pieter:
Hello,
I'm trying to write a simple module for the haskell web server (hws- cgi).
And I would like to write a simple module that maintains some kind of state for a session.
But I'm I write I cannot do this in pure Haskell ? Without adopting the sources of the Haskell web server ?
I'll examplify to make it more concrete :
The requestHandler function a module has to implement has this signature.
requestHandler :: ServerState -> ServerRequest -> IO (Maybe Response)
Let 's assume I have this implementation
requestHandler st _ = return $ Just $ mkRequest where mkRequest = okResponse (serverConfig st) mkBody hs True mkBody = HereItIs " This is a test" hs = mkHeaders [contentTypeHeader "text/html"]
And I would like the response to include, for example, a number indicating the number of calls that has been handled by the module.
I would concider using an Mvar but I can't "remember" the mvar between requests.
Am I right to assume that the interface of the requestHandler method has to be adapted ? Or that serverstate has to be adopted so that it can act as a datastore ?
I don't think so. You could, for example store the count on disk, and read it back in. Or you could simulate a disk store by using a mutable variable, hidden in your module: module M (requestHandler) where import Control.Concurrent.MVar import System.IO.Unsafe -- -- A threadsafe mutable variable, internal to this module. Rather -- than use, say, a disk file as storage, we can keep the count here. -- countRef :: MVar Int countRef = unsafePerformIO $ newMVar 0 {-# NOINLINE countRef #-} ------------------------------------------------------------------------ -- And a quick example: type Response = Int requestHandler :: IO (Maybe Response) requestHandler = do n <- modifyMVar countRef $ \c -> return (c+1, c) print $ "received " ++ show n ++ " requests." return $ case n of 0 -> Nothing _ -> Just n *Main> requestHandler "received 0 requests." Nothing *Main> requestHandler "received 1 requests." Just 1 *Main> requestHandler "received 2 requests." Just 2 *Main> requestHandler "received 3 requests." Just 3 This seems simpler than writing the count to disk. And as long as you stay in IO, perfectly safe. In the longer term, you might want to look at state-parameterised plugins for the HWS. We do this in lambdabot, associating a private state type with each plugin, which is managed by the main server. The plugins can then get or set internal state, without resorting to local mutable variables. -- Don