ResourcePool.hs

newtype Pool r = Pool (TVar (Seq r))

newPool :: [r] -> IO (Pool r)
newPool = fmap Pool . newTVarIO . Seq.fromList

getResource :: Pool r -> IO r
getResource (Pool pool) = STM.atomically do
  rs <- STM.readTVar pool
  case rs of
    Seq.Empty -> STM.retry
    r :<| rs' -> writeTVar pool rs' $> r

putResource :: Pool r -> r -> IO ()
putResource (Pool pool) r = STM.atomically $ STM.modifyTVar' pool (:|> r)

-- Will block until resource is available
withResource :: Pool r -> (r -> IO a) -> IO a
withResource pool = bracket (getResource pool) (putResource pool)