MVar swap

[Simon Marlow]

> I took a look at the implementation of the MVar function swap 
> and I am 
> just woundering where the atomicy comes from.
> 
> -- |Swap the contents of an 'MVar' for a new value.
> swapMVar :: MVar a -> a -> IO a
> swapMVar mvar new =
>    block $ do
>      old <- takeMVar mvar
>      putMVar mvar new
>      return old
> 
> Is it really just the Exception block, that makes it not 
> interruptable?

swapMVar is only atomic with respect to other well-behaved threads.  By
well-behaved I mean that they only do takeMVar followed by putMVar on
the MVar, never a putMVar on its own.  Does that answer your question?

Cheers,
	Simon