median.hs

import Data.List (partition)

average2 :: (Fractional a) => a -> a -> a
average2 x y = (x + y) / 2

median :: Ord a => [a] -> Maybe a
median xs
  | even n = Nothing
  | otherwise = select (n `div` 2) xs
  where
    n = length xs

select :: Ord a => Int -> [a] -> Maybe a
select _ [] = Nothing
select k (x:xs) = case compare k m of
  LT -> select k lt
  EQ -> Just x
  GT -> select (k - m - 1) gte
  where
     m = length lt 
     (lt, gte) = partition (< x) xs

main :: IO ()
main = do
  let v = [1,2,3,4,5]
  print $ median v