On 05/12/2006, at 1:00 PM, Benjamin Franksen wrote:
Bernie Pope wrote:
If you want a global variable then you can use something like:
import System.IO.Unsafe (unsafePerformIO)
global = unsafePerformIO (newIORef [])
But this is often regarded as bad programming style (depends who you talk to).
Besides, isn't this example /really/ unsafe? I thought, at least the IORef has to be monomorphic or else type safety is lost?
Perhaps your question is rhetorical, but in case it is not, then yes, we ought to make it a monomorphic type. This little example seg-faults on my mac, and no doubt on other machines as well: import System.IO.Unsafe (unsafePerformIO) import Data.IORef global = unsafePerformIO (newIORef []) main = do modifyIORef global (id :) x <- readIORef global print ((head x + 1) :: Int) It writes the identity function onto the front of the global variable, and then reads it back as an int, and tries to do addition on it. Cheers, Bernie.