On Thu, Oct 22, 2009 at 1:32 PM, David Menendez <d...@zednenem.com> wrote: > On Thu, Oct 22, 2009 at 2:23 AM, Gregory Crosswhite > <gcr...@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.
This is exactly what happened in the original example. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
