モナドがくれたもの

LT駆動開発18の発表資料です。
https://github.com/LTDD/Sessions/wiki/LT%E9%A7%86%E5%8B%95%E9%96%8B%E7%99%BA18

解説など
http://blog.eiel.info/blog/2015/09/05/ltdd-18/

Transcript

1. モナドがくれたもの 2015-09-05 LTۦಈ։ൃ18
2. 10年後の8月、
3. 10年後の8月、 未来を変えられるのを信じて。
4. ͻΉΒ ͱ΋ͻ͜ Scala ೉͍͠ɻ Haskell೉͍͠ɻ Ͱ΋ɺͨͿΜ೉͍͠ͱ͜Ζ͕ҧ͏ɻ
5. ࠓ݄ͷπΠʔτ https://twitter.com/mrtc0/status/637823757550153729
6. ࠓ݄ͷπΠʔτ https://twitter.com/mrtc0/status/637823757550153729
7. ࠓ݄ͷπΠʔτ https://twitter.com/mrtc0/status/637823757550153729 爆発炎上しました
8. Ϟφυͬͯ ͳʹ͕خ͍͠Μ͚ͩͬ
9. ボイラープレートコード を除去できる
10. コンピュータが得意なことは？
11. 同じことをくりかえす
12. シンプルなコードをかくには？
13. くりかえしをみつけること
14. みつけた「くりかえし」に 名前をつけておくと便利
15. > モナド <
16. ྫࣔ͸ཧղͷࢼۚੴ
17. 値を保存しておく場所をつくる • 変数a には 2が束縛されている • 変数b には 3が束縛さてている • 変数c には 4が束縛されている • これを表現するデータ構造
18. ม਺Λอଘ͢Δςʔϒϧ import Data.Map as M hiding (map, foldr) table = M.fromList [("a",2), ("b",3), ("c",4)]
19. ม਺aͷ஋͸Կ? import Data.Map as M hiding (map, foldr) table = M.fromList [("a",2), ("b",3), (“c",4)] lookupA :: Maybe Int lookupA = M.lookup "a" table -- => Just 2
20. ม਺aͷ஋͸Կ? import Data.Map as M hiding (map, foldr) table = M.fromList [("a",2), ("b",3), (“c",4)] lookupA :: Maybe Int lookupA = M.lookup "a" table -- => Just 2 Just is 何？
21. テーブルの中には 値がないかもしれない
22. Justはテーブルに 値があったことを示す
23. ม਺zͷ஋͸ʁ table = M.fromList [("a",2), ("b",3), (“c",4)] lookupZ :: Maybe Int lookupZ = M.lookup "z" table -- => Nothing
24. ม਺zͷ஋͸ʁ table = M.fromList [("a",2), ("b",3), (“c",4)] lookupZ :: Maybe Int lookupZ = M.lookup "z" table -- => Nothing 変数z はないよ
25. Nothing は 値がなかったことを示す
26. tableの中には 期待した変数の情報はないかも しれない…
27. 課題1 • 変数a に1を足した値は？
28. ͳΕͯΔߟ͑ํͰ -- a ͸औΕͳ͍͔΋͠Εͳ͍ͷͰ͜ΕΛߟྀ͢Δ aPlus1 :: Maybe Int aPlus1 = let a = M.lookup "a" table in if isJust a then Just (1 + (fromJust a)) else Nothing aPlus1 -- => Just 3
29. ͳΕͯΔߟ͑ํͰ -- a ͸औΕͳ͍͔΋͠Εͳ͍ͷͰ͜ΕΛߟྀ͢Δ aPlus1 :: Maybe Int aPlus1 = let a = M.lookup "a" table in if isJust a then Just (1 + (fromJust a)) else Nothing aPlus1 -- => Just 3 a が Justか確認
30. ͳΕͯΔߟ͑ํͰ -- a ͸औΕͳ͍͔΋͠Εͳ͍ͷͰ͜ΕΛߟྀ͢Δ aPlus1 :: Maybe Int aPlus1 = let a = M.lookup "a" table in if isJust a then Just (1 + (fromJust a)) else Nothing aPlus1 -- => Just 3 a が Justか確認 Just をはずして1を足す
31. ͳΕͯΔߟ͑ํͰ -- a ͸औΕͳ͍͔΋͠Εͳ͍ͷͰ͜ΕΛߟྀ͢Δ aPlus1 :: Maybe Int aPlus1 = let a = M.lookup "a" table in if isJust a then Just (1 + (fromJust a)) else Nothing aPlus1 -- => Just 3 a が Justか確認 Just をはずして1を足す a がなかった場合と区別するために Justをつける
32. ύλʔϯϚονΛ͔ͭͬͨॻ͖ํ aPlus1' :: Maybe Int aPlus1' = case M.lookup "a" table of Nothing -> Nothing Just a -> Just (a + 1)
33. ύλʔϯϚονΛ͔ͭͬͨॻ͖ํ aPlus1' :: Maybe Int aPlus1' = case M.lookup "a" table of Nothing -> Nothing Just a -> Just (a + 1) 変数aがなかったら Nothing
34. ύλʔϯϚονΛ͔ͭͬͨॻ͖ํ aPlus1' :: Maybe Int aPlus1' = case M.lookup "a" table of Nothing -> Nothing Just a -> Just (a + 1) 変数aがなかったら Nothing 変数aがあったら aと名付ける
35. ύλʔϯϚονΛ͔ͭͬͨॻ͖ํ aPlus1' :: Maybe Int aPlus1' = case M.lookup "a" table of Nothing -> Nothing Just a -> Just (a + 1) 変数aがなかったら Nothing 変数aがあったら aと名付ける aに1を足してJustにする
36. 抽象化してみよう
37. とにかく、俺はaに1を足したいん だ。JustとかNothingとかしら ねーよ。上手いことなんとかやっ てくれ。
38. fmap aPlus1'' = fmap (\a -> a + 1) (M.lookup "a" table) aPlus1’’ -- => Just 3
39. fmap aPlus1'' = fmap (\a -> a + 1) (M.lookup "a" table) aPlus1’’ -- => Just 3 変数aがあったら a + 1をする
40. fmap aPlus1'' = fmap (\a -> a + 1) (M.lookup "a" table) aPlus1’’ -- => Just 3 変数aがあったら a + 1をする JustやNothingはでてこない (結果には出る)
41. fmap aPlus1'' = fmap (\a -> a + 1) (M.lookup "a" table) aPlus1’’ -- => Just 3 変数aがあったら a + 1をする JustやNothingはでてこない (結果には出る) a + 1 が興味の中心
42. 課題2 • 変数a と 変数b の値の和は？
43. a + b aPlusB :: Maybe Int aPlusB = case M.lookup "a" table of Nothing -> Nothing Just a -> case M.lookup "b" table of Nothing -> Nothing Just b -> Just (a + b) -- Just
44. a + b aPlusB :: Maybe Int aPlusB = case M.lookup "a" table of Nothing -> Nothing Just a -> case M.lookup "b" table of Nothing -> Nothing Just b -> Just (a + b) -- Just Just と Nothingの確認をひたすら…
45. ந৅Խ…ʁ -- ந৅Խ? Just (Just 5) ͭΒ͍… aPlusB' :: Maybe (Maybe Int) aPlusB' = fmap (\a -> fmap (\b -> a + b ) (M.lookup "b" table) ) (M.lookup "a" table) aPlusB’ -- => Just (Just 5)
46. joinΛಋೖ aPlusB'' :: Maybe Int aPlusB'' = join . fmap (\a -> fmap (\b -> a + b ) (M.lookup "b" table) ) $ (M.lookup "a" table)
47. 課題3 • 変数a と 変数b と 変数cの和は？
48. a + b + c aPlusBPlusC :: Maybe Int aPlusBPlusC = case M.lookup "a" table of Nothing -> Nothing Just a -> case M.lookup "b" table of Nothing -> Nothing Just b -> case M.lookup "c" table of Nothing -> Nothing Just c -> Just $ a + b + c
49. a + b + c aPlusBPlusC :: Maybe Int aPlusBPlusC = case M.lookup "a" table of Nothing -> Nothing Just a -> case M.lookup "b" table of Nothing -> Nothing Just b -> case M.lookup "c" table of Nothing -> Nothing Just c -> Just $ a + b + c Just と Nothingの確認をひたすら…
50. だんだんつらくなってきた
51. ந৅Խ aPlusBPlusC' :: Maybe (Maybe (Maybe Int)) aPlusBPlusC' = fmap (\a -> fmap (\b -> fmap (\c -> a + b + c ) (M.lookup "c" table) ) (M.lookup "b" table) ) (M.lookup "a" table) aPlusBPlusC' -- Just (Just (Just 9))
52. ந৅Խ aPlusBPlusC' :: Maybe (Maybe (Maybe Int)) aPlusBPlusC' = fmap (\a -> fmap (\b -> fmap (\c -> a + b + c ) (M.lookup "c" table) ) (M.lookup "b" table) ) (M.lookup "a" table) aPlusBPlusC' -- Just (Just (Just 9)) 綺麗な形をしているけど Just (Just (Just 9)) がつらい
53. joinͷಋೖ aPlusBPlusC_' :: Maybe Int aPlusBPlusC_' = join . fmap (\a -> join . fmap (\b -> fmap (\c -> a + b + c ) (M.lookup "c" table) ) $ (M.lookup "b" table) ) $ (M.lookup "a" table)
54. joinͷಋೖ aPlusBPlusC_' :: Maybe Int aPlusBPlusC_' = join . fmap (\a -> join . fmap (\b -> fmap (\c -> a + b + c ) (M.lookup "c" table) ) $ (M.lookup "b" table) ) $ (M.lookup "a" table) Just 9 になったけど 最深部だけ違う…おしい
55. ͘Γ͔͑ͨ͢Ίʹগ͠৑௕ʹ aPlusBPlusC'' :: Maybe Int aPlusBPlusC'' = join . fmap (\a -> join . fmap (\b -> join . fmap (\c -> Just $ a + b + c ) $ (M.lookup "c" table) ) $ (M.lookup "b" table) ) $ (M.lookup "a" table)
56. ͘Γ͔͑ͨ͢Ίʹগ͠৑௕ʹ aPlusBPlusC'' :: Maybe Int aPlusBPlusC'' = join . fmap (\a -> join . fmap (\b -> join . fmap (\c -> Just $ a + b + c ) $ (M.lookup "c" table) ) $ (M.lookup "b" table) ) $ (M.lookup "a" table) くりかえし同じことをしている
57. くりかえしを うまく隠せないだろうか？
58. • join . map を >>= におきかえる • Just を return におきかえる
59. ந৅Խ aPlusBPlusC''' :: Maybe Int aPlusBPlusC''' = (>>= (\a -> (>>= (\b -> (>>= (\c -> return $ a + b + c )) (M.lookup "c" table) )) (M.lookup "b" table) )) (M.lookup "a" table)
60. ੔ཧ͢Δ aPlusBPlusC'''' :: Maybe Int aPlusBPlusC'''' = M.lookup "a" table >>= \a -> M.lookup "b" table >>= \b -> M.lookup "c" table >>= \c -> return $ a + b + c
61. do ߏจΛ࢖͏ aPlusBPlusC''''' :: Maybe Int aPlusBPlusC''''' = do a <- M.lookup "a" table b <- M.lookup "b" table c <- M.lookup "c" table return $ a + b + c
62. ݩͷͱൺֱ aPlusBPlusC :: Maybe Int aPlusBPlusC = case M.lookup "a" table of Nothing -> Nothing Just a -> case M.lookup "b" table of Nothing -> Nothing Just b -> case M.lookup "c" table of Nothing -> Nothing Just c -> Just $ a + b + c
63. M.lookupͷ݁ՌΛҾ਺Ͱ͏͚ͱΔΑ͏ʹ aPlusBPlusC_ mA mB mC = do a <- mA b <- mB c <- mC return $ a + b + c
64. ͔ͭͬͯΈΔ aPlusBPlusC_ (M.lookup "a" table) (M.lookup "b" table) (M.lookup "c" table) — Just 9 aPlusBPlusC_ (Just 2) (Just 3) (Just 4) — Just 9 aPlusBPlusC_ Nothing (Just 3) (Just 4) — Nothing aPlusBPlusC_ (Just 2) Nothing (Just 4) — Nothing
65. ந৅Խ͍ͯ͠ΔͷͰϦετͰ΋ aPlusBPlusC_ [2] [3] [4] — [9] aPlusBPlusC_ [] [3] [4] — [] aPlusBPlusC_ [2] [] [4] — [] aPlusBPlusC_ [2,3] [3] [4] — [9,10] aPlusBPlusC_ [2,3] [3,4] [4] — [9,10,10,11]
66. ந৅Խ͍ͯ͠ΔͷͰϦετͰ΋ aPlusBPlusC_ [2] [3] [4] — [9] aPlusBPlusC_ [] [3] [4] — [] aPlusBPlusC_ [2] [] [4] — [] aPlusBPlusC_ [2,3] [3] [4] — [9,10] aPlusBPlusC_ [2,3] [3,4] [4] — [9,10,10,11] [2+3+4, 2+4+4, 3+3+4, 3+4+4]
67. in Ruby [2,3].map do |a| [3,4].map do |b| [4].map do |c| a + b + c end end end
68. OOPͬΆ͘ omap = flip fmap listAPlusBPlusC :: [] ([] ([] Int)) listAPlusBPlusC = [2,3] `omap` \a -> [3,4] `omap` \b -> [4,5] `omap` \c -> a + b + c
69. [] Int ͕͍͍ͳ͊ listAPlusBPlusC' :: [Int] listAPlusBPlusC' = [2,3] >>= \a -> [3,4] >>= \b -> [4] >>= \c -> return $ a + b + c
70. do ߏจͰɻ listAPlusBPlusC'' = do a <- [2,3] b <- [3,4] c <- [4,5] return $ a + b + c
71. >>=ͷத਎ Maybe൛ mA >>= f = case mA of Nothing = Nothing Just a = Just (f a)
72. >>= ͷத਎ Ϧετ൛ mA >>= f = case mA of [] -> [] x:xs -> go (f x) (xs >>= f) where go [] acc = acc go (y:ys) acc = y : (go ys acc)
73. 課題4 • >>= を展開してみよう • Maybe • List
74. ՝୊༻ aPlusBPlusC_ mA mB mC = do mA >>= \a -> mB >>= \b -> mC >>= \c -> return $ a + b + c
75. Maybe aPlusBPlusC__ mA mB mC = case mA of Nothing -> Nothing Just a -> case mB of Nothing -> Nothing Just b -> case mC of Nothing -> Nothing Just c -> Just (a + b + c)
76. case mA of [] -> [] xa:xas -> go ((\a -> case mB of [] -> [] xb:xbs -> go ((\b -> case mC of [] -> [] x:xs -> go ((\c -> a + b + c) x) (xs >>= (\c -> a + b + c)) where go [] acc = acc go (y:ys) acc = y : (go ys acc) ) xb) (xbs >>= (\b -> case mC of [] -> [] x:xs -> go ((\c -> a + b + c) x) (xs >>= (\c -> a + b + c)) where go [] acc = acc go (y:ys) acc = y : (go ys acc) )) where gob [] accb = accb gob (yb:ybs) accb = y : (go ybs accb) ) xa) (xas >>= (\a -> )) where go [] acca = acca
77. case mA of [] -> [] xa:xas -> go ((\a -> case mB of [] -> [] xb:xbs -> go ((\b -> case mC of [] -> [] x:xs -> go ((\c -> a + b + c) x) (xs >>= (\c -> a + b + c)) where go [] acc = acc go (y:ys) acc = y : (go ys acc) ) xb) (xbs >>= (\b -> case mC of [] -> [] x:xs -> go ((\c -> a + b + c) x) (xs >>= (\c -> a + b + c)) where go [] acc = acc go (y:ys) acc = y : (go ys acc) )) where gob [] accb = accb gob (yb:ybs) accb = y : (go ybs accb) ) xa) (xas >>= (\a -> )) where go [] acca = acca イメージです。
78. ·ͱΊ
79. まとめ • コンテナにはfmapが便利 • ただしネストする場合は • Just (Just (Just 9))のようなことに • Just 9 とみなせるなら • >>= が便利
80. ルールに従うことで 必要なことを勝手にやってくれる
81. 最高のプログラミングを…
82. 次回予告
83. IOモナドへ… そ し て