poppler worker thread design

popplerthread.hs

import Control.Concurrent
import Control.Concurrent.STM
import Data.Monoid
import Graphics.Rendering.Cairo 
import Graphics.UI.Gtk.Poppler.Document 
import Graphics.UI.Gtk.Poppler.Page
import System.Directory
import System.FilePath

popplerGetDocFromFile :: FilePath -> IO (Maybe Document)
popplerGetDocFromFile fp = 
  documentNewFromFile ("file://localhost" <> fp) Nothing 

popplerGetPageFromDoc :: Document -> Int -> IO Page
popplerGetPageFromDoc doc pn = do 
  pg <- documentGetPage doc (pn-1) 
  return pg 

main :: IO () 
main = do 
  pwd <- getCurrentDirectory 
  tvar <- atomically $ newTVar [1] 
  withPDFSurface "test.pdf" 610 720 $ \sfc -> do 
    mvar <- newEmptyMVar 
    Just doc <- popplerGetDocFromFile (pwd </> "1311.4247v1.pdf")
    let go = do
          threadDelay 1000000
          lst <- atomically $ readTVar tvar
          case lst of 
            [] -> putMVar mvar ()
            p:ps -> do 
              pg <- popplerGetPageFromDoc doc p
              renderWith sfc (pageRender pg)
              atomically  $ writeTVar tvar ps
              go
    forkIO go
    -- 
    atomically $ do
      lst <- readTVar tvar 
      writeTVar tvar (lst ++ [2])
    atomically $ do
      lst <- readTVar tvar 
      writeTVar tvar (lst ++ [3])
    takeMVar mvar
  return ()