7つの言語7つの世界 Haskell セルフスタディ1日目

「７つの言語 ７つの世界」第8章、Haskell（ハスケル）言語

７つの言語 ７つの世界

• Ruby
• Io
• Prolog
• Scala
• Erlang
• Cloijure
• Haskell

セルフスタディ1日目

allEven

• allEven にはいろいろな書き方がある.何通りの書き方を見つけられるか試してみよ.

リスト内包表記を使ったバージョン

allEven :: [Integer] -> [Integer]
allEven list = [x | x <- list, even x]Code language: Haskell (haskell)

リストを逆順

• リストを受け取って,同じリスト(の各項の並び)を逆順にして返す関数を書け.

標準のreverse関数があります。reverse関数を自前で書きなさいと考えて、

reverseList :: [t] -> [t]
reverseList [] = []
reverseList (h:t) = reverseList t ++ [h]Code language: Haskell (haskell)

*Main> reverseList [1, 2]
[2,1]
*Main> reverseList [[1], [2]]
[[2],[1]]
*Main> reverseList ["1", "2"]
["2","1"]Code language: Haskell (haskell)

black , white , blue , yellow , red

• black , white , blue , yellow , red から 2 色を取った組み合わせを表す 2 要素のタプルをすべて作成する関数を書け.順番は関係ないので,例えば, (black, blue)と (blue, black) はどちらか一方のみを含めるようにすること.

p243の let crew = ["Kirk", "Spock", "McCoy"] のくだりを流用して、

colors :: [String]
colors = ["black", "white", "blue", "yellow", "red"]
    
colorPairs :: [(String,String)]
colorPairs = [(a, b) | a <- colors, b <- colors, a < b]Code language: Haskell (haskell)

*Main> colors
["black","white","blue","yellow","red"]

*Main> colorPairs 
[("black","white"),("black","blue"),("black","yellow"),("black","red"),("white","yellow"),("blue","white"),("blue","yellow"),("blue","red"),("red","white"),("red","yellow")]
Code language: Haskell (haskell)

12x12の乗算表

•リスト内包表記を用いて,子供が使う乗算表を作れ.この表は, 3 要素のタプルのリストとして表現できる.その場合,各タプルの最初の 2 要素は 1 〜 12 の整数, 3番目の要素は最初の 2 要素を掛け合わせた値となる.

次のリストを作る関数を作ります。

[
(1,1,1),(1,2,2),(1,3,3),,,,(1,12,12),
(2,1,2),(2,2,4),(2,3,6),,,,(2,12,24),
(12,1,12),(12,2,24),(12,3,36),,,,(12,12,144)
]Code language: Haskell (haskell)

mulitiplicationTable = [mulitiplicationRow y | y <- [1 .. 12]]
mulitiplicationRow y = [(y, x, x * y) | x <- [1 .. 12]]Code language: Haskell (haskell)

さきほどのリストを表示する際に、先頭行に、x = 1, 2,,, 12、各行の先頭列に y = 1, 2,,,12 を表示して、交差するセルには、x * y の結果を表示するようにしてみます。

	1	2	3	4	5	6	7	8	9	10	11	12	
1	1	2	3	4	5	6	7	8	9	10	11	12	
2	2	4	6	8	10	12	14	16	18	20	22	24	
3	3	6	9	12	15	18	21	24	27	30	33	36	
4	4	8	12	16	20	24	28	32	36	40	44	48	
5	5	10	15	20	25	30	35	40	45	50	55	60	
6	6	12	18	24	30	36	42	48	54	60	66	72	
7	7	14	21	28	35	42	49	56	63	70	77	84	
8	8	16	24	32	40	48	56	64	72	80	88	96	
9	9	18	27	36	45	54	63	72	81	90	99	108	
10	10	20	30	40	50	60	70	80	90	100	110	120	
11	11	22	33	44	55	66	77	88	99	110	121	132	
12	12	24	36	48	60	72	84	96	108	120	132	144	Code language: plaintext (plaintext)

module Main where
    main :: IO ()
    main = return ()

    mulitiplicationTable = [mulitiplicationRow y | y <- [1 .. 12]]
    mulitiplicationRow y = [(y, x, x * y) | x <- [1 .. 12]]

    printMulticationTalbe :: IO ()
    printMulticationTalbe = do
        printList (makeHeader mulitiplicationTable)
        printTable (makeBody mulitiplicationTable)

    makeHeader (h:t) = do
        "" : [show x | (y, x, z) <- h]

    makeBody [] = []
    makeBody (h:t) = do
        let y = [show y | (y, x, z) <- h, x == 1]
        let cells = [show z | (y, x, z) <- h]
        let line = y ++ cells
        line : makeBody t

    printCell :: String -> IO ()
    printCell x = do
        putStr x
        putStr "\t"

    printList :: [String] -> IO ()
    printList [] = do
        putStrLn ""

    printList (h:t) = do
        printCell h
        printList t
    
    printTable :: [[String]] -> IO ()
    printTable [] = do
        putStr ""

    printTable (h:t) = do
        printList h
        printTable tCode language: Haskell (haskell)

地図塗り分け問題

• Haskell を使って,地図塗り分け問題( 4.2 「地図の塗り分け」 ( p. 78 ) )を解いてみよ.

リスト内包表記を使って、隣接する色が違う条件を指定しました。

module Main where
    main :: IO ()
    main = return ()

    coloring = do
        let colors = ["red", "green", "blue"]
        [(tennessee, mississippi, alabama, georgia, florida) |
            tennessee <- colors, mississippi <- colors, alabama <- colors, georgia <- colors, florida <- colors,
            tennessee /= mississippi,
            tennessee /= alabama,
            tennessee /= georgia,
            mississippi /= alabama,
            alabama /= georgia,
            alabama /= florida,
            georgia /= florida
            ]
Code language: Haskell (haskell)

*Main> coloring
[("red","green","blue","green","red"),
("red","blue","green","blue","red"),
("green","red","blue","red","green"),
("green","blue","red","blue","green"),
("blue","red","green","red","blue"),
("blue","green","red","green","blue")]Code language: Haskell (haskell)

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