[Haskell-cafe] Re: Tying a simple circularly STM linked list

[John Ky]

Thanks Chris,

The undefined works for me.

-John

On Wed, Jan 7, 2009 at 11:11 AM, ChrisK <haskell at 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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