gistfile1.hs

newtype BlockingLazyGenerator a = BLG (MVar (a, BlockingLazyGenerator a))

pushBLG :: BlockingLazyGenerator a -> a -> IO (BlockingLazyGenerator a)
pushBLG (BLG blg) val = do
    newBLG <- fmap BLG newEmptyMVar
    res    <- tryPutMVar blg (val, newBLG)
    when (not res) $ error "tried to put a BLG more than once"
    return newBLG
    
unsafeTailBLG :: BlockingLazyGenerator a -> [a]
unsafeTailBLG (BLG b) = unsafeInterleaveIO $ do
    (v, rem) <- readMVar b
    return (v:unsafeInterleaveIO rem)
             
newBLG :: IO (BlockingLazyGenerator a, [a])
newBLG = do
    blg <- fmap BLG newEmptyMVar
    return (blg, unsafeTailBLG blg)