[Haskell-cafe] Re: Beginner: IORef constructor?

Bernie Pope bjpop at csse.unimelb.edu.au
Wed Dec 6 01:40:38 EST 2006

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

    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.


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