Another approach, still lazy. But without stack blowup.

gistfile1.hs

{-# LANGUAGE BangPatterns #-}
module T where

import Control.Concurrent.MVar

main = do
  countMVar <- newMVar (\ count -> count):: IO (MVar (Int -> Int))

  let
    process :: IO ()
    process = do
      cmd <- getLine
      modifyMVar_ countMVar $ \ countEater -> return $ \ count -> countEater $! count + 1
      if cmd == "out"
        then modifyMVar_ countMVar $ \ countEater -> do
          let
            now = countEater 0
          putStrLn $ show now
          return $ \ count -> now + count
        else return ()
      process
  
  process