Re: [Haskell-cafe] why cannot i get the value of a IORef variable ?

For clarity, one trick that uses "unsafePerformIO" which you may have seen posted on this list earlier today is the following way of creating a globally visible IORef:

import Data.IORef import System.IO.Unsafe

*** counter = unsafePerformIO $ newIORef 0 ***

next = do  modifyIORef counter (+1)  readIORef counter

This is still unsafe but it can evidently be slightly improved with NOINLINE pragma: {-# NOINLINE counter #-} counter = unsafePerformIO $ newIORef 0 without said pragma the counter could be initialized numerous times, as I understand it. All this said, I hope people avoid unsafePerformIO for mutable globals - reader monad, state monad, and partial application should be sufficient tools. Thomas