# 99 questions/Solutions/6

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Dan.krejsa (Talk | contribs) (Add a more efficient palindrome checker) |
(simple example of reader monad) |

## Revision as of 18:00, 14 June 2011

(*) 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

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