import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
 
rev ::  UArray Int Int -> UArray Int Int
rev ary = runSTUArray $ do
  ary <- thaw ary :: ST s (STUArray s Int Int)
  (l,r) <- getBounds ary
  rev' ary l r ((r - l + 1) `div` 2) 0
  return ary
  where
    rev' ary init end len n
      | n < len = do
          let (a,b) = (init + n, end - n)
          x <- readArray ary a
          y <- readArray ary b
          writeArray ary a y
          writeArray ary b x
          rev' ary init end len (n+1)
      | otherwise = return ()
 
size = 1000000
main = print $ (rev $ runSTUArray $ newListArray (1, size) [1..size]) ! 1

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