Monads you've already put in production (without knowing it)

Transcript

1. Monads you are already using in prod Tejas Dinkar nilenso
2. about.me • Hi, I’m Tejas • Nilenso: Partner • twitter: tdinkar • github: gja
3. Serious Pony
4. Online Abuse
5. Trouble at the Koolaid Point http://seriouspony.com/trouble-at-the-koolaid-point/ https://storify.com/adriarichards/telling-my-troll-story-because- kathy-sierra-left-t
6. If you think you understand Monads, you don't understand Monads.
8. This talk is inaccurate and will make a mathematician cry
10. Goal of this talk For you to say “Oh yeah, I’ve used that hack”
12. Monads • Programmable Semicolons • Used to hide plumbing away from you • You can say Monads in almost any sentence and people will think you are smart
14. Values Value
15. Monads Value Box
16. Mysore Masala Monad M onad Value
17. Monads Value Box
18. Monads • Monads define two functions • return takes a value and puts it in a box • bind takes a box & function f, returning f(value) • it is expected that the function returns a box
19. Value Value Another Value Value Function return bind
20. Our Function Signatures Value f(value)
21. Some math (√4) + 5
22. Some math (√4) + 5 3 or 7!
23. Value 4
24. Monad [4]
25. [alive, dead]
26. ruby! x = [1, 2, 3] y = x.map { |x| x + 1 } # y = [2, 3, 4]
27. return Value Value return
28. return def m_return(x) [x] end # m_return(4) => [4]
29. The functions Value f(value)
30. Square Root fn def sqrt(x) s = Math.sqrt(x) [s, -s] end # sqrt(4) => [2, -2]
31. Increment Fn def inc_5(x) [x + 5] end # inc_5(1) => [6]
32. Bind Functions Another Value Value Function bind
33. Bind Function x = m_return(4) y = x.????? { |p| sqrt(p) } # I want [-2, 2]
34. Bind Function x = m_return(4) y = x.map {|p| sqrt(p) } # y => [[2, -2]] # ^—— Box in a box?
35. Bind Function x = m_return(4) y = x.mapcat {|p| sqrt(p) } # y => [2, -2]
36. Putting it together m_return(4) .mapcat {|p| sqrt(p)} .mapcat {|p| inc_5(p)} # => [3, 7]
37. You have invented the List Monad, used to model non-determinism Congrats
38. Turtles all the way down
39. A small constraint • Let’s do a bit of a self imposed constraint on this • Functions must return either 0 or 1 elements • (we’ll only model positive integers here)
40. return - stays the same
41. bind - stays the same x = m_return(4) y = x.mapcat { |p| inc_5(p) } # y => 9
42. Square Root Fn def sqrt(x) if (x < 0) return [] #error else [Math.sqrt(x)] end end # sqrt(4) => [2] # sqrt(-1) => []
43. Describe in English There is a list passed to each step Maybe this list has just one element, or Maybe it has none
45. The Maybe Monad • The intent is to short circuit computation • The value of the `box’ is None, or Just(Value) • You can think of it as a type-safe nil / null
46. try def try(x, f) if x == nil return f(x) else return nil end end # 4.try { |x| x + 5 } => 9 # nil.try {|x| x + 5 } => nil
48. Let’s start over • The Monad Laws • Left Identity • Right Identity • Associativity
49. Left Identity m_return(a).bind(f) == f(a)
50. Right Identity m.bind(m_return) == m
51. Associativity m.bind(f).bind(g) == m.bind(x -> f(x).bind(g))
52. Store Computation
53. The State Monad • Rest of the world - State Machine (sorta) • The value inside the box f(state) => [r new-state] • Particularly useful in pure languages like Haskell • Let’s build a stack
54. The functions Value f(value)
55. The functions (f(value) state) [new-value, new-state]
56. push def push(val) lambda { |state| new_state = state.push(val) [value, new_state] } end
57. pop def pop() lambda { |state| val = state.pop() [val, state] } end
58. def double_top() lambda { |state| top = state.pop() [2 * top, state.push(2*top)] } end double_top
59. return def m_return(x) lambda { |state| [x, state] } end
60. bind def bind(mv, f) lambda { |state| v, temp_state = mv(state) state_fn = f(v) state_fn(temp_state) } end
61. example # Not working code ! m_return(4) .bind(a -> push(a)) .bind(b -> push(b + 1)) .bind(c -> double_top()) .bind(d -> sum_top2()) .bind(e -> pop())
63. Associativity m.bind(f).bind(g) == m.bind(x => f(x).bind(g))
64. turn this # Not working code ! m_return(4) .bind(a -> push(a)) .bind(b -> push(b + 1)) .bind(c -> double_top()) .bind(d -> sum_top2()) .bind(e -> pop())
65. into this m_return(4) .bind(a -> push(a) .bind(b -> push(b + 1) .bind(c -> double_top() .bind(d -> sum_top() .bind(e -> pop())))))
66. done with ruby
67. imagine # Not working code state_monad { a <- m_return(4) b <- push(a) c <- push(b + 1) d <- double_top() e <- sum_top2() pop() }
68. Back to List m_return(4) .mapcat {|p| sqrt(p)} .mapcat {|p| inc_5(p)} # => [3, 7]
69. Back to List m_return(4) .mapcat {|a| sqrt(a) .mapcat {|b| inc_5(b)}} # => [3, 7]
70. Back to List list_monad { a <- m_return(4) b <- sqrt(a) c <- inc_5(b) c }
71. On to Clojure • this is an example from clojure.net • the state is a vector containing every function we’ve called so far
72. (defn inc-s [x] (fn [state] [(inc x) (conj state :inc)]))
73. in clojure (defn inc-s [x] (fn [state] [(inc x) (conj state :inc)])) (defn do-things [x] (domonad state-m [a (inc-s x) b (double-s a) c (dec-s b) d (dec-s c)] d)) ! ((do-things 7) []) => [14 [:inc :double :dec :dec]]
74. state monad in Clojure (defmonad state-m "Monad describing stateful computations. The monadic values have the structure (fn [old-state] [result new-state])." [m-result (fn m-result-state [v] (fn [s] [v s])) m-bind (fn m-bind-state [mv f] (fn [s] (let [[v ss] (mv s)] ((f v) ss)))) ])
75. state monad in Haskell inc = state (\st -> let st' = st +1 in (st’,st')) inc3 = do x <- inc y <- inc z <- inc return z
76. Finally, IO
77. IOMonad • rand-int(100) is non deterministic !
78. ay-yo
79. IOMonad • rand-int(100) is non deterministic • rand-int(100, seed = 42) is deterministic • monadic value: f(world) => [value, world-after-io]
80. IOMonad • puts() just `appends to a buffer’ in the real world • How does gets() return different strings? • gets() returns a fixed value based on the `world’
81. Image Credits http://www.myfoodarama.com/2010/11/masala- dosa.html http://www.clojure.net/2012/02/10/State/ http://www.cafepress.com/ +no_place_like_home_ruby_slippers_3x5_area_rug, 796646161 http://www.netizens-stalbans.co.uk/installs-and- upgrades.html.htm http://www.hpcorporategroup.com/what-is-the-life- box.html
82. Thank You MANY QUESTIONS? VERY MONAD SO FUNCTIONAL Y NO CLOJURE? TEJAS@GJA.IN @tdinkar WOW WOW WOW MUCH EASY SUPER SIMPLE