递归

因为递归本身也很熟悉了，所以不再细讲，主要列几个Haskell中应用递归的函数样例。注意，由于Haskell中用递归来实现循环的功能，所以有些递归看着有些别扭，需要适应。

求数列中最大值

maximum' :: (Ord a) => [a] -> a
maximum' [] = error "maximum of empty list"
maximum' [x] = x
maximum' (x:xs) = max x (maximum' xs)

replicate

replicate取一个整数x和一个量y，返回一个包含x个y的列表。

replicate' :: Int -> a -> [a]
replicate' n x
true| n <= 0	= []
    | otherwise = x:replicate' (n-1) x

take

take x可以从列表中取出前x个元素，形成另一个列表

take' :: (Num i, Ord i) => i -> [a] -> [a]
take' n _
true| n <= 0 = []
take' _ []	 = []
take' n (x:xs) = x : take' (n-1) xs

reverse

reserve函数取一个列表作为参数，返回反转的列表。

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reserve' :: [a] => [a]
reverse' [] = []
reverse' (x:xs) = reverse' xs ++ [x]

zip

zip可以取两个列表为参数，将其中的每个数捆绑在一起，形成一个二元组的列表

zip' :: [a] -> [b] -> [(a,b)]
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y) : zip' xs ys

qsort

使用递归的方法，我们可以很容易的构造出qsort函数--快速排序。

qsort :: (Ord a) => [a] -> [a]
qsort [] = []
qsort (x:xs) = left ++ [x] ++ right
truewhere left  = qsort [a | a <- xs, a <= x]
    	  right = qsort [a | a <- xs, a > x]

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