haskell:
I am writing a web application server in Haskell. I would like to be able to modify the app on the fly. Simplyfing the app server, it would look like this:
appServer appMVar reqChan state = do req <- readChan reqChan app <- readMVar appMVar (state',resp) <- return $ app state req forkIO $ doResp req resp appServer appMVar reqChan state'
The app would get loaded as follows:
updateApp writeLog appMVar moduleName fnName = do mbApp <- (getApp >>= Just) `catch` (\err -> writeLog err >> return Nothing) maybe (return ()) (overWriteMVar appMVar) mbApp threadDelay 1000 updateApp appMVar moduleName fnName where getApp = ghci_load_module moduleName >>= flip ghci_getName fnName
overWriteMVar mvar val = if isEmptyMVar appMVar then putMVar appMVar app else swapMVar appMVar app {-- Is there a way to overwrite an MVar without risking blocking by another thread filling it before the putMVar?
Note it would be really nice if:
* ghci_load_module would use an already cached module if the underlying source file has not changed (and therefore cost little performance-wise).
* the returned code would be compiled with lots of optimization, etc.
* Bonus: it would verify that the loaded function is type consistent with channel but tolerate bigger data types so that if the prior app assumed state was
data State = Foo | Bar
It would not be an error if the new app handled a state that looked like:
data State = Foo | Bar | Baz
The result would be a haskell server where all the haskell source acts like asp, jsp, php pages, but its all type safe and you don't have to define a standard page type as in HSP.
I am aware that there is a DynamicLoader project:
http://www.dtek.chalmers.se/~d00ram/dynamic/
That lets you load "object files." But I would really like something that loads source files instead....
Please look at hs-plugins: http://www.cse.unsw.edu.au/~dons/hs-plugins and the accompanying paper: http://www.cse.unsw.edu.au/~dons/hs-plugins/paper hs-plugins is already being used at Chalmers for their Haskell Server Pages project, which sounds a lot like what you're describing. You can ask Niklas Broberg about this. Cheers, Don