counter.hs

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types (status200)
import Network.Wai.Handler.Warp (run)
import Control.Monad.Trans (liftIO, lift)
import Data.IORef (newIORef, atomicModifyIORef)
import Data.ByteString.Lazy.UTF8 (fromString)

application counter request = do
  count <- lift $ incCount counter
  return $ responseLBS status200 [("Content-type", "text/html")] $ fromString $ show count

incCount counter = atomicModifyIORef counter (\c -> (c+1, c))

main = do 
  counter <- newIORef 0
  run 3000 $ application counter