On Oct 22, 2009, at 11:32 AM, David Menendez wrote:
On Thu, Oct 22, 2009 at 2:23 AM, Gregory Crosswhite
<gcross@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@zednenem.com>
<http://www.eyrie.org/~zednenem/>
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