This is how we add 10 to every element in a list

add10.hs

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

while ::
  Monad f =>
  f Bool
  -> f a
  -> f ()
while p a =
  p >>= (`when` (a >> while p a))
  
map' ::
  (a -> b)
  -> [a]
  -> [b] 
map' f list =
  runST $ do
    l <- var id
    c <- var list
    while
      (notEmpty <$> at c)
      (do m <- at l
          d <- at c
          l .= (m . (f (head d):))
          c .= tail d)
    ($[]) <$> at l

add10 ::
  [Int]
  -> [Int]
add10 =
  map' (+10)

-- support
notEmpty = not . null
at = readSTRef
var = newSTRef
(.=) = writeSTRef
infixr 4 .= 

Wicked.