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_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
