[Haskell-cafe] STM, orElse and timed read from a channel

[Tomasz Zielonka]

On Mon, Dec 05, 2005 at 10:50:13AM -0000, Simon Peyton-Jones wrote:
> | Yes. Perhaps it would be easier to do it cleanly than arbitrary
> | top-level IO bindings?
> 
> It turns out to be easy to provide
> 
> 	newTVarIO :: a -> IO (TVar a)
> 
> which you can call from inside 'unsafePerformIO'.  That means you can
> allocate top-level TVars without fuss.
> 
> The whole question of top-level IO remains open, but this will address
> the immediate question very nicely.  

Unfortunately, in some cases, as in the TimeVar.hs example, the
initialization phase is a bit more complex - here I also spawn
a new thread.

But of course it would be a nice thing for the remaining 95% of cases.

Speaking about the whole thing of nested atomically blocks, now that I
know how to work around it, I can live with it.

> I've also added a SourceForge bug to remind us to fail more tidily if
> you do atomically inside atomically.

Great!

I have an idea going a bit further, but hopefully still quite easy to
implement. But I am not sure that it would be a sane thing to do:

    On each "atomically" check if the current thread is already inside
    "atomically". If so, then instead of rising an exception we could
    spawn a new thread to perform the "atomically" block. The
    result/exception would be propagated to the original thread on a
    traditional MVar.

I think it could be implemented mostly in Haskell. The only missing
thing is an IO-primitive:

    isInAtomicallyBlock :: IO Bool

But I fear that doing it this way could introduce a race condition.

Best regards
Tomasz

-- 
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