MVar swap

[Georg Martius]

Hi,

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?

Thanks!
Georg
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