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Haskellで学ぶ関数型言語

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The document describes a Haskell program that translates characters in one string to characters in another string. It defines a translate function that maps characters from the first string (set1) to the corresponding characters in the second string (set2). A translateString function applies the translate function to a given string, and the main function gets the set1 and set2 strings from arguments, reads stdin, applies translateString, and writes the result to stdout, catching any errors.

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Haskell
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Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
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Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
1 public class Counter { 2 3 private int count = 0; 4 5 /** 6 * . 7 * @param value 8 * @return 9 */ 10 public int up(int value) { 11 count += value; 12 return count; 13 } 14 }
1 public class Counter { 2 3 private int count = 0; 4 5 /** 6 * . 7 * @param value 8 * @return 9 */ 10 public int up(int value) { 11 count += value; 12 return count; 13 } 14 }
1 import java.util.Scanner; 2 3 public class IOSample { 4 5 public static void main(String[] args) { 6 7 Scanner scan = new Scanner(System.in); 8 9 System.out.println(" "); 10 String str = scan.next(); // 11 System.out.println(" "+ str); 12 } 13 } ( )
1 import java.util.Scanner; 2 3 public class IOSample { 4 5 public static void main(String[] args) { 6 7 Scanner scan = new Scanner(System.in); 8 9 System.out.println(" "); 10 String str = scan.next(); // 11 System.out.println(" "+ str); 12 } 13 } ( )
Haskell Concurrent Clean OCaml Erlang Scala Lisp(Scheme) F#
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
What's faster than C++, more concise than Perl, more regular than Python, more flexible than Ruby, more typeful than C#, more robust than Java, and has absolutely nothing in common with PHP? It's Haskell! -- Autrijus Tang( ) C++ Perl Python Ruby C# Java PHP Haskell
Haskellで学ぶ関数型言語
1 cond :: Bool 2 cond = True 3 4 price :: Int 5 price = 2800 6 7 pi :: Float 8 pi = 3.141592653589793 9 10 greeting :: String 11 greeting = "Hello, Haskell!" 12 13 letter :: Char 14 letter = 'C'
‘A’ ‘B’ ‘C’ ‘D’ 1 bools = [True, False, True] :: [Bool] 2 3 -- "ABCD" 4 letters = ['A', 'B', 'C', 'D'] :: [Char] 5 6 -- 7 empty = [] :: [Int] 8 9 -- 1 10 notEmpty = [[]] :: [[Float]]
1 -- 2 (False, 'A') :: (Bool, Char) 3 4 -- ( ) 5 ("Name", True, 123) :: (String, Bool, Int) 6 7 -- 8 ('a', (False, 'b')) :: (Char, (Bool, Char)) 9 10 -- 11 (['a', 'b', ‘c’], [False, True]) :: ([Char], [Bool]) 12 13 -- 14 [('a', False), ('b', True)] :: [(Char, Bool)]
1 double :: Int -> Int 2 double x = x + x 3 4 add :: (Int, Int) -> Int 5 add (x, y) = x + y 6 7 mult :: Int -> Int -> Int -> Int 8 mult x y z = x * y * z > double 3 6 > add (1, 7) 8 > mult 7 8 9 504
1 add :: (Int, Int) -> Int 2 add (x, y) = x + y 3 4 -- Int -> (Int -> Int) Haskell 5 add' :: Int -> Int -> Int 6 add' x y = x + y 1 7 8 mult10 :: Int -> Int -> Int 9 mult10 = mult 10 10 11 mult30 :: Int -> Int 12 mult30 = mult 10 3 > mult10 3 8 240 > mult30 5 150
1 twice :: (Int -> Int) -> Int -> Int 2 twice f x = f (f x) 3 Int 4 isDigit :: Char -> Bool Int 5 isDigit c = c >= '0' && c <= '9' 6 7 -- map :: (a -> b) -> [a] -> [b] > twice (*2) 3 12 > map (+1) [1, 2, 3, 4] [2, 3, 4, 5] > map isDigit ['a', '0', 'b', '9'] [False, True, False, True]
> length [1, 3, 5, 7] 4 > length [True, False] 2 > length [length, length, length] 3 length :: [a] -> Int map :: (a -> b) -> [a] -> [b]
> 1 + 2 3 > (*) 1.1 2.3 -- 1.1 * 2.3 2.53 > 10 `div` 2 -- div 10 2 5 1 (+) :: Num a => a -> a -> a 2 (*) :: Num a => a -> a -> a 3 negate :: Num a => a -> a 4 abs :: Num a => a -> a
1 -- ( Ord, Show, Read ) 2 class Eq a where 3 (==) :: a -> a -> Bool 4 (/=) :: a -> a -> Bool 5 6 -- MyType Eq 7 instance Eq MyType where 8 (MyType a) == (MyType a') = (a == a')
1 -- [λxy. x+y] 2 add x y = x + y 3 add' = x -> (y -> x + y) 4 add'' = x y -> x + y 5 6 -- 7 -- const x y = x 8 const :: a -> (b -> a) 9 const x = _ -> x 10 11 -- > map (x -> x * 2 + 1) [1, 3, 5, 7] [3, 7, 11, 15]
> [x^2 | x <- [1..5]] [1,4,9,16,25] > [(x, y) | x <- [1,2,3], y <- [4,5]] [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)] 1 -- 2 factors :: Int -> [Int] 3 factors n = [x | x <- [1..n], n `mod` x == 0] 4 -- 5 primes :: Int -> [Int] 6 primes n = [x | x <- [2..n], factors x == [1, x]] > factors 100 [1,2,4,5,10,20,25,50,100] > primes 50 [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47]
1 -- 2 factorial :: Int -> Int 3 factorial 0 = 1 4 factorial n = n * factorial (n - 1) 5 6 -- 7 product :: Num a => [a] -> a 8 product [] = 1 9 product (n:ns) = n * product ns 1 2 3 4 1:[2,3,4] 4:[]
1 qsort :: Ord a => [a] -> [a] 2 qsort [] = [] 3 qsort (x:xs) = qsort smaller ++ [x] ++ qsort larger 4 where 5 smaller = [a|a <- xs, a <= x] 6 larger = [a|a <- xs, a > x] > qsort [100, 23, 37, 3, 55, 68, 49] [3,23,37,49,55,68,100]
1 even :: Int -> Bool even 4 2 even 0 = True = {even } 3 even n = odd (n - 1) odd 3 4 = {odd } 5 odd :: Int -> Bool even 2 6 odd 0 = False = {even } 7 odd n = even (n - 1) odd 1 = {odd } > even 4 even 0 True = {even } > odd 4 True False
Haskellで学ぶ関数型言語
1 -- ! 2 summary :: [Integer] -> Integer 3 summary [] = 0 4 summary (n:ns) = n + summary ns 5 6 -- ( ) 7 summary' :: [Integer] -> Integer 8 summary' xs = f xs 0 9 where 10 f [] sum = sum 11 f (y:ys) sum = f ys (y + sum)
mult (x, y) = x * y ? > mult (1+2, 2+3) ? mult (1+2, 2+3) mult (1+2, 2+3) = { + } = {mult } mult (3, 2+3) (1+2) * (2+3) = {+ } = { + } mult (3, 5) 3 * (2+3) = {mult } = {+ } 3 * 5 3 * 5 = {* } = {* } 15 15
inf :: Int inf = 1 + inf fst (0, inf) fst (0, inf) = {inf } = {fst } fst (0, 1+inf) 0 = {inf } fst (0, 1+(1+inf)) = {inf } fst (0, 1+(1+(1+inf))) = {inf } !!
1 ones :: [Int] 2 ones = 1 : ones 3 4 -- ( ) 5 sieve :: [Int] -> [Int] 6 sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p /= 0] 7 8 primes :: [Int] 9 primes = sieve [2..] head ones = {head ones } > head ones head (1:ones) 1 = {head } > take 10 primes 1 [2,3,5,7,11,13,17,19,23,29]
Mac OS X 10.7.1 tarai :: Int -> Int -> Int -> Int 1.8 GHz Intel Core i7 tarai x y z 4 GB 1333 MHz DDR3 | x <= y = y | otherwise = tarai (tarai (x-1) y z) > tarai 16 10 0 (tarai (y-1) z x) 16 (tarai (z-1) x y) Haskell private int tarai(int x, int y, int z) { :107039ms if (x <= y) { return y; :45,722,185,221 } else { return tarai(tarai(x-1, y, z), tarai(y-1, z, x), tarai(z-1, x, y)); } } Java
Monad
Haskell (※ )
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
6 -- set1 set2 7 translate :: String -> String -> Char -> Char 8 translate [] _ c = c 9 translate (x:xs) [] c = if x == c then ' ' else translate xs [] c 10 translate (x:xs) [y] c = if x == c then y else translate xs [y] c 11 translate (x:xs) (y:ys) c = if x == c then y else translate xs ys c 12 13 -- 14 translateString :: String -> String -> String -> String 15 translateString set1 set2 str = map (translate set1 set2) str 16 17 usage :: IOError -> IO () 18 usage e = do putStrLn "Usage: ex14 set1 set2" 19 putStrLn "Translates characters in set1 on stdin to the corresponding" 20 putStrLn "characters from set2 and writes the translation to stdout." 21 22 -- 23 main :: IO () 24 main = (do [set1,set2] <- getArgs 25 contents <- hGetContents stdin 26 putStr $ translateString set1 set2 contents) `catchError` usage
6 -- set1 set2 7 translate :: String -> String -> Char -> Char 8 translate [] _ c = c 9 translate (x:xs) [] c = if x == c then ' ' else translate xs [] c 10 translate (x:xs) [y] c = if x == c then y else translate xs [y] c 11 translate (x:xs) (y:ys) c = if x == c then y else translate xs ys c 12 13 -- 14 translateString :: String -> String -> String -> String 15 translateString set1 set2 str = map (translate set1 set2) str 16 17 usage :: IOError -> IO () 18 usage e = do putStrLn "Usage: ex14 set1 set2" 19 putStrLn "Translates characters in set1 on stdin to the corresponding" 20 putStrLn "characters from set2 and writes the translation to stdout." 21 22 -- 23 main :: IO () 24 main = (do [set1,set2] <- getArgs 25 contents <- hGetContents stdin 26 putStr $ translateString set1 set2 contents) `catchError` usage
Monad
1 class Monad m where 2 (>>=) :: m a -> (a -> m b) -> m b 3 return :: a -> m a 4 5 -- data Bool = False | True 6 data Maybe a = Nothing | Just a 7 8 instance Monad Maybe where 9 return = Just ( ) 10 fail = Nothing 11 Nothing >>= f = Nothing Just Nothing 12 (Just x) >>= f = f x 1 Maybe Int
lookup :: Eq a => a -> [(a, b)] -> Maybe b players = [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")] > lookup 11 players Just "Darvish" > lookup 212 players Nothing
1 pacific = [("Fs", [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")]), 2 ("SH", [(3, "Matsunaka"), (21, "Wada"), (47, "Sugiuchi")]), 3 ("Es", [(18, "Tanaka"), (21, "Iwakuma"), (32, "Matsui")])] 4 5 lookupName :: String -> Int -> Maybe String 6 lookupName t n = case (lookup t pacific) of 7 Just players -> lookup n players 8 Nothing -> Nothing > lookupName "Fs" 41 Just "Inaba" …
-- ( ) class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a instance Monad Maybe where return = Just fail = Nothing Nothing >>= f = Nothing (Just x) >>= f = f x Nothing Maybe Nothing >>= Int -> ? = Just Maybe Maybe Int >>= Int -> ? = ?
1 pacific = [("Fs", [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")]), 2 ("SH", [(3, "Matsunaka"), (21, "Wada"), (47, "Sugiuchi")]), 3 ("Es", [(18, "Tanaka"), (21, "Iwakuma"), (32, "Matsui")])] 4 5 lookupName :: String -> Int -> Maybe String 6 lookupName t n = case (lookup t pacific) of 7 Just players -> lookup n players 8 Nothing -> Nothing 9 10 lookupName' t n = lookup t pacific >>= lookup n > lookupName' "Es" 18 Just "Tanaka" [(Int, b)] -> Maybe b > lookupName' "Fs" 18 Nothing Maybe [(Int, String)] > lookupName' "DH" 21 Nothing -- (>>=) :: m a -> (a -> m b) -> m b lookup :: Eq a => a -> [(a, b)] -> Maybe b
Maybe Maybe Maybe Mayb a >>= a -> b >>= b -> c >>= c -> d
IO() String -> IO() main = putStrLn "Hello, World!" main = let a = putStrLn "Hello" in (a >> a) IO a
IO String String -> IO() main = getContents >>= putStrLn IO IO IO () = Str >>= Str -> ()
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語
Haskellで学ぶ関数型言語

More Related Content

Haskellで学ぶ関数型言語

  • 2.
  • 7. ?
  • 12. 1 public class Counter { 2 3 private int count = 0; 4 5 /** 6 * . 7 * @param value 8 * @return 9 */ 10 public int up(int value) { 11 count += value; 12 return count; 13 } 14 }
  • 13. 1 public class Counter { 2 3 private int count = 0; 4 5 /** 6 * . 7 * @param value 8 * @return 9 */ 10 public int up(int value) { 11 count += value; 12 return count; 13 } 14 }
  • 14. 1 import java.util.Scanner; 2 3 public class IOSample { 4 5 public static void main(String[] args) { 6 7 Scanner scan = new Scanner(System.in); 8 9 System.out.println(" "); 10 String str = scan.next(); // 11 System.out.println(" "+ str); 12 } 13 } ( )
  • 15. 1 import java.util.Scanner; 2 3 public class IOSample { 4 5 public static void main(String[] args) { 6 7 Scanner scan = new Scanner(System.in); 8 9 System.out.println(" "); 10 String str = scan.next(); // 11 System.out.println(" "+ str); 12 } 13 } ( )
  • 16. Haskell Concurrent Clean OCaml Erlang Scala Lisp(Scheme) F#
  • 19. What's faster than C++, more concise than Perl, more regular than Python, more flexible than Ruby, more typeful than C#, more robust than Java, and has absolutely nothing in common with PHP? It's Haskell! -- Autrijus Tang( ) C++ Perl Python Ruby C# Java PHP Haskell
  • 21. 1 cond :: Bool 2 cond = True 3 4 price :: Int 5 price = 2800 6 7 pi :: Float 8 pi = 3.141592653589793 9 10 greeting :: String 11 greeting = "Hello, Haskell!" 12 13 letter :: Char 14 letter = 'C'
  • 22. ‘A’ ‘B’ ‘C’ ‘D’ 1 bools = [True, False, True] :: [Bool] 2 3 -- "ABCD" 4 letters = ['A', 'B', 'C', 'D'] :: [Char] 5 6 -- 7 empty = [] :: [Int] 8 9 -- 1 10 notEmpty = [[]] :: [[Float]]
  • 23. 1 -- 2 (False, 'A') :: (Bool, Char) 3 4 -- ( ) 5 ("Name", True, 123) :: (String, Bool, Int) 6 7 -- 8 ('a', (False, 'b')) :: (Char, (Bool, Char)) 9 10 -- 11 (['a', 'b', ‘c’], [False, True]) :: ([Char], [Bool]) 12 13 -- 14 [('a', False), ('b', True)] :: [(Char, Bool)]
  • 24. 1 double :: Int -> Int 2 double x = x + x 3 4 add :: (Int, Int) -> Int 5 add (x, y) = x + y 6 7 mult :: Int -> Int -> Int -> Int 8 mult x y z = x * y * z > double 3 6 > add (1, 7) 8 > mult 7 8 9 504
  • 25. 1 add :: (Int, Int) -> Int 2 add (x, y) = x + y 3 4 -- Int -> (Int -> Int) Haskell 5 add' :: Int -> Int -> Int 6 add' x y = x + y 1 7 8 mult10 :: Int -> Int -> Int 9 mult10 = mult 10 10 11 mult30 :: Int -> Int 12 mult30 = mult 10 3 > mult10 3 8 240 > mult30 5 150
  • 26. 1 twice :: (Int -> Int) -> Int -> Int 2 twice f x = f (f x) 3 Int 4 isDigit :: Char -> Bool Int 5 isDigit c = c >= '0' && c <= '9' 6 7 -- map :: (a -> b) -> [a] -> [b] > twice (*2) 3 12 > map (+1) [1, 2, 3, 4] [2, 3, 4, 5] > map isDigit ['a', '0', 'b', '9'] [False, True, False, True]
  • 27. > length [1, 3, 5, 7] 4 > length [True, False] 2 > length [length, length, length] 3 length :: [a] -> Int map :: (a -> b) -> [a] -> [b]
  • 28. > 1 + 2 3 > (*) 1.1 2.3 -- 1.1 * 2.3 2.53 > 10 `div` 2 -- div 10 2 5 1 (+) :: Num a => a -> a -> a 2 (*) :: Num a => a -> a -> a 3 negate :: Num a => a -> a 4 abs :: Num a => a -> a
  • 29. 1 -- ( Ord, Show, Read ) 2 class Eq a where 3 (==) :: a -> a -> Bool 4 (/=) :: a -> a -> Bool 5 6 -- MyType Eq 7 instance Eq MyType where 8 (MyType a) == (MyType a') = (a == a')
  • 30. 1 -- [λxy. x+y] 2 add x y = x + y 3 add' = x -> (y -> x + y) 4 add'' = x y -> x + y 5 6 -- 7 -- const x y = x 8 const :: a -> (b -> a) 9 const x = _ -> x 10 11 -- > map (x -> x * 2 + 1) [1, 3, 5, 7] [3, 7, 11, 15]
  • 31. > [x^2 | x <- [1..5]] [1,4,9,16,25] > [(x, y) | x <- [1,2,3], y <- [4,5]] [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)] 1 -- 2 factors :: Int -> [Int] 3 factors n = [x | x <- [1..n], n `mod` x == 0] 4 -- 5 primes :: Int -> [Int] 6 primes n = [x | x <- [2..n], factors x == [1, x]] > factors 100 [1,2,4,5,10,20,25,50,100] > primes 50 [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47]
  • 32. 1 -- 2 factorial :: Int -> Int 3 factorial 0 = 1 4 factorial n = n * factorial (n - 1) 5 6 -- 7 product :: Num a => [a] -> a 8 product [] = 1 9 product (n:ns) = n * product ns 1 2 3 4 1:[2,3,4] 4:[]
  • 33. 1 qsort :: Ord a => [a] -> [a] 2 qsort [] = [] 3 qsort (x:xs) = qsort smaller ++ [x] ++ qsort larger 4 where 5 smaller = [a|a <- xs, a <= x] 6 larger = [a|a <- xs, a > x] > qsort [100, 23, 37, 3, 55, 68, 49] [3,23,37,49,55,68,100]
  • 34. 1 even :: Int -> Bool even 4 2 even 0 = True = {even } 3 even n = odd (n - 1) odd 3 4 = {odd } 5 odd :: Int -> Bool even 2 6 odd 0 = False = {even } 7 odd n = even (n - 1) odd 1 = {odd } > even 4 even 0 True = {even } > odd 4 True False
  • 36. 1 -- ! 2 summary :: [Integer] -> Integer 3 summary [] = 0 4 summary (n:ns) = n + summary ns 5 6 -- ( ) 7 summary' :: [Integer] -> Integer 8 summary' xs = f xs 0 9 where 10 f [] sum = sum 11 f (y:ys) sum = f ys (y + sum)
  • 37. mult (x, y) = x * y ? > mult (1+2, 2+3) ? mult (1+2, 2+3) mult (1+2, 2+3) = { + } = {mult } mult (3, 2+3) (1+2) * (2+3) = {+ } = { + } mult (3, 5) 3 * (2+3) = {mult } = {+ } 3 * 5 3 * 5 = {* } = {* } 15 15
  • 38. inf :: Int inf = 1 + inf fst (0, inf) fst (0, inf) = {inf } = {fst } fst (0, 1+inf) 0 = {inf } fst (0, 1+(1+inf)) = {inf } fst (0, 1+(1+(1+inf))) = {inf } !!
  • 39. 1 ones :: [Int] 2 ones = 1 : ones 3 4 -- ( ) 5 sieve :: [Int] -> [Int] 6 sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p /= 0] 7 8 primes :: [Int] 9 primes = sieve [2..] head ones = {head ones } > head ones head (1:ones) 1 = {head } > take 10 primes 1 [2,3,5,7,11,13,17,19,23,29]
  • 40. Mac OS X 10.7.1 tarai :: Int -> Int -> Int -> Int 1.8 GHz Intel Core i7 tarai x y z 4 GB 1333 MHz DDR3 | x <= y = y | otherwise = tarai (tarai (x-1) y z) > tarai 16 10 0 (tarai (y-1) z x) 16 (tarai (z-1) x y) Haskell private int tarai(int x, int y, int z) { :107039ms if (x <= y) { return y; :45,722,185,221 } else { return tarai(tarai(x-1, y, z), tarai(y-1, z, x), tarai(z-1, x, y)); } } Java
  • 41. Monad
  • 42. Haskell (※ )
  • 47. 6 -- set1 set2 7 translate :: String -> String -> Char -> Char 8 translate [] _ c = c 9 translate (x:xs) [] c = if x == c then ' ' else translate xs [] c 10 translate (x:xs) [y] c = if x == c then y else translate xs [y] c 11 translate (x:xs) (y:ys) c = if x == c then y else translate xs ys c 12 13 -- 14 translateString :: String -> String -> String -> String 15 translateString set1 set2 str = map (translate set1 set2) str 16 17 usage :: IOError -> IO () 18 usage e = do putStrLn "Usage: ex14 set1 set2" 19 putStrLn "Translates characters in set1 on stdin to the corresponding" 20 putStrLn "characters from set2 and writes the translation to stdout." 21 22 -- 23 main :: IO () 24 main = (do [set1,set2] <- getArgs 25 contents <- hGetContents stdin 26 putStr $ translateString set1 set2 contents) `catchError` usage
  • 48. 6 -- set1 set2 7 translate :: String -> String -> Char -> Char 8 translate [] _ c = c 9 translate (x:xs) [] c = if x == c then ' ' else translate xs [] c 10 translate (x:xs) [y] c = if x == c then y else translate xs [y] c 11 translate (x:xs) (y:ys) c = if x == c then y else translate xs ys c 12 13 -- 14 translateString :: String -> String -> String -> String 15 translateString set1 set2 str = map (translate set1 set2) str 16 17 usage :: IOError -> IO () 18 usage e = do putStrLn "Usage: ex14 set1 set2" 19 putStrLn "Translates characters in set1 on stdin to the corresponding" 20 putStrLn "characters from set2 and writes the translation to stdout." 21 22 -- 23 main :: IO () 24 main = (do [set1,set2] <- getArgs 25 contents <- hGetContents stdin 26 putStr $ translateString set1 set2 contents) `catchError` usage
  • 49. Monad
  • 50. 1 class Monad m where 2 (>>=) :: m a -> (a -> m b) -> m b 3 return :: a -> m a 4 5 -- data Bool = False | True 6 data Maybe a = Nothing | Just a 7 8 instance Monad Maybe where 9 return = Just ( ) 10 fail = Nothing 11 Nothing >>= f = Nothing Just Nothing 12 (Just x) >>= f = f x 1 Maybe Int
  • 51. lookup :: Eq a => a -> [(a, b)] -> Maybe b players = [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")] > lookup 11 players Just "Darvish" > lookup 212 players Nothing
  • 52. 1 pacific = [("Fs", [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")]), 2 ("SH", [(3, "Matsunaka"), (21, "Wada"), (47, "Sugiuchi")]), 3 ("Es", [(18, "Tanaka"), (21, "Iwakuma"), (32, "Matsui")])] 4 5 lookupName :: String -> Int -> Maybe String 6 lookupName t n = case (lookup t pacific) of 7 Just players -> lookup n players 8 Nothing -> Nothing > lookupName "Fs" 41 Just "Inaba" …
  • 53. -- ( ) class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a instance Monad Maybe where return = Just fail = Nothing Nothing >>= f = Nothing (Just x) >>= f = f x Nothing Maybe Nothing >>= Int -> ? = Just Maybe Maybe Int >>= Int -> ? = ?
  • 54. 1 pacific = [("Fs", [(6, "Nakata"), (11, "Darvish"), (41, "Inaba")]), 2 ("SH", [(3, "Matsunaka"), (21, "Wada"), (47, "Sugiuchi")]), 3 ("Es", [(18, "Tanaka"), (21, "Iwakuma"), (32, "Matsui")])] 4 5 lookupName :: String -> Int -> Maybe String 6 lookupName t n = case (lookup t pacific) of 7 Just players -> lookup n players 8 Nothing -> Nothing 9 10 lookupName' t n = lookup t pacific >>= lookup n > lookupName' "Es" 18 Just "Tanaka" [(Int, b)] -> Maybe b > lookupName' "Fs" 18 Nothing Maybe [(Int, String)] > lookupName' "DH" 21 Nothing -- (>>=) :: m a -> (a -> m b) -> m b lookup :: Eq a => a -> [(a, b)] -> Maybe b
  • 55. Maybe Maybe Maybe Mayb a >>= a -> b >>= b -> c >>= c -> d
  • 56. IO() String -> IO() main = putStrLn "Hello, World!" main = let a = putStrLn "Hello" in (a >> a) IO a
  • 57. IO String String -> IO() main = getContents >>= putStrLn IO IO IO () = Str >>= Str -> ()

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