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1. Awesomely descriptive JavaScript with Monads

2. Theory

3. Monad laws return a >>= f ≡ f a m >>= return ≡ m (m >>= f) >>= g ≡ m >>= (x -> f x >>= g)

4. Monad laws

5. Practical theory

7. V E G E T A B L E A L U E

10. P => M<S> f

11. => unit P => M

12. ( , ) => bind ( M , P => M<S> ) => M<S>

13. ( M , P => M<S> ) => M<S>M P => M<S> M<S> extract value bind function m(x) x x => m(y) m(y) bind

14. Monad laws (JavaScript) bind( unit(v) , f ) ≡ f(v) bind( monad , unit ) ≡ monad bind( bind( monad , f ) , g ) ≡ bind( monad, v => bind( f(v) , g ) )

15. monet.js

16. UNIT Identity(x) Maybe.Some(x) Maybe.None() Either.Left(e) Either.Right(x) List(x, Nil) monad.bind(f) monad.flatMap(f) monad.chain(f) BIND

17. Monad laws (monet.js / Identity) Identity(v).flatMap(f) ≡ f(v) id.flatMap(Identity) ≡ id id.flatMap(f).flatMap( g ) ≡ id.flatMap(v => f(v).flatMap(g))

18. Practice: Identity

20. var pepperInABowl = getBowlOf('pepper', 'red'); var pepper = pepperInABowl.get(); var slicedPepperInABowl = slice(pepper); var tomatoInABowl = getBowlOf('tomato'); var tomato = tomatoInABowl.get(); var slicedTomatoInABowl = slice(tomato); var slicedPepper = slicedPepperInABowl.get(); var slicedTomato = slicedTomatoInABowl.get(); var mixInABowl = mix(slicedPepper, slicedTomato);

22. const mixInABowl = getBowlOf('pepper') .flatMap(slice) .faltMap(silcedPepper => getBowlOf('tomato') .flatMap(slice) .faltMap(mixedTomato => mix(silcedPepper , mixedTomato)));

24. But HOW ?

25. interface Bowl<V> { get(): V; } getBowlOf<V>(name: string): Bowl<V>; slice<S>(vegetable: Vege): Bowl<S>; mix<M>(...ingredients): Bowl<M>;

26. var pepperInABowl = getBowlOf('pepper'); var pepper = pepperInABowl.get(); var slicedPepperInABowl = slice(pepper); var tomatoInABowl = getBowlOf('tomato'); var tomato = tomatoInABowl.get(); var slicedTomatoInABowl = slice(tomato); var slicedPepper = slicedPepperInABowl.get(); var slicedTomato = slicedTomatoInABowl.get(); var mixInABowl = mix(slicedPepper, slicedTomato);

30. monet.js Identity( … )

31. type Op<A, B> = (val:A) => Bowl; interface Bowl<V> { get(): V; flatMap<S>(f: Op<V, S>): Bowl<S>; }

32. goo.gl/dh9DrZ

34. const slicedPepperInABowl = getBowlOf('pepper').flatMap(slice);

36. const slicedPepperInABowl = getBowlOf('pepper').flatMap(slice); const slicedTomatoInABowl = getBowlOf('tomato').flatMap(slice);

38. const slicedPepperInABowl = getBowlOf('pepper').flatMap(slice); const mixInABowl = getBowlOf('pepper') .flatMap(slice) .faltMap(silcedPepper => mix(silcedPepper , slicedPepperInABowl.get()));

39. const slicedPepperInABowl = getBowlOf('pepper').flatMap(slice); const mixInABowl = getBowlOf('pepper') .flatMap(slice) .flatMap(silcedPepper => slicedPepperInABowl .flatMap(mixedTomato => mix(silcedPepper , mixedTomato)));

40. const mixInABowl = getBowlOf('pepper') .flatMap(slice) .faltMap(silcedPepper => getBowlOf('tomato') .flatMap(slice) .flatMap(mixedTomato => mix(silcedPepper , mixedTomato)));

41. var pepperInABowl = getBowlOf('pepper', 'red'); var pepper = pepperInABowl.get(); var slicedPepperInABowl = slice(pepper); var tomatoInABowl = getBowlOf('tomato'); var tomato = tomatoInABowl.get(); var slicedTomatoInABowl = slice(tomato); var slicedPepper = slicedPepperInABowl.get(); var slicedTomato = slicedTomatoInABowl.get(); var mixInABowl = mix(slicedPepper, slicedTomato);

42. Practice: Maybe

45. let spiceMix; const salt = getSpice('salt'); let coriander = getSpice('coriander'); if (coriander) { coriander = crush(coriander); } if (coriander && salt && isPowdered(salt)) { spiceMix = mixSpice(salt , coriander); } spiceMix = vegeta;

46. let spiceMix; const salt = getSpice('salt'); let coriander = getSpice('coriander'); if (coriander) { coriander = crush(coriander); } if (coriander && salt && isPowdered(salt)) { spiceMix = mixSpice(salt , coriander); } spiceMix = vegeta;

47. const spiceMix = getSpice('salt').filter(isPowdered) .flatMap(salt => getSpice('coriander').map(crush) .flatMap(coriander => mixSpice(salt , coriadner))) .orJust(vegeta);

48. But HOW ?

49. getSpice<V>(name: string): Maybe<V>; crush(spice: S): S; isPowdered(spice: S): boolean;

54. monet.js Maybe

55. interface Maybe<T> { flatMap<V>(fn: (val: T) => Maybe<V>): Maybe<V>; map<V>(fn: (val: T) => V): Maybe<V>; filter(fn: (val: T) => boolean): Maybe<T>; orJust(val: T): T; }

56. const silkySalt = getSpice('salt').filter(isPowdered); const crushedCoriander = getSpice('coriander').map(crush); const spiceMixMaybe = silkySalt.flatMap(salt => crushedCoriander.flatMap(coriander => mixSpice( salt , coriadner ))); const spiceMix = spiceMixMaybe.orJust(vegeta);

57. const crushedCoriander = getSpice('coriander').map(crush); const spiceMixMaybe = getSpice('salt').filter(isPowdered).flatMap(salt => crushedCoriander.flatMap(coriander => mixSpice( salt , coriadner ))); const spiceMix = spiceMixMaybe.orJust(vegeta);

58. const spiceMixMaybe = getSpice('salt').filter(isPowdered).flatMap(salt => getSpice('coriander').map(crush).flatMap(coriander => mixSpice( salt , coriadner ))); const spiceMix = spiceMixMaybe.orJust(vegeta);

59. const spiceMixMaybe = getSpice('salt').filter(isPowdered) .flatMap(salt => getSpice('coriander').map(crush) .flatMap(coriander => mixSpice( salt , coriadner ))); const spiceMix = spiceMixMaybe.orJust(vegeta);

60. const spiceMix = getSpice('salt').filter(isPowdered) .flatMap(salt => getSpice('coriander').map(crush) .flatMap(coriander => mixSpice( salt , coriadner ))); .orJust(vegeta)

61. const spiceMix = getSpice('salt') .filter(isPowdered) .flatMap(salt => getSpice('coriander') .map(crush) .flatMap(coriander => mixSpice(salt , coriadner))) .orJust(vegeta);

62. null

63. getSpice<V>(name: string): Maybe<V> { let spice = locker.get(name); if (spice != null) { return Some(spice); } return None(); }

64. getSpice<V>(name: string): Maybe<V> { const spice = locker.get(name); if (spice != null) { return Some(spice); } return None(); }

65. getSpice<V>(name: string): Maybe<V> { const spice = locker.get(name); if (spice != null) { return … ? } return None(); }

66. monet.js Some( … )

68. monet.js None()

70. monet.js .fromNull()

71. getSpice<V>(name: string): Maybe<V> { return Maybe.fromNull( locker.get(name) ); }

73. goo.gl/cvGVeo

74. const saladBowl = mixInABowl .flatMap(mixedVegetables => mix(mixedVegetables, spiceMix));

75. e a t i t

76. Monads Identity Maybe Either Validation List NEL Reader Free IO ?

77. Monads Identity Maybe Either Validation List NEL Reader Free IO Promise

78. T H X

79. Jakub . Strojewski @gmail.com u l f r y k

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Awesomely descriptive JavaScript with monads

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Awesomely descriptive JavaScript with monads

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