-- Communicating Haskell Processes.
-- Copyright (c) 2008, University of Kent.
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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_

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

flipMaybe :: Maybe a -> Maybe ()
flipMaybe (Just _) = Nothing
flipMaybe Nothing = Just ()