Tony's problem

lazy.hs

reverse'' :: Int -> [a] -> [a] -> [a]
reverse'' n [] rs = drop n rs
reverse'' n as rs = h : t
  where 
    h = case as of (a : as) -> head $ reverse'' n as (a : rs)
    t = case as of (a : as) -> reverse'' (n + 1) as (a : rs)

reverse' :: [a] -> [a]
reverse' x = reverse'' 0 x []

main :: IO ()
main = do
  print $ reverse' $ [1..10]
  print $ length . take 1 . reverse' . reverse' $ [1..]
  print $ length . take 50000 . reverse' . reverse' $ [1..]