EVars.hs

{-# LANGUAGE DeriveDataTypeable #-}
module Transient.EVars where

import Transient.Base
import Transient.Internals(runTransState,onNothing, EventF(..), killChildren)
import qualified Data.Map as M
import Data.Typeable

import Control.Concurrent
import Control.Applicative
import Control.Concurrent.STM
import Control.Monad.IO.Class
import Control.Exception(SomeException)

import Data.List(nub)
import Control.Monad.State



data EVar a= EVar  (TChan (StreamData a)) deriving  Typeable


-- | creates an EVar.
--
-- Evars are event vars. `writeEVar` trigger the execution of all the continuations associated to the  `readEVar` of this variable
-- (the code that is after them).
--
-- It is like the publish-subscribe pattern but without inversion of control, since a readEVar can be inserted at any place in the
-- Transient flow.
--
-- EVars are created upstream and can be used to communicate two sub-threads of the monad. Following the Transient philosophy they
-- do not block his own thread if used with alternative operators, unlike the IORefs and TVars. And unlike STM vars, that are composable,
-- they wait for their respective events, while TVars execute the whole expression when any variable is modified.
--
-- The execution continues after the writeEVar when all subscribers have been executed.
--
-- Now the continuations are executed in parallel.
--
-- see https://www.fpcomplete.com/user/agocorona/publish-subscribe-variables-transient-effects-v
--

newEVar ::  TransIO (EVar a)
newEVar  = Transient $ do
   id <- genId
   ref <-liftIO  newBroadcastTChanIO
   return . Just $ EVar  ref

-- | delete al the subscriptions for an evar.
cleanEVar :: EVar a -> TransIO ()
cleanEVar (EVar  ref1)= liftIO $ atomically $ do
    writeTChan  ref1 SDone


-- | read the EVar. It only succeed when the EVar is being updated
-- The continuation gets registered to be executed whenever the variable is updated.
--
-- if readEVar is re-executed in any kind of loop, since each continuation is different, this will register
-- again. The effect is that the continuation will be executed multiple times
-- To avoid multiple registrations, use `cleanEVar`
readEVar :: EVar a -> TransIO a
readEVar (EVar  ref1)=  do
     tchan <-  liftIO . atomically $ dupTChan ref1
     r <- parallel $ atomically $  readTChan tchan

     case r of
        SDone -> empty
        SMore x -> return x
        SLast x -> return x
        SError e -> empty
--              error $ "readEVar: "++ show e

-- |  update the EVar and execute all readEVar blocks with "last in-first out" priority
--
writeEVar (EVar  ref1) x= liftIO $ atomically $ do
       writeTChan  ref1 $ SMore x


-- | write the EVar and drop all the `readEVar` handlers.
--
-- It is like a combination of `writeEVar` and `cleanEVar`
lastWriteEVar (EVar ref1) x= liftIO $ atomically $ do
       writeTChan  ref1 $ SLast x


---- Finalization
--
--
--type FinishReason= Maybe SomeException
--
--
--
--data Finish= Finish (EVar FinishReason) deriving Typeable
--
---- | initialize the event variable for finalization.
---- all the following computations in different threads will share it
---- it also isolate this event from other branches that may have his own finish variable
--initFinish :: TransIO Finish
--initFinish= do
--      fin <-  newEVar
--      let f = Finish fin
--      setData  f
--      return  f
--
---- | set a computation to be called when the finish event happens
--onFinish :: (FinishReason ->TransIO ()) -> TransIO ()
--onFinish  close=  do
--       Finish finish <- getSData <|> initFinish
--       e <-  freeThreads $ readEVar finish
--       close e  -- !!> "CLOSE"
--       stop
--     <|>
--       return ()
--
--
--
---- | trigger the event, so this closes all the resources
--finish :: FinishReason -> TransIO ()
--finish e= do
--    liftIO $ putStr  "finish: " >> print e
--    Finish finish <- getSData <|> initFinish
--    lastWriteEVar finish e
--
---- | deregister all the finalization actions.
---- A initFinish is needed to register actions again
--unFinish= do
--    Finish fin <- getSData
--    cleanEVar fin    -- !!> "DELEVAR"
--   <|> return ()   -- !!> "NOT DELEVAR"
--
--
---- | kill all the processes generated by the parameter when finish event occurs
--killOnFinish comp= do
--
--   chs <- liftIO $ newTVarIO []
--   onFinish $ const $ liftIO $ killChildren chs   -- !> "killOnFinish event"
--   r <- comp
--   modify $ \ s -> s{children= chs}
--   return r
--
---- | trigger finish when the stream data return SDone
--checkFinalize v=
--           case v of
--              SDone ->  finish Nothing >> stop
--              SLast x ->  return x
--              SError e -> liftIO ( print e) >> finish Nothing >> stop
--              SMore x -> return x