[Haskell-cafe] STM problems

[Michael Speer]

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