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

On Thu, Oct 22, 2009 at 1:32 PM, David Menendez wrote:

On Thu, Oct 22, 2009 at 2:23 AM, Gregory Crosswhite wrote:

...

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 ***

Danger! If the monomorphism restriction is disabled, this ends up creating a value of type forall a. Num a => IORef a, which can be used to break type safety.

More generally,

cell :: IORef a cell = unsafePerformIO $ newIORef undefined

unsafeCoerce :: a -> b unsafeCoerce x = unsafePerformIO $ do    writeIORef cell x    readIORef cell

This way lies segmentation faults. That "unsafe" is there for a reason.

This is exactly what happened in the original example.