<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">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. <div><br></div><div>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:</div><div><br></div><div>withMVar state $ \db -> do</div><div> maybeValue <- get db rdOpts key</div><div> put db wrOpts key $ maybe init incr maybeValue</div><div><br></div><div>Now with the HEAD version, the get and put operations have types:</div><div><br></div><div><a name="v:get" class="def" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; font-weight: bold; color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">get</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> :: </span><a href="Database-LevelDB.html#t:MonadResource" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">MonadResource</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> m => </span><a href="Database-LevelDB.html#t:DB" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">DB</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> </span><a href="Database-LevelDB.html#t:ReadOptions" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">ReadOptions</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> </span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/bytestring-0.9.1.10/Data-ByteString.html#t:ByteString" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">ByteString</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> m (</span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/base-4.3.1.0/Data-Maybe.html#t:Maybe" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">Maybe</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> </span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/bytestring-0.9.1.10/Data-ByteString.html#t:ByteString" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">ByteString</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; ">)</span></div><div><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "><a name="v:put" class="def" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; font-weight: bold; color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">put</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> :: </span><a href="Database-LevelDB.html#t:MonadResource" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">MonadResource</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> m => </span><a href="Database-LevelDB.html#t:DB" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">DB</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> </span><a href="Database-LevelDB.html#t:WriteOptions" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">WriteOptions</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> </span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/bytestring-0.9.1.10/Data-ByteString.html#t:ByteString" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">ByteString</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> </span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/bytestring-0.9.1.10/Data-ByteString.html#t:ByteString" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">ByteString</a><span style="color: rgb(0, 0, 0); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(240, 240, 240); display: inline !important; float: none; "> -> m </span><a href="/usr/local/Cellar/ghc/7.0.4/share/doc/ghc/html/libraries/ghc-prim-0.2.0.0/GHC-Unit.html#t:-40--41-" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; text-decoration: none; color: rgb(171, 105, 84); font-family: monospace; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 16px; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">()</a></span></div><div><br></div><div>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.)</div><div><br></div><div>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:</div><div><br></div><div><div>withMVarR :: MVar a -> (a -> ResourceT IO b) -> ResourceT IO b</div><div>withMVarR m io = do</div><div> -- mask $ \restore -> do</div><div> a <- lift $ takeMVar m</div><div> b <- {-restore-} (io a) `onExceptionR` (lift $ putMVar m a)</div><div> lift $ putMVar m a</div><div> return b</div></div><div><br></div><div><div><div><div>onExceptionR :: ResourceT IO a -> ResourceT IO b -> ResourceT IO a</div><div>onExceptionR a b = do</div><div> runResourceT $ transResourceT (\a -> transResourceT (onException a) b) a</div></div></div></div><div><br></div><div>Maybe I need to create a "HasMVar" class analogous to HasRef? (<a href="http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-Monad-Trans-Resource.html#t:HasRef">http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-Monad-Trans-Resource.html#t:HasRef</a>)</div><div><br></div><div>I'd appreciate any suggestions on how to make these fit together. </div><div><br></div><div>Warren</div></body></html>