Re: [Gtk2hs-users] Strange behaviour with MVars

[Axel S. <Axe...@in...>]

Hi Toni,


On 17.08.2012, at 16:09, Toni Cebrián wrote:

> Yes, but why having and empty MVar freezes everything. I have the value 
> of interest in st, in fact putStrLn prints that value, but the painting 
> doesn't happen.
> 

The semantics of an MVar is that it execution blocks when you try to read it when it's empty.

Cheers,
Axel

> And regarding the pattern, I thought that the State monad would be the 
> way to go instead of doing imperative coding in Haskell.
> 
> Regards.
> Toni.
> 
> El 17/08/12 15:47, wag...@se... escribió:
>> Quoting Toni Cebrián <an...@gm...>:
>> 
>>> Hi,
>>> 
>>>     Instead of using IORef I'm using MVar as the main entry point to
>>> state in my Gtk2Hs application. But I'm having strange behaviours
>>> and I'm not able to identify the reason. If I write a handler for
>>> the onExpose event like:
>>> 
>>>     onExpose canvas $ \_ -> (do
>>>                                 st <- readMVar ui
>>>                                 putStrLn st
>>>                                 myPaint st drawin
>>>                             )
>>> everything works fine, but if instead of *readMVar* I use
>>> *takeMVar*, the application freezes and the painting code in
>>> "myPaint" doesn't show although the putStrLn was printed correctly.
>>> Does anybody know what I'm doing wrong here?
>> The source of readMVar tells the story:
>> 
>> readMVar m =
>>    mask_ $ do
>>      a <- takeMVar m
>>      putMVar m a
>>      return a
>> 
>> Ignoring "mask_" for the moment, the point is that readMVar leaves the
>> MVar full, and takeMVar doesn't.
>> 
>>> PS. I came to this problem after posting this question
>>> <http://stackoverflow.com/questions/12002814/how-to-deal-with-application-state-in-gtk2hs>
>>> in SO. Maybe there is a better approach rather than using MVars and IORefs?
>> You seem pretty confident that there's a better way than using
>> pointers despite everything you read saying otherwise!
>> 
>> ~d
>> 
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> 
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