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.
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