[Haskell-cafe] why cannot i get the value of a IORef variable ?

[Tim Wawrzynczak]

Well, I apologize for starting this whole thread which involves so many dead
kittens :(  I was just trying to help answer a question :)
I guess I assumed too much.. that someone would think to be careful when
using a function with the word 'unsafe' in it...

So, be warned, all Haskellers!  Be careful when using any function that
starts with the word 'unsafe'!  You may kill a kitten!  And I guess this
says something about using 'unsafe' functions...
http://upload.wikimedia.org/wikipedia/en/1/11/God-kills-kitten.jpg (NSFW)...

Cheers and sorry all,
Tim


On Thu, Oct 22, 2009 at 1:59 PM, Gregory Crosswhite <
gcross at phys.washington.edu> wrote:

> Yes, I was once taught that "Every time you use unsafePerformIO, God kills
> a kitten,"  so every time I consider using it I first ask myself:  is this
> really worth an innocent kitten's life?
>
> Cheers,
> Greg
>
>
> On Oct 22, 2009, at 11:32 AM, David Menendez wrote:
>
>  On Thu, Oct 22, 2009 at 2:23 AM, Gregory Crosswhite
>> <gcross at 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 at zednenem.com>
>> <http://www.eyrie.org/~zednenem/ <http://www.eyrie.org/%7Ezednenem/>>
>>
>
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