找出数字是否是回文

Question

I'm relatively new at Haskell and I have no idea why this won't compile. What I'm trying to do is create a function that checks if a number in string form is a palindrome by creating a helper function that does that. Thanks!

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

Errors:

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

It doesn't compile because digitToInt takes a Char and returns an Int .

You're giving it a list ( s :: [Char] ) and treating the result as another list ( helper takes a list).