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(simple example of reader monad)
Current revision (20:49, 30 November 2012) (edit) (undo)
(Added solution using Control.Arrows fan out operator.)
 
(5 intermediate revisions not shown.)
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isPalindrome''' :: (Eq a) => [a] -> Bool
isPalindrome''' :: (Eq a) => [a] -> Bool
isPalindrome''' = Control.Monad.liftM2 (==) id reverse
isPalindrome''' = Control.Monad.liftM2 (==) id reverse
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</haskell>
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Or even:
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<haskell>
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isPalindrome'''' :: (Eq a) => [a] -> Bool
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isPalindrome'''' = (==) Control.Applicative.<*> reverse
</haskell>
</haskell>
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p rev (x:xs) [_] = rev == xs
p rev (x:xs) [_] = rev == xs
p rev xs [] = rev == xs
p rev xs [] = rev == xs
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</haskell>
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Here's one using foldr and zipWith.
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<haskell>
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palindrome :: (Eq a) => [a] -> Bool
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palindrome xs = foldr (&&) True $ zipWith (==) xs (reverse xs)
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palindrome' xs = and $ zipWith (==) xs (reverse xs) -- same, but easier
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</haskell>
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<haskell>
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isPalindrome list = take half_len list == reverse (drop (half_len + (len `mod` 2)) list)
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where
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len = length list
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half_len = len `div` 2
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isPalindrome' list = f_part == reverse s_part
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where
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len = length list
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half_len = len `div` 2
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(f_part, s_part') = splitAt half_len list
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s_part = drop (len `mod` 2) s_part'
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</haskell>
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Using Control.Arrows (&&&) fan out operator.
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With monomorphism restriction:
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<haskell>
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isPalindrome1 xs = (uncurry (==) . (id &&& reverse)) xs
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</haskell>
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Point free with no monomorphism restriction:
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<haskell>
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isPalindrome1 = (uncurry (==) . (id &&& reverse))
</haskell>
</haskell>

Current revision

(*) 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'


Using Control.Arrows (&&&) fan out operator.

With monomorphism restriction: 

isPalindrome1 xs = (uncurry (==) . (id &&& reverse)) xs

Point free with no monomorphism restriction: 

isPalindrome1 = (uncurry (==) . (id &&& reverse))
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