# 99 questions/Solutions/6

(*) Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).

```isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome xs = xs == (reverse xs)```
```isPalindrome' []  = True
isPalindrome' [_] = True
isPalindrome' xs  = (head xs) == (last xs) && (isPalindrome' \$ init \$ tail xs)```

Here's one to show it done in a fold just for the fun of it. Do note that it is less efficient then the previous 2 though.

```isPalindrome'' :: (Eq a) => [a] -> Bool
isPalindrome'' xs = foldl (\acc (a,b) -> if a == b then acc else False) True input
where
input = zip xs (reverse xs)```

Another one just for fun:

```isPalindrome''' :: (Eq a) => [a] -> Bool
isPalindrome''' = Control.Monad.liftM2 (==) id reverse```

Or even:

```isPalindrome'''' :: (Eq a) => [a] -> Bool
isPalindrome'''' = (==) Control.Applicative.<*> reverse```

Here's one that does half as many compares:

```palindrome :: (Eq a) => [a] -> Bool
palindrome xs = p [] xs xs
where p rev (x:xs) (_:_:ys) = p (x:rev) xs ys
p rev (x:xs) [_] = rev == xs
p rev xs [] = rev == xs```

Here's one using foldr and zipWith.

```palindrome :: (Eq a) => [a] -> Bool
palindrome xs = foldr (&&) True \$ zipWith (==) xs (reverse xs)
palindrome' xs = and \$ zipWith (==) xs (reverse xs) -- same, but easier```

```isPalindrome list = take half_len list == reverse (drop (half_len + (len `mod` 2)) list)
where
len = length list
half_len = len `div` 2

isPalindrome' list = f_part == reverse s_part
where
len = length list
half_len = len `div` 2
(f_part, s_part') = splitAt half_len list
s_part = drop (len `mod` 2) s_part'```