On Wed, Jan 7, 2009 at 11:11 AM, ChrisK <haskell@list.mightyreason.com> wrote:

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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