{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
-- | An extensible effect that wraps 'Control.Monad.Log.MonadLog' into an extensible effect.
module Control.Eff.Log
  ( handleLogsWith
  , Logs(..)
  , logMessageFreeEff
  , logMsg
  , module ExtLog
  , LogChannel()
  , logChannelPutIO
  , forwardLogsToChannel
  , forkLogChannel
  , joinLogChannel
  , logChannelBracket
  )
where

import           Control.Concurrent
import           Control.Concurrent.STM
import           Control.Eff                   as Eff
import           Control.Exception              ( bracket )
import           Control.Monad                  ( void
                                                , when
                                                )
import           Control.Monad.Log             as ExtLog
                                         hiding ( )
import           Control.Monad.Trans.Control
import           Data.Kind
import qualified Control.Eff.Lift              as Eff
import qualified Control.Monad.Log             as Log

-- | The 'Eff'ect type to wrap 'ExtLog.MonadLog'.
-- This is a
data Logs message a where
  LogMessageFree :: (forall n . Monoid n => (message -> n) -> n) -> Logs message ()

-- | Effectful version of the /strange/ 'ExtLog.logMessageFree' function.
logMessageFreeEff
  :: Member (Logs message) r
  => (forall n . Monoid n => (message -> n) -> n)
  -> Eff r ()
logMessageFreeEff foldMapish = send (LogMessageFree foldMapish)

-- | Effectful version of the 'ExtLog.logMessage' function.
logMsg :: Member (Logs m) r => m -> Eff r ()
logMsg msg = logMessageFreeEff ($ msg)

-- | Handle 'Logs' effects using 'Log.LoggingT' 'Log.Handler's.
handleLogsWith
  :: forall m r message a
   . (Monad m, SetMember Eff.Lift (Eff.Lift m) r)
  => Eff (Logs message ': r) a
  -> (forall b . (Log.Handler m message -> m b) -> m b)
  -> Eff r a
handleLogsWith actionThatLogs foldHandler = Eff.handle_relay return
                                                             go
                                                             actionThatLogs
 where
  go :: Logs message b -> (b -> Eff r c) -> Eff r c
  go (LogMessageFree foldMapish) k =
    Eff.lift (foldHandler (Log.runLoggingT (Log.logMessageFree foldMapish)))
      >>= k

-- * Concurrent Logging

-- | Input queue for a concurrent logger.
data LogChannel message =
  LogChannel { fromLogChannel :: TQueue message
             , logChannelOpen :: TVar Bool
             , logChannelThread :: ThreadId
             }

-- | Send the log messages to a 'LogChannel'.
forwardLogsToChannel
  :: forall r message a
   . (SetMember Eff.Lift (Eff.Lift IO) r)
  => LogChannel message
  -> Eff (Logs message ': r) a
  -> Eff r a
forwardLogsToChannel logChan actionThatLogs = do
  handleLogsWith actionThatLogs
                 (\withHandler -> withHandler (logChannelPutIO logChan))

-- | Enqueue a log message into a log channel
logChannelPutIO :: LogChannel message -> message -> IO ()
logChannelPutIO c m = atomically
  (do
    isOpen <- readTVar (logChannelOpen c)
    when isOpen (writeTQueue (fromLogChannel c) m)
  )

-- | Fork 'LogChannel' backed by a process that repeatedly receives log messages
-- sent by 'forwardLogstochannel' or 'logChannelPutIO'. The process logs by
-- invoken the given IO action. To stop and terminate a 'LogChannel' invoke
-- 'joinLogChannel'.
forkLogChannel
  :: forall message
   . (message -> IO ())
  -> Maybe message
  -> IO (LogChannel message)
forkLogChannel handle mFirstMsg = do
  (msgQ, isOpenV) <- atomically
    (do
      tq <- newTQueue
      v  <- newTVar True
      mapM_ (writeTQueue tq) mFirstMsg
      return (tq, v)
    )
  thread <- forkFinally (logLoop msgQ isOpenV) (const (cleanUp msgQ isOpenV))
  return (LogChannel msgQ isOpenV thread)
 where
  cleanUp :: TQueue message -> TVar Bool -> IO ()
  cleanUp tq isOpenVar =
    atomically
        (do
          writeTVar isOpenVar False
          flushTQueue tq
        )
      >>= mapM_ handle
  logLoop :: TQueue message -> TVar Bool -> IO ()
  logLoop tq isOpenVar = do
    mMsg <- atomically
      (do
        isOpen <- readTVar isOpenVar
        if isOpen then Just <$> readTQueue tq else return Nothing
      )
    case mMsg of
      Just msg -> do
        handle msg
        logLoop tq isOpenVar
      Nothing -> return ()

-- | Close a log channel. Subsequent loggin requests will no be handled any
-- more.
joinLogChannel :: Maybe message -> LogChannel message -> IO ()
joinLogChannel closeLogMessage (LogChannel tq isOpenVar thread) = do
  wasOpen <- atomically
    (do
      isOpen <- readTVar isOpenVar
      if isOpen
        then do
          writeTVar isOpenVar False
          mapM_ (writeTQueue tq) closeLogMessage
          return True
        else return False
    )
  when wasOpen (killThread thread)


-- | Wrap 'LogChannel' creation and destruction around a monad action in
-- 'bracket'y manner.
logChannelBracket
  :: Maybe message
  -> Maybe message
  -> (LogChannel message -> IO a)
  -> LoggingT message IO a
logChannelBracket mWelcome mGoodbye f = control
  (\runInIO -> do
    myTId <- myThreadId
    let logHandler = void . runInIO . logMessage
    bracket (forkLogChannel logHandler mWelcome) (joinLogChannel mGoodbye) f
  )