{-# OPTIONS_GHC -XImplicitParams #-}
module Process where

import MBox
import Control.Exception
import System.IO

data Process a = Process  (a -> IO (Maybe ()))      -- write message
                          (IO Bool)                 -- is open?
                          (IO (Maybe Bool))         -- is empty?

-- a process is a build-run-in-my-private-world style process, similar to what Erlang uses, it is 
-- intentional that no forkIO/forkOS is done in runProcess - it is the responsibility ofthe caller
-- proc is bracket(ed) to guarantee that on any exit, including exceptions the mail box is closed
-- Unfortunately there is a possibility of resource leaks through mboxes. If a Process structure 
-- is kept somewhere outside runProcess, it won't be garbage collected. It remains to be seen how
-- best to reduce, or eliminate, the risk of that. The problem is that the Process structure
-- *must* be passed outside, otherwise it will be a very lonely thread....

runProcess :: Process (Process a) -> (Process a -> IO ()) -> IO ()
runProcess (Process sendBack isOpen isEmpty) proc = do
    mbox <- newMBox
    let ?receive = readMBox mbox
    let ?tryReceive = tryReadMBox mbox
    let ?unGet = unGetMBox mbox
    let p = (Process (writeMBox mbox) (isOpenMBox mbox) (isEmptyMBox mbox))
    --should check, for safety, alternatively could take a don't care attitude
    ok <- sendBack p
    case ok of
        Just _ -> bracket 
                    (return p) 
                    (\_ -> (closeMBox mbox)) 
                    proc
        Nothing -> (closeMBox mbox)