Short example of how to use IORef. IORef is not a global value, it is a container that must be passed, but its contents can be changed.

tutorial1.hs

module Main where

import Data.IORef

-- runhaskell tutorial1.hs
{-
create IORef with newIORef
newIORef :: a -> IO (IORef a)
  IORef is a container that has something of type a
  the value in IORef can change

writeIORef :: IORef a -> a -> IO ()
  change the value of the object in IORef
  must be the same type

modifyIORef :: IORef a -> (a -> a) -> IO ()
  apply a pure function to the value in IORef

get value out of IORef
readIORef :: IORef a -> IO a
  notice that it returns IO
  the value is removes its IORef wrapper 
  and it gains an IO one, you have to be in IO to use IORefs
-}

main :: IO ()
main = do
    -- create a new IORef
    stringRef <- newIORef $ ""
    string1 <- readIORef stringRef
    print string1
    
    -- change the value inside stringRef
    writeIORef stringRef "A"
    string2 <- readIORef stringRef
    print string2
    -- now it is apparent that the value inside of stringRef has changed

tutorial2.hs

module Main where

import Data.IORef
import System.IO

-- runhaskell tutorial2.hs

-- to access an IORef you still have to pass it manually
-- and to alter it you need to be in an IO monad, since 
-- modifyIORef and writeIORef all return IO ()

step :: IORef Int -> IO ()
step counter = do
  -- pass a function (a->a) that modifies the value inside
  -- the IORef
  modifyIORef counter (+1) 
  return ()

infiniteLoop :: IORef Int -> IO ()
infiniteLoop counter = do
  -- wait for user to push enter
  _ <- getLine
  c <- readIORef counter
  print c
  step counter
  infiniteLoop counter 

{- counter is not a global var
   brokenInfiniteLoop does not know that counter exists
brokenInfiniteLoop :: IO ()
brokenInfiniteLoop = do
  _ <- getLine
  c <- readIORef counter
  print c
  step counter
  brokenInfiniteLoop
-}

main = do
  counter <- newIORef 0
  infiniteLoop counter
  -- run infinitely, wait for user to push enter before showing incremented counter