如何用Haskell实现回文字符串的前半部分提取函数？

需求：编写halfPalindrome函数，判断输入字符串是否为回文：若是，用Just返回其前半部分；若不是，则返回Nothing。

示例：

• halfPalindrome "boy" → Nothing
• halfPalindrome "abba" → Just "ab"

最初尝试的代码（存在问题） #

halfPalindrome :: String -> Maybe String
halfPalindrome x = 
    if x == reverse x 
        then 
            Just half 
    else Nothing 

            
half :: [a] -> ([a], [a])
half xs = (take 1 xs, drop 1 xs) 
    where 1 = div (length xs) 2

修正后的可行代码 #

完整版本 #

halfPalindrome :: String -> Maybe String
halfPalindrome x
  | x == reverse x = let (a, _) = half x in Just a
  | otherwise = Nothing

first_half :: [a] -> [a]
first_half = (\xs -> case xs of
            [] -> []
            xs -> take ((length xs) `div` 2 ) xs)

second_half :: [a] -> [a]
second_half = (\xs -> case xs of
            [] -> []
            xs -> drop ((length xs) `div` 2 ) xs)

half :: [a] -> ([a],[a])
half = (\xs -> case xs of
            [] -> ([],[])
            xs -> (first_half xs, second_half xs))

简化版本 #

halfPalindrome :: String -> Maybe String
halfPalindrome x 
    | x == reverse x = let (a, _) = half x in Just a
    | otherwise = Nothing

half :: [a] -> ([a], [a]) 
half xs = 
    ((take s xs), (drop s xs))
    where
        s = (length xs ) `div` 2

内容的提问来源于stack exchange，提问作者ipholie46