Merge sort in Haskell

sort :: (Ord a) => [a] -> [a]
sort [] = []
sort xs = head . mergesort . split $ xs

split :: [a] -> [[a]]
split = map (:[])

mergesort :: (Ord a) => [[a]] -> [[a]]
mergesort []           = []
mergesort [x]          = [x]
mergesort (xs:ys:rest) = mergesort $ merge xs ys : mergesort rest

merge :: (Ord a) => [a] -> [a] -> [a]
merge xs []                      = xs
merge [] xs                      = xs
merge first@(x:xs) second@(y:ys) = if x < y then x : merge xs second
                                   else y : merge first ys