[Haskell-cafe] Current situation regarding global IORefs

Brian Hulley brianh at metamilk.com
Fri Apr 21 10:34:25 EDT 2006

Robert Dockins wrote:
> On Apr 21, 2006, at 9:56 AM, Brian Hulley wrote:
>> Hi -
>> I've run into the global mutable state problem described in http://
>> www.haskell.org/hawiki/GlobalMutableState
>> Since the page was last edited in March last year, I'm wondering if
>> there have been any developments or further thoughts on how to
>> safely create top level IORefs since they are absolutely essential
>> for the library I'm writing.
>> For my library, which implements a GUI, I have a Manager module
>> which keeps track of which control currently has the keyboard focus
>> etc, and I don't want to have to pass round the state of the
>> manager to every control since this would be monstrously
>> inconvenient and a total waste of space/time, so at the moment I'm
>> reduced to:
>>     module Manager where
>>     keyboard :: IORef (Maybe Control)
>>     {-# NOINLINE keyboard #-}
>>     keyboard = unsafePerformIO $ newIORef Nothing
>> The problem is that I don't know if this is guaranteed to be
>> completely safe for all Haskell compilers or even for all future
>> versions of ghc (?)
> RE: the technique itself, you should also compile the module with -
> fno-cse.


> RE: the design, Isn't that bit of state local to a dialog/window/
> control group or something?  I understand that top level state is a
> problem in general that needs some sort of solution, but I'm not sure
> it's the right hammer here....

There is only one GUI for the application and only one control in it can 
have the keyboard focus so it seems natural to use global state here, but I 
suppose I could also look into using a state monad. The advantage (perhaps 
also disadvantage ;-) ) of global state is that it allows me to easily 
convert all my old C++ singleton classes to Haskell modules...

> As far as I know, the only recent developments in this area are a
> rumor from the Simons that they are working on some sort of thread-
> local state which (under some sets of design decisions) can fill the
> needs of top level state.  If you press them, they might be willing
> to give some details about this.

I was kind of hoping that there would just be a safe, simple way to create a 
top level monomorphic IORef without having to use a pragma etc.

Thanks, Brian. 

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