ST Monad Example

STMonad.hs

-- ST Monad Example

import Control.Monad.ST (ST)
import Control.Monad.ST (runST)
import Data.STRef (STRef)
import Data.STRef (modifySTRef)
import Data.STRef (newSTRef)
import Data.STRef (readSTRef)

prog :: Int -> ST s Int
prog i = do
  r <- newSTRef i
  modifySTRef r (+ 2)
  modifySTRef r (* 3)
  readSTRef r

test :: Int -> Int
test i = runST (prog i)

-- export error
prog2 :: Int -> ST s (STRef s Int)
prog2 i = do
  newSTRef i
-- test2 :: Int -> STRef s Int
-- test2 i = runST (prog2 i)