Poison.hs

-- Communicating Haskell Processes.
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module Control.Concurrent.CHP.Poison where

import Control.Concurrent.STM

-- | A Maybe-like poison wrapper.
data WithPoison a = PoisonItem | NoPoison a deriving (Eq, Show)

instance Functor WithPoison where
  fmap f (NoPoison x) = NoPoison $ f x
  fmap _ PoisonItem = PoisonItem

instance Monad WithPoison where
  return = NoPoison
  PoisonItem >>= _ = PoisonItem
  NoPoison x >>= f = f x

mergeWithPoison :: [WithPoison a] -> WithPoison ()
mergeWithPoison = sequence_

waitForJustOrPoison :: TVar (WithPoison (Maybe a)) -> STM (WithPoison a)
waitForJustOrPoison tv = do x <- readTVar tv
                            case x of
                              PoisonItem -> return PoisonItem
                              NoPoison Nothing -> retry
                              NoPoison (Just y) -> return $ NoPoison y

waitForNothingOrPoison :: TVar (WithPoison (Maybe a)) -> STM (WithPoison ())
waitForNothingOrPoison tv = do x <- readTVar tv
                               case x of
                                 PoisonItem -> return PoisonItem
                                 NoPoison (Just _) -> retry
                                 NoPoison Nothing -> return $ NoPoison ()