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

[David Menendez]

On Thu, Oct 22, 2009 at 2:23 AM, Gregory Crosswhite
<gcross at phys.washington.edu> 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.

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>