gcs function using a while loop in haskell

gistfile1.hs

import Control.Monad
import Control.Monad.ST
import Data.STRef

gcd ::
  Integer
  -> Integer
  -> Integer
gcd a b = 
  runST $
    do a' <- newSTRef a
       b' <- newSTRef b
       while ((/= 0) . snd) 
             (a' `read2` b')
             (\(x, y) -> 
               b' |= (x `mod` y) >> 
               a' |= y) 
             fst






-- BEGIN peripherals

read2 ::
  STRef s a
  -> STRef s b
  -> ST s (a, b)
read2 a b =
  liftM2 (,) (readSTRef a) (readSTRef b)

while ::
  Monad m =>
  (t -> Bool)
  -> m t
  -> (t -> m a)
  -> (t -> b)
  -> m b
while p s z e =
  do a <- s
     z a
     b <- s
     if p b
       then while p s z e
       else return (e b)

(|=) ::
  STRef s a
  -> a
  -> ST s ()
(|=) =
  writeSTRef