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
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