don't use reverse

palindrome.hs

isPalindrome :: String -> Bool
isPalindrome str = isPalindrome' str [] (len `div` 2)
  where
    len = length str
    isPalindrome' [] _ _ = True
    isPalindrome' xs ys 0 = if len `mod` 2 == 0 then xs == ys else (tail xs) == ys
    isPalindrome' (x:xs) ys n = isPalindrome' xs (x:ys) (n - 1)