Using MVar as a closure value

mvar_as_closure.hs

{-# LANGUAGE KindSignatures #-}                                                                                                                                                                                                                
{-# LANGUAGE RankNTypes #-}                                                                                                                                                                                                                    
{-# LANGUAGE OverloadedStrings #-}                                                                                                                                                                                                             
                                                                                                                                                                                                                         
import Control.Concurrent.MVar                                                                                                                                                                                                                 
                                                                                                                                                                                                                                               
testMVar :: IO Int                                                                                                                                                                                                                             
testMVar = do                                                                                                                                                                                                                                  
  value <- newMVar $ ([10, 100, 1000]::[Int])                                                                                                                                                                                                  
  let f = modifyMVar value $ \l -> case l of                                                                                                                                                                                                   
        [] -> return ([], 99)                                                                                                                                                                                                                  
        (x:xs) -> return (xs, x)                                                                                                                                                                                                               
  useF f                                                                                                                                                                                                                                       
                                                                                                                                                                                                                                               
--useF :: forall (m :: * -> *) b. (Monad m, Num b) => m b -> m b                                                                                                                                                                               
useF :: forall (m :: * -> *) b. (Monad m, Num b) => m b -> m b                                                                                                                                                                                 
useF x = do                                                                                                                                                                                                                                    
  x1 <- x                                                                                                                                                                                                                                      
  x2 <- x                                                                                                                                                                                                                                      
  x3 <- x                                                                                                                                                                                                                                      
  x4 <- x                                                                                                                                                                                                                                      
  return (x1 + x2 + x3 + x4)                                                                                                                                                                                                                   
                                                                                                                                                                                                                                               
main :: IO ()                                                                                                                                                                                                                                  
main = do                                                                                                                                                                                                                                      
  --r <- useF $ testMVar                                                                                                                                                                                                                       
  r <- testMVar                                                                                                                                                                                                                                
  putStrLn $ show r