在研究Haskell的函数式编程

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发表于 2012-8-1 00:32:21 | 显示全部楼层 |阅读模式

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本帖最后由轻舟过于 2012-8-1 00:55 编辑

最近两天做了Haskell 99题中的前20道
函数式编程解决问题的思路还真是不一样
还有函数式编程是不是研究Lisp更好一点。。

题目链接在这里：http://www.haskell.org/haskellwiki/99_questions
相应的还有Prolog、Lisp、Perl、OCaml的99题

论坛的SyntaxHighlighter插件怎么没有Haskell的语法高亮

[mw_shl_code=text,true]-- #1
myLast :: [a] -> a
myLast (x:[]) = x
myLast (x:xs) = myLast xs

-- #2
myButLast :: [a] -> a
myButLast (x1:x2:[]) = x1
myButLast (x1:x2:xs) = myButLast (x2:xs)

-- #3
elementAt :: [a] -> Int -> a
elementAt (x:xs) 1 = x
elementAt (x:xs) idx = elementAt xs (idx - 1)

-- #4
myLength :: [a] -> Int
myLength xs = foldl (+) 0 $ map (\x -> 1) xs

-- #5
myReverse :: [a] -> [a]
myReverse = foldl (\xs x-> x:xs) []

-- #6
isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome xs = xs == myReverse xs

data NestedList a = Elem a | List [NestedList a] deriving (Show)
-- #7
flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List lst) = foldl (++) [] $ map flatten lst

-- #8
compress :: (Eq a) => [a] -> [a]
compress [] = []
compress all@(x:xs) = (head $ takeWhile (==x) all):compress(dropWhile (==x) all)

-- #9
pack :: (Eq a) => [a] -> [[a]]
pack [] = []
pack all@(x:xs) = (takeWhile (==x) all):pack(dropWhile (==x) all)

-- #10
encode :: (Eq a) => [a] -> [(Int, a)]
encode [] = []
encode all@(x:xs) = ((myLength $ takeWhile (==x) all), x):encode(dropWhile (==x) all)

data RunLengthCode a = Single a | Multiple Int a deriving (Show)

-- #11
encodeModified :: (Eq a) => [a] -> [RunLengthCode a]
encodeModified [] = []
encodeModified lst = map toRunLengthCode $ encode lst
      where toRunLengthCode pair = if fst pair == 1 then (Single (snd pair)) else (Multiple (fst pair) (snd pair))

-- #12
decodeModified :: [RunLengthCode a] -> [a]
decodeModified [] = []
decodeModified ((Single x):xs) = [x] ++ decodeModified(xs)
decodeModified ((Multiple len x):xs) = (replicate len x) ++ decodeModified(xs)

-- #13
encodeDirect :: (Eq a) => [a] -> [RunLengthCode a]
encodeDirect [] = []
encodeDirect all@(x:xs) = code:encodeDirect(dropWhile (==x) all)
      where       len = myLength $ takeWhile (==x) all
            code = if len == 1 then (Single x) else (Multiple len x)

-- #14
dupli :: [a] -> [a]
dupli [] = []
dupli (x:xs) = [x, x] ++ dupli(xs)

-- #15
repli :: [a] -> Int -> [a]
repli [] _ = []
repli (x:xs) cnt = (replicate cnt x) ++ (repli xs cnt)

-- #16
{-
- dropEvery = flip $ \n -> map snd . filter ((n/=) . fst) . zip (cycle [1..n])
-}
dropEvery :: [a] -> Int -> [a]
dropEvery lst cnt = map snd $ filter (\p -> (fst p) `mod` cnt /= 0) $ zip [1..] lst

-- #17
split :: [a] -> Int -> ([a], [a])
split lst len = (take len lst, drop len lst)

-- #18
slice :: [a] -> Int -> Int -> [a]
slice lst start end = fst $ split (drop (start - 1) lst) (end - start + 1)

-- #19
rotate :: [a] -> Int -> [a]
rotate lst num = (drop len lst) ++ (take len lst)
      where       lstLen = length lst
            len = (num `mod` lstLen + lstLen) `mod` lstLen

-- #20
removeAt :: Int -> [a] -> (a, [a])
removeAt num lst = (head lstTail, (take (num - 1) lst) ++ (tail lstTail))
      where lstTail = drop (num - 1) lst
[/mw_shl_code]

然后还有spoj上的TEST
[mw_shl_code=text,true]main = do
line <- getLine
if line == "42"
then getContents
else do
putStrLn line
main[/mw_shl_code]

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在研究Haskell的函数式编程

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