[Haskell-cafe] using ResourceT with MVars

[Warren Harris]

I would like to use LevelDB in my code, and the HEAD version of leveldb-haskell now uses runResourceT to manage open db connections and iterators. I would also like to run multiple threads, and coordinate them for transaction-like behavior, i.e. read and writing data without interruption. LevelDB doesn't support transactions itself, so the alternative would seem to be using MVars for mutual exclusion. 

With the 0.0.3 version of leveldb-haskell, the get and put operations were in the IO monad, which made it trivial to write something like this:

withMVar state $ \db -> do
  maybeValue <- get db rdOpts key
  put db wrOpts key $ maybe init incr maybeValue

Now with the HEAD version, the get and put operations have types:

get :: MonadResource m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString)
put :: MonadResource m => DB -> WriteOptions -> ByteString -> ByteString -> m ()

I don't think it makes sense in general to lift the withMVar into MonadResource and use runResourceT to run its body in IO. This would not satisfy the intention behind using ResourceT -- resource allocated inside the withMVar body would be deallocated when the withMVar scope is exited, rather than being managed by the outermost ResourceT. (Although, in this case we aren't allocating any resources, so maybe it's a moot point for my simple example.)

I also don't think it makes sense to use takeMVar and putMVar instead of withMVars as this would subvert the exception handling that withMVars provides. And my attempts to define a withMVarR (in the ResourceT IO monad) also seem to require runResourceT:

withMVarR :: MVar a -> (a -> ResourceT IO b) -> ResourceT IO b
withMVarR m io = do
  -- mask $ \restore -> do
    a <- lift $ takeMVar m
    b <- {-restore-} (io a) `onExceptionR` (lift $ putMVar m a)
    lift $ putMVar m a
    return b

onExceptionR :: ResourceT IO a -> ResourceT IO b -> ResourceT IO a
onExceptionR a b = do
  runResourceT $ transResourceT (\a -> transResourceT (onException a) b) a

Maybe I need to create a "HasMVar" class analogous to HasRef? (http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-Monad-Trans-Resource.html#t:HasRef)

I'd appreciate any suggestions on how to make these fit together. 

Warren
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