Main.hs

module Main where

import Data.IORef                        
                                         
data Counter = Counter { x :: IORef Int }

makeCounter :: Int -> IO Counter        
makeCounter i = do iref <- newIORef i   
                   return (Counter iref)


incCounter :: Int -> Counter -> IO ()            
incCounter i (Counter c) = do modifyIORef c (+ i)

showCounter :: Counter -> IO ()               
showCounter (Counter c) = do c' <- readIORef c
                             print(c')        

main :: IO ()
main = do
  c <- makeCounter 1
  showCounter c
  incCounter 1 c
  showCounter c