[Haskell-cafe] Observer pattern in Haskell?

Andy Gimblett haskell at gimbo.org.uk
Mon Nov 9 13:15:52 EST 2009

On 9 Nov 2009, at 17:41, Neil Brown wrote:

> Just to clarify -- I meant access to another MVar.  Basically, if I  
> do this:
> do v <- newMVar
>   addObserver sub (putMVar v)
> If when the observers are run, the MVar v (that I've allocated) is  
> non-empty, my code will block until it is empty, which will also  
> block all the subsequent observers from being run (and block the  
> code that called setValue) until the MVar is cleared by another  
> thread.  So my one poorly-written observer could deadlock (or cause  
> stutter in) the system, whereas in the forkIO version, this observer  
> would be fine -- it would block in its own new thread.

Ah yes, of course - I understand.  Of course, there's nothing really  
to stop application authors doing such things in the main thread  
too...  ;-)

Thanks for the clarification,


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