demostrate monad transformers and forever

a.hs

import Control.Monad
import Control.Monad.Trans.Class -- from `transformers`
import Control.Monad.Trans.Maybe -- from `transformers`
import Data.IORef

main :: IO ()
main = do
  i <- newIORef (0 :: Int)
  void $ runMaybeT $ forever $ do
    n <- lift (readIORef i)
    if n < 5
    then lift $ do
      print n
      modifyIORef i (+1)
    else
      MaybeT $ return Nothing

even better, with guard

import Control.Monad
import Control.Monad.Trans.Class -- from `transformers`
import Control.Monad.Trans.Maybe -- from `transformers`
import Data.IORef

main :: IO ()
main = do
  i <- newIORef (0 :: Int)
  void $ runMaybeT $ forever $ do
    n <- lift (readIORef i)
    guard (n < 5)
    lift $ do
      print n
      modifyIORef i (+1)

see: https://en.wikibooks.org/wiki/Haskell/Monad_transformers