相关疑难解决方法(0)

isPalindrome :: [a] -> Bool
isPalindrome xs = case xs of 
                        [] -> True 
                        [x] -> True
                        a -> (last a) == (head a) && (isPalindrome (drop 1 (take (length a - 1) a)))

main = do
    print (show (isPalindrome "blaho"))

结果是

No instance for (Eq a)
  arising from a use of `=='
In the first argument of `(&&)', namely `(last a) == (head a)'
In the expression:
  (last a) == (head a)
  && (isPalindrome (drop 1 (take (length a …