revTake-revDrop.hs

-- O(n) revTake and revDrop using list-numeral alike Church-numeral
-- (from Richard O'Keefe's post to haskell-cafe, i.e.
--  http://www.haskell.org/pipermail/haskell-cafe/2010-September/083905.html)

--  |          l           |
--  +----------------+-----+
--  |    l-n (_:m)      n  |

revTake n xs = drop' (drop n xs) xs
  where drop' (_:m) (_:xs) = drop' m xs
        drop'  _     xs    = xs

revDrop n xs = take' (drop n xs) xs
  where take' (_:m) (x:xs) = x : take' m xs
        take'  _     _     = []