module TList (TList, newTList, readTList, writeTList) where

import Control.Concurrent.STM

-- <<TList
newtype TList a = TList (TVar [a])

newTList :: STM (TList a)
newTList = do
  v  <- newTVar []
  return (TList v)

writeTList :: TList a -> a -> STM ()
writeTList (TList v) a = do
  list <- readTVar v
  writeTVar v (list ++ [a])

readTList :: TList a -> STM a
readTList (TList v) = do
  xs <- readTVar v
  case xs of
    []      -> retry
    (x:xs') -> do
      writeTVar v xs'
      return x
-- >>