I am playing a writing a simple STM hashtable. I expected the following code to alter the TVars in it, but they appear immutable. I assume I am simply doing something wrong, but I am not sure what it is. Any pointers would be greatly appreciated. import Control.Concurrent.STM import Control.Concurrent import Array data HashTable a b = HashTable (a -> Int) (STM (Array Int (TVar (Maybe b)))) mkHashTable :: ( a -> Int ) -> Int -> HashTable a b mkHashTable hashfn numBuckets | numBuckets > 0 = do HashTable hashfn $ do vs <- mapM (\v -> newTVar Nothing >>= \tv -> return (v,tv) ) [1..numBuckets] return $ array (1,numBuckets) vs readHashTable :: HashTable a b -> a -> STM (Maybe b) readHashTable hashtable key = let (HashTable hashfn table) = hashtable in table >>= \tb -> readTVar (tb ! hashfn key) writeHashTable :: HashTable a b -> a -> (Maybe b) -> STM () writeHashTable hashtable key value = let (HashTable hashfn table) = hashtable in table >>= \tb -> writeTVar (tb ! hashfn key) value >> return () main = let ht = mkHashTable id 10 :: HashTable Int String in do v <- atomically $ do mapM_ (\n -> writeHashTable ht n (Just $ "hello " ++ show n)) [1..10] mapM (\n -> readHashTable ht n) [1..10] print v