找出數字是否是回文

Question

我在Haskell相對較新，我不知道為什么它無法編譯。 我正在嘗試做的是創建一個函數，該函數通過創建一個輔助函數來檢查字符串形式的數字是否是回文。 謝謝！

isPalindrome :: [Char] -> Bool
isPalindrome s = (helper . Char.digitToInt) s
    where helper [] = True
          helper [x] = True
          helper (x:xs) = x == (last xs) && (helper . init) xs

錯誤：

euler.hs:29:28: error:
• Couldn't match type ‘Int’ with ‘[a0]’
  Expected type: Char -> [a0]
    Actual type: Char -> Int
• In the second argument of ‘(.)’, namely ‘Char.digitToInt’
  In the expression: helper . Char.digitToInt
  In the expression: (helper . Char.digitToInt) s
euler.hs:29:45: error:
• Couldn't match expected type ‘Char’ with actual type ‘[Char]’
• In the first argument of ‘helper . Char.digitToInt’, namely ‘s’
  In the expression: (helper . Char.digitToInt) s
  In an equation for ‘isPalindrome’:
      isPalindrome s
        = (helper . Char.digitToInt) s
        where
            helper [] = True
            helper [x] = True
            helper (x : xs) = x == (last xs) && (helper . init) xs

Answer 1

它不會編譯，因為digitToInt接受一個Char並返回一個Int 。

您給它一個列表（ s :: [Char] ），並將結果視為另一個列表（ helper獲取一個列表）。