. const add = (x, y) => x + y; add(2,

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

    3) === 5; add(2, 3) === 5; add(2, 3) === 5; Referentially transparent
  22. × let name = 'Jeremy'; const getName = () =>

    name; const setName = (newName) => { name = newName; }; const printUpperName = () => { console.log(name.toUpperCase()); };
  23. × let name = 'Jeremy'; const getName = () =>

    name; const setName = (newName) => { name = newName; }; const printUpperName = () => { console.log(name.toUpperCase()); };
  24. × let name = 'Jeremy'; const getName = () =>

    name; const setName = (newName) => { name = newName; }; const printUpperName = () => { console.log(name.toUpperCase()); };
  25. × let name = 'Jeremy'; const getName = () =>

    name; const setName = (newName) => { name = newName; }; const printUpperName = () => { console.log(name.toUpperCase()); };
  26. describe('api', () => { beforeEach(() => mockConsoleLog()); afterEach(() => restoreConsoleLog());

    it('sets and prints the name', () => { printUpperName(); expect(console.log).calledWith('JEREMY'); setName('Jet'); printUpperName(); expect(console.log).calledWith('JET'); }); }); ×
  27. const upperName = (name) => name.toUpperCase(); describe('api', () => {

    it('returns an uppercase name', () => { expect(upperName('Jeremy')).to.equal('JEREMY'); expect(upperName('Jet')).to.equal('JET'); }); }); ✓
  28. function doubleNumbers(numbers) { const doubled = []; const l =

    numbers.length; for (let i = 0; i < l; i++) { doubled.push(numbers[i] * 2); } return doubled; } doubleNumbers([1, 2, 3]); // [2, 4, 6]
  29. const hobbies = [ 'programming', 'reading', 'music', ]; const firstTwo

    = hobbies.splice(0, 2); console.log(firstTwo); // ['programming', 'reading'] console.log(hobbies); // ['music']
  30. const hobbies = [ 'programming', 'reading', 'music', ]; const firstTwo

    = hobbies.splice(0, 2); console.log(firstTwo); // ['programming', 'reading'] console.log(hobbies); // ['music'] Slight typo + mutability
  31. class Point { constructor(x, y) { this.x = x; this.y

    = y; } moveBy(dx, dy) { this.x += dx; this.y += dy; } } const point = new Point(0, 0); point.moveBy(5, 5); point.moveBy(-2, 2); console.log([point.x, point.y]); // [3, 7] ×
  32. const createPoint = (x, y) => Object.freeze([x, y]); const movePointBy

    = ([x, y], dx, dy) => { return Object.freeze([x + dx, y + dy]); }; let point = createPoint(0, 0); point = movePointBy(point, 5, 5); point = movePointBy(point, -2, 2); console.log(point); // [3, 7] ✓ (*or object-oriented without mutation)
  33. const createPoint = (x, y) => Object.freeze([x, y]); const movePointBy

    = ([x, y], dx, dy) => { return Object.freeze([x + dx, y + dy]); }; let point = createPoint(0, 0); point = movePointBy(point, 5, 5); point = movePointBy(point, -2, 2); console.log(point); // [3, 7] ✓ (*or object-oriented without mutation)
  34. const createPoint = (x, y) => Object.freeze([x, y]); const movePointBy

    = ([x, y], dx, dy) => { return Object.freeze([x + dx, y + dy]); }; let point = createPoint(0, 0); point = movePointBy(point, 5, 5); point = movePointBy(point, -2, 2); console.log(point); // [3, 7] ✓ (*or object-oriented without mutation)
  35. const createPoint = (x, y) => Object.freeze([x, y]); const movePointBy

    = ([x, y], dx, dy) => { return Object.freeze([x + dx, y + dy]); }; let point = createPoint(0, 0); point = movePointBy(point, 5, 5); point = movePointBy(point, -2, 2); console.log(point); // [3, 7] ✓ (*or object-oriented without mutation)
  36. CONS VERBOSE MORE OBJECT CREATION* MORE GARBAGE COLLECTION* MORE MEMORY

    USAGE* *Alleviated with libs like Immutable.js
  37. const multiply = (x, y) => x * y; function

    add(x, y) { return x + y; } const addAlias = add; const evens = [1, 2, 3].map(n => n * 2);
  38. const multiply = (x, y) => x * y; function

    add(x, y) { return x + y; } const addAlias = add; const evens = [1, 2, 3].map(n => n * 2);
  39. const multiply = (x, y) => x * y; function

    add(x, y) { return x + y; } const addAlias = add; const evens = [1, 2, 3].map(n => n * 2);
  40. const multiply = (x, y) => x * y; function

    add(x, y) { return x + y; } const addAlias = add; const evens = [1, 2, 3].map(n => n * 2);
  41. const createAdder = (x) => { return (y) => x

    + y; }; const add3 = createAdder(3); add3(2) === 5; add3(3) === 6;
  42. const createAdder = (x) => { return (y) => x

    + y; }; const add3 = createAdder(3); add3(2) === 5; add3(3) === 6;
  43. const request = (options) => { return fetch(options.url, options) .then(resp

    => resp.json()); }; const usersPromise = request({ url: '/users', headers: { 'X-Custom': 'mykey' } }); const tasksPromise = request({ url: '/tasks', headers: { 'X-Custom': 'mykey' } });
  44. const request = (options) => { return fetch(options.url, options) .then(resp

    => resp.json()); }; const usersPromise = request({ url: '/users', headers: { 'X-Custom': 'mykey' } }); const tasksPromise = request({ url: '/tasks', headers: { 'X-Custom': 'mykey' } }); Repetitive
  45. const createRequester = (options) => { return (otherOptions) => {

    return request(Object.assign( {}, options, otherOptions )); }; }; const customRequest = createRequester({ headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  46. const createRequester = (options) => { return (otherOptions) => {

    return request(Object.assign( {}, options, otherOptions )); }; }; const customRequest = createRequester({ headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  47. const createAdder = (x) => { return (y) => x

    + y; }; const createRequester = (options) => { return (otherOptions) => { return request(Object.assign( {}, options, otherOptions )); }; }; RECALL
  48. const add = (x, y) => x + y; const

    add3 = partial(add, 3); add3(2) === 5;
  49. const add = (x, y) => x + y; const

    add3 = partial(add, 3); add3(2) === 5;
  50. const request = (defaults, options) => { options = Object.assign({},

    defaults, options); return fetch(options.url, options) .then(resp => resp.json()); }; const customRequest = partial(request, { headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  51. const request = (defaults, options) => { options = Object.assign({},

    defaults, options); return fetch(options.url, options) .then(resp => resp.json()); }; const customRequest = partial(request, { headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  52. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  53. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  54. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  55. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  56. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  57. const partialFromBind = (fn, ...args) => { return fn.bind(null, ...args);

    }; const partial = (fn, ...args) => { return (...otherArgs) => { return fn(...args, ...otherArgs) }; };
  58. const add3 = add(3); add3(2) === 5; const customRequest =

    request({ headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  59. const add3 = add(3); add3(2) === 5; const customRequest =

    request({ headers: { 'X-Custom': 'mykey' } }); const usersPromise = customRequest({ url: '/users' }); const tasksPromise = customRequest({ url: '/tasks' });
  60. const add = x => y => x + y;

    function add(x) { return function(y) { return x + y; }; }
  61. const add = x => y => x + y;

    function add(x) { return function(y) { return x + y; }; }
  62. const add = x => y => x + y;

    function add(x) { return function(y) { return x + y; }; }
  63. const request = defaults => options => { options =

    Object.assign( {}, defaults, options ); return fetch(options.url, options) .then(resp => resp.json()); };
  64. const request = defaults => options => { options =

    Object.assign( {}, defaults, options ); return fetch(options.url, options) .then(resp => resp.json()); };
  65. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  66. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  67. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  68. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  69. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  70. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  71. const map = fn => array => array.map(fn); const multiply

    = x => y => x * y; const pluck = key => object => object[key]; const discount = multiply(0.98); const tax = multiply(1.0925); const customRequest = request({ headers: { 'X-Custom': 'mykey' } }); customRequest({ url: '/cart/items' }) .then(map(pluck('price'))) .then(map(discount)) .then(map(tax));
  72. [ { price: 5 }, { price: 10 }, {

    price: 3 }, ] map(pluck('price')) [ 5, 10, 3, ] item.price
  73. const processWord = compose(hyphenate, reverse, toUpperCase); const words = [

    'hello', 'functional', 'programming' ]; const newWords = words.map(processWord); console.log(newWords); // ['OL-LEH, 'LANOI-TCNUF', 'GNIMM-ARGORP']
  74. const processWord = compose(hyphenate, reverse, toUpperCase); const words = [

    'hello', 'functional', 'programming' ]; const newWords = words.map(processWord); console.log(newWords); // ['OL-LEH, 'LANOI-TCNUF', 'GNIMM-ARGORP']
  75. const compose = (...fns) => arg => ( fns.reduceRight( (result,

    fn) => fn(result), arg ) ); fns = [hyphenate, reverse, toUpperCase] arg = result = 'hello'
  76. const compose = (...fns) => arg => ( fns.reduceRight( (result,

    fn) => fn(result), arg ) ); fns = [hyphenate, reverse] result = 'HELLO'
  77. const compose = (...fns) => arg => ( fns.reduceRight( (result,

    fn) => fn(result), arg ) ); fns = [hyphenate] result = 'OLLEH'
  78. const compose = (...fns) => arg => ( fns.reduceRight( (result,

    fn) => fn(result), arg ) ); fns = [] result = 'OL-LEH'
  79. const factorial = (n) => { let result = 1;

    while (n > 1) { result *= n; n--; } return result; };
  80. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); };
  81. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; Recursive call
  82. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; Base case
  83. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1;
  84. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2;
  85. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6;
  86. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24;
  87. STEPS Find the recurrence
 (n × n-1 × n-2 ×

    … 1)
 Find the base case
 (n < 2)
  88. const value = factorial(100000); console.log(value); // ??? WHAT IS THE

    RESULT? RangeError: Maximum call stack size exceeded
  89. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> Call Stack
  90. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) Call Stack
  91. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) factorial(3) Call Stack
  92. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) factorial(3) factorial(2) Call Stack
  93. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) factorial(3) factorial(2) factorial(1) Call Stack
  94. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) factorial(3) factorial(2) Call Stack
  95. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) factorial(3) Call Stack
  96. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> factorial(4) Call Stack
  97. factorial(4); 4 * factorial(3); 4 * 3 * factorial(2); 4

    * 3 * 2 * factorial(1); 4 * 3 * 2 * 1; 4 * 3 * 2; 4 * 6; 24; <main> Call Stack
  98. const value = factorial(100000); console.log(value); // ??? 100,000 calls =

    100,000 stack frames 1 stack frame ≈ 48B Max stack usage ≈ 1MB 100,000 x 48 / 1024 / 1024 = 4.58MB > 1MB
  99. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; UNOPTIMIZABLE
  100. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; 1 UNOPTIMIZABLE
  101. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; 1 UNOPTIMIZABLE 2
  102. const factorial = (n) => { if (n < 2)

    { return 1; } return n * factorial(n - 1); }; 1 UNOPTIMIZABLE 2 3
  103. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; OPTIMIZABLE
  104. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; OPTIMIZABLE
  105. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; OPTIMIZABLE
  106. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; OPTIMIZABLE
  107. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; 1 OPTIMIZABLE
  108. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; 1 2 OPTIMIZABLE
  109. const factorial = (n, accum = 1) => { if

    (n < 2) { return accum; } return factorial(n - 1, n * accum); }; 1 2 3 OPTIMIZABLE