You can use "undefined" or "error ..." :
{-# LANGUAGE RecursiveDo #-} import Control.Concurrent.STM import Control.Monad.Fix
-- Transactional loop. A loop is a circular link list. data Loop a = ItemLink { item :: a , prev :: TVar (Loop a) , next :: TVar (Loop a) }
-- Create a new empty transactional loop. newLoop :: a -> STM (TVar (Loop a)) newLoop item = do tLoop <- newTVar undefined writeTVar tLoop (ItemLink item tLoop tLoop) return tLoop
Hmmm.. STM does not have a MonadFix instance. But IO does:
-- Use MonadFix instance of newLoopIO newLoopIO :: a -> IO (TVar (Loop a)) newLoopIO item = mfix (\ tLoop -> newTVarIO (ItemLink item tLoop tLoop))
But mfix (like fix) is difficult to read in large amounts, so there is "mdo":
-- Use RecursiveDo notation newLoopMDO :: a -> IO (TVar (Loop a)) newLoopMDO item = mdo tLoop <- newTVarIO (ItemLink item tLoop tLoop) return tLoop
Cheers, Chris