On Wed, 2010-09-08 at 09:53 +0200, Alberto G. Corona wrote:

Ah, ok. I though that your intention was to restrict the class definition for the IO monad . Goood.

Maybe in the long term, it make sense to include this type class in some monad related package such is mtl, since references are part of many user monads. I think also that some default instances can be defined for transformed monads.

Yes - they can. In fact: instance (MonadTrans t, Monad (t m), Reference r m) => Reference r (t m) where newRef = lift . newRef readRef = lift . readRef writeRef r = lift . writeRef r instance (MonadIO m, Reference r IO) => Reference r m where newRef = liftIO . newRef readRef = liftIO . readRef writeRef r = liftIO . writeRef r

It is pleasant for me to read and write something like

x <- readRef rx

rather than

x<- readMyMonadRef sx

Such generalizatiopns may be also good for extracting more common patterns among different monads.

Actually the primary need was to create mutable single-linked list for iteratee-parsec. I didn't wanted to restrict myself to ST or IO so I choose to add class for them. Regards