Dynamic Source Loading

S. Alexander Jacobson haskell at alexjacobson.com
Mon Oct 25 23:18:51 EDT 2004

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 =
      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 =
        mbApp <- (getApp  >>= Just) `catch`
		 (\err -> writeLog err >> return Nothing)
        maybe (return ()) (overWriteMVar appMVar) mbApp
	threadDelay 1000
	updateApp appMVar moduleName fnName
        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

* 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:


That lets you load "object files."  But I would
really like something that loads source files

Is this possible?


S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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