```変な処理があれば修正してください。

*ループ1 [#a13940a0]
[[問題文>練習問題#eb2c4338]]
times 0 action = return ()
times n action = action >> times (n-1) action

main = do
5 `times` putStrLn "Hello World!"

main = putStr \$ "おっぱい！" >> "Hello World!\n"

*ループ2 [#ob0d5bdb]
[[問題文>練習問題#eb2c4338]]
import System.Environment

times 0 action = return ()
times n action = action >> times (n-1) action

main = do
x <- getArgs
(read (x !! 0)) `times` putStrLn "Hello World!"

import System.Environment

main :: IO ()
main = fmap (read . head) getArgs >>= \n -> forM_ [1..n] \$ putStrLn . ("Hello World! "++) . show

*FizzBuzz [#f4c6d1f7]
[[問題文>練習問題#t52e5a48]]
main = do
mapM_ putStrLn (map fizzBuzz [1..100])

fizzBuzz :: Integer -> String
fizzBuzz n
| n `mod` 15 == 0 = "FizzBuzz"
| n `mod`  5 == 0 = "Buzz"
| n `mod`  3 == 0 = "Fizz"
| otherwise       = show n

fizzbuzz :: [String]
fizzbuzz = zipWith3 f [1..] fizz buzz
where
f i x y = if null x && null y then show i else x ++ y
fizz = cycle ["", "", "Fizz"]
buzz = cycle ["", "", "", "", "Buzz"]

fizzbuzz' :: [String]
fizzbuzz' = map f [1..]
where
f i = let x = g i in if null x then show i else x
g x = concat [ s | (i, s) <- [(3, "Fizz"), (5, "Buzz")], mod x i == 0 ]

main :: IO ()
main = mapM_ putStrLn \$ take 30 fizzbuzz

*素数列挙 [#ld4986f8]
[[問題文>練習問題#o8db2119]]
primesTo m = 2 : sieve [3, 5..m] where
sieve [] = []
sieve (p:xs) = p : sieve (xs `minus` [p*p, p*p+2*p..m])

minus (x:xs) (y:ys) = case (compare x y) of
LT -> x : minus xs (y:ys)
EQ ->     minus xs ys
GT ->     minus (x:xs) ys
minus xs _ = xs

main = do
print \$ primesTo (floor 1e6)

*うるう年判定 [#rc63b4a5]
[[問題文>練習問題#i0d67516]]
isleap :: Integer -> Bool
isleap y
| y `mod` 400 == 0 = True
| y `mod` 100 == 0 = False
| y `mod`   4 == 0 = True
| otherwise        = False

main = do
putStrLn "西暦を入力してください"
x <- getLine
putStrLn (x ++ "年はうるう年" ++ if isleap(read(x)) then "です" else "ではありません" )

*フィボナッチ数列 [#mf294cb0]
[[問題文>練習問題#p59f794f]]
import System.Environment

fibo = 0 : 1 : zipWith (+) fibo (tail fibo)

main = do
x <- getArgs
putStrLn (show (fibo !! read (x !! 0)))

*累乗1 [#b0e3fd20]
[[問題文>練習問題#x6e30de3]]
pow :: Integer -> Integer -> Integer
pow _ 0 = 1
pow 0 _ = 0
pow a b = a * pow a (b-1)

main = do
putStrLn (show (pow 2 10))

*累乗2 [#w1eaeb0e]
[[問題文>練習問題#x6e30de3]]
pow :: Integer -> Integer -> Integer
pow 0 _ = 0
pow _ 0 = 1
pow a b = pow' 1 a b
where
pow' x a 0 = x
pow' x a b
| even b = pow' x (a*a) (div b 2)
| otherwise = pow' (x*a) a (b-1)

main = do
putStrLn (show (pow 2 1000007))

*クイックソート [#w101147e]
[[問題文>練習問題(アルゴリズム編)#q8e0119e]]
quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
let smallerSorted = quicksort[a | a<-xs, a <= x]
biggerSorted  = quicksort[a | a<-xs, a >  x]
in smallerSorted ++ [x] ++ biggerSorted

main :: IO()
main = print \$ quicksort [5, 7, 9, 2, 1, 3, 8, 4, 6, 0]

*世界のナベアツ問題 [#p9107c97]
[[問題文>練習問題#t52a5a48]]
nabeatsu :: Integer -> String
nabeatsu x = if mod x 3 == 0 || elem '3' s then "Aho" else s where s = show x

main :: IO ()
main = putStrLn . unwords \$ map nabeatsu [1..100]

*Caesar暗号解読 [#v9844940]
[[問題文>練習問題#h54a0395]]
import Data.List (isInfixOf)

next :: String -> Char -> Char
next (x:xs) = \c -> if x == c then head xs else next xs c
next _ = id

analyze :: String -> String
analyze = head . filter (isInfixOf keyword) . iterate (map \$ next chars)
where chars   = "-abcdefghijklmnopqrstuvwxyz .,-"
keyword = "person"

main :: IO ()
main =  putStrLn \$ analyze encoded
where encoded = "qdq-gi.q-a ziatmxxitmdqibtqi-ustbi ri.qmoqrcxi.qbubu zir -ibtq\
\i-qp-qaai ripmymsqkir -ibtqi-qy dmxi ri.cnxuoi rruoumxakir -ib\
\tqiqzmobyqzbkii-q.qmxi -imyqzpyqzbi rixmeaki -puzmzoqai -i-qsc\
\xmbu zaimzpir -i btq-iymbbq-a;iz -iatmxximzgi.q-a zinqiuzimzgi\
\emgipuao-uyuzmbqpimsmuzabir -ia. za -uzsiacotiimi.qbubu zj"

*数当てゲーム [#u56f4eea]
[[問題文>練習問題#d082e883]]
import System.IO
import System.Random

game :: Int -> IO ()
game x = do
us <- getLine
case reads us of
[(u, _)] -> when (x /= u) \$ do
putStrLn . ("too "++) . (++"!") \$ if x < u then "big" else "small"
game x
_ -> putStrLn "invalid input!" >> game x

main :: IO ()
main = do hSetBuffering stdout NoBuffering
putStrLn "input a number!"
getStdGen >>= game . fst . randomR (1,100)
putStrLn "right!"

tailZero :: (Num a) => [a] -> [a]
tailZero = zipWith (*) (1 : repeat 0)

main :: IO ()
main = print \$ tailZero [3,5,2,4,2]

*シェルピンスキーのギャスケット [#r0c059cc]
import Control.Applicative

pascal :: [[Integer]]
pascal = [1] : map (zipWith (+) <\$> ([0] ++) <*> (++ [0])) pascal

sierpinski :: Int -> String
sierpinski = unlines . map (map draw) . flip take pascal
where draw x = if odd x then '*' else ' '

main :: IO ()
main = putStr \$ sierpinski 32

*カレンダー出力 [#j1958644]
[[問題文>練習問題#i5e3e061]]
import System.Environment

ff :: Int -> Int -> Int
ff y m
| m < 3 = ff (y-1) (m+12)
| otherwise = 365*y + (y`div`4) - (y`div`100) + (y`div`400) + ((306*(m+1))`div`10) - 428 + 1

calendar :: Int -> Int -> String
calendar y m
| y < 1 || m < 1 || 12 < m = error "calendar"
| otherwise = unlines . ("Su Mo Tu We Th Fr Sa":) . split7 \$ map toStr calList
where calList   = [1 - (ff y m) `mod` 7 .. ff y (m+1) - ff y m]
toStr x   = if 0 < x  then sp x \$ show x else "  "
sp x      = if x < 10 then (' ':) else id
split7 [] = []
split7 xs = let (as, bs) = splitAt 7 xs in unwords as : split7 bs

main :: IO ()
main = fmap (map read) getArgs >>= \[y,m] -> putStr \$ calendar y m

*ハノイの塔 [#y79a264b]
import System.Environment

hanoi :: Integer -> a -> a -> a -> [(a, a)]
hanoi 0 _ _ _ = []
hanoi n a b c = hanoi (n-1) a c b ++ [(a,b)] ++ hanoi (n-1) c b a

main :: IO ()
main = do
[x] <- fmap (map read) getArgs
let f = \(i,(a,b)) -> putStrLn \$ show i ++ ": " ++ a ++ "->" ++ b
mapM_ f . zip [1..] \$ hanoi x "A" "B" "C"

*転置行列 [#b483a0f8]
[[問題文>練習問題#q9f27a17]]
import Data.List (transpose)

*英単語しりとりプログラム [#d52f1308]
[[問題文>練習問題#x38ec247]]
import System.IO
import System.Random
import qualified Data.Set as S
import qualified Data.Map as M

type User    = Bool
type Initial = Char
type Dict    = S.Set String
type Used    = M.Map String (Int, User)

game :: User -> Initial -> Dict -> Used -> IO ()
game user c dic used = do
word <- if user then userInput c dic else randomSelect c dic used
case word of
[] -> putStrLn \$ winMsg usedSize
_  -> case M.lookup word used of
Just (n, u) -> putStrLn \$ loseMsg n u usedSize
Nothing     -> game (not user) (last word) dic \$ M.insert word (usedSize + 1, user) used
where usedSize = M.size used
winMsg n = "S: まいりました！あなたの勝ちです。今回のしりとりでは "
++ show n ++ " 個の単語を使用しました。"
loseMsg n u m = "S: その言葉は " ++ show n ++ " 回目に "
++ (if u then "あなた" else "わたし") ++ " が使用しています。わたしの勝ちです。\
\今回のしりとりでは " ++ show m ++ " 個の単語を使用しました。"

userInput :: Initial -> Dict -> IO String
userInput c dic = do
putStr "U: "
s <- getLine
if not (null s) && head s /= c
then putStrLn ("S: " ++ [c] ++ "から初めてください") >> userInput c dic
else if S.member s dic
then return s
else putStrLn "S: 辞書に存在しない単語です。" >> userInput c dic

randomSelect :: Initial -> Dict -> Used -> IO String
randomSelect c dic used = do
gen <- getStdGen
newStdGen
let sub  = S.filter (\x -> head x == c && M.notMember x used) dic
idx  = fst \$ randomR (0, S.size sub - 1) gen
word = if S.null sub then [] else S.toList sub !! idx
when (word /= []) . putStrLn \$ "S: " ++ word
return word

start :: IO ()
start = do
gen <- getStdGen
content <- readFile "word.dic"
let user = fst \$ random gen
inic = fst \$ randomR ('a','z') gen
dic  = S.fromList \$ lines content
when user . putStrLn \$ "S: " ++ [inic] ++ "から初めてください"
game user inic dic M.empty

main :: IO ()
main = hSetBuffering stdout NoBuffering >> hSetEncoding stdout utf8 >> start

*線形合同法 [#r62dac48]
[[問題文>練習問題#pc7a3f04]]
import Data.Ratio

lcgs :: Integral a => a -> a -> a -> a -> [a]
lcgs a b m x | not st    = error "lcgs"
| otherwise = x' : lcgs a b m x'
where st = m > a && m > b && a > 0 && b >= 0
x' = (a * x + b) `mod` m

main :: IO ()
main = mapM_ (print . fromRational) xs >> putStr "avg = " >> print (fromRational avg)
where a = 997; b = 1; m = 65536; x = 12345; n = 100
xs  = take n . map (% m) \$ lcgs a b m x
avg = sum xs / fromIntegral n

*配列から大きい要素とその添字を求めるプログラム [#vaeff67a]
[[問題文>練習問題#u170a96a]]
import Data.List (sortBy)
import Data.Function (on)

topThree :: [Int] -> [(Int,Int)]
topThree = take 3 . reverse . sortBy (on compare snd) . zip [0..]

main :: IO ()
main = mapM_ (putStrLn . f) . topThree \$ [12,6,8,3,10,1,0,9]
where f = \(i,x) -> show i ++ " -> " ++ show x

*迷路を解くプログラム [#vf7037b3]
[[問題文>練習問題#d4135691]]
-- 参考ページ
-- Programming Praxis – Dijkstra’s Algorithm | Bonsai Code
-- <http://bonsaicode.wordpress.com/2011/01/04/programming-praxis-dijkstra%E2%80%99s-algorithm/>

import Data.Function (on)
import Data.List (delete, minimumBy)
import Data.Map ((!), fromList, fromListWith, adjust, keys, Map)

type Pos  = Int
type Cost = Int

dijkstra :: Pos -> Map Pos [(Pos, Cost)] -> Map Pos (Cost, Maybe Pos)
dijkstra source graph = let vs = keys graph in
f (fromList [(v, (if v == source then 0 else maxBound, Nothing)) | v <- vs]) vs
where f ds [] = ds
f ds q  = f (foldr relax ds \$ graph ! m) (delete m q)
where m = minimumBy (on compare (ds!)) q
relax (e,d) = adjust (min (fst (ds ! m) + d, Just m)) e

build :: [Pos] -> Map Pos [(Pos, Cost)]
build = fromListWith (++) . map f
where f r = let next = filter (\x -> 0 < x && x < 99) \$ map (r+) [-1,1,-10,10]
in  (r, zip next \$ repeat 1)

shortestPath :: Pos -> Pos -> Map Pos [(Pos, Cost)] -> [Pos]
shortestPath from to graph = f to
where f x = x : maybe [] f (snd \$ dijkstra from graph ! x)

answer :: String -> String
answer mazeLine = drawPath
where maze         = zip [0..] mazeLine
road         = foldr (\(i,c) a -> if c == ' ' then i : a else a) [] maze
[start,goal] = filter wall road
where wall i = (0 < i && i < 9) || (90 < i && i < 99) || (elem i \$ [0,10..90] ++ [9,19..99])
path         = shortestPath start goal \$ build road
drawPath     = map (\(i,c) -> if elem i path then '+' else c) maze

main :: IO ()
main = mapM_ putStrLn . split10 . answer . concat =<< replicateM 10 getLine
where split10 [] = []
split10 xs = let (as, bs) = splitAt 10 xs in as : split10 bs

*クリティカルパス [#r67a0c46]
[[問題文>練習問題#o006da2d]]
import Control.Arrow
import Data.Function (on)
import Data.List (sortBy, intercalate, groupBy)
import qualified Data.Map as M

paths :: (Num c, Ord k) => M.Map k [(k, c)] -> ([k], c) -> [([k], c)]
paths graph path@(nodes, cost) =
maybe [path] (>>= paths graph . ((:nodes) *** (+cost))) \$ M.lookup (head nodes) graph

criticalPaths :: [(a, Int)] -> [(a, Int)]
criticalPaths = last . groupBy (on (==) snd) . sortBy (on compare snd)

main :: IO ()
main = do
[_, link] <- fmap (map read . words) getLine
input     <- replicateM link getLine
let buildGraph = M.fromListWith (++) . map ((\[[s],[t],c] -> (s, [(t, read c)])) . words)
f (path, cost) = (intercalate " -> " . map (:"") \$ reverse path) ++ "\n" ++ show cost
mapM_ (putStrLn . f) . criticalPaths \$ paths (buildGraph input) ("A", 0)

*ひらがな2文字をランダムで出力するプログラム [#l32d6dde]
[[問題文>練習問題#s98f80e0]]
import System.IO
import System.Random

main :: IO ()
main = hSetEncoding stdout utf8 >> getStdGen >>= putStrLn . take 2 . randomRs ('あ', 'ん')

*平方根を求めてみる [#nc756426]
[[問題文>練習問題#y8402045]]
import Control.Arrow

sqrt' :: Fractional a => a -> a
sqrt' = snd . (!!111) . iterate (fst &&& (/2) . uncurry(+) . (snd &&& uncurry(/))) <<< id &&& id

main = print \$ sqrt' 3
```