> Can someone help me understand how this works? I've been
> reading the paper "Asynchronous Exceptions in Haskell". This
> gives a combinator
>
> finally :: IO a -> IO b -> IO s
>
> finally a b =
> block (do {
> r <- catch (unblock a) (\e -> do { b; throw e });
> b;
> return r; })
>
> Now suppose we have
>
> finally (putStrLn "Test Started") (putStrLn "Test Terminated")
>
> then looking at the semantics, putStrLn can become stuck and
> therefore can be interrupted. So the interrupt could occur
> whilst "Test Terminated" is being output and we could end up with
>
> Test Started
> Test Term
>
> Is this what could happen? If so, is there a way of making
> sure that "Test Terminated" is output?
Yes, that's correct. putStrLn is interruptible, so you can't force it
to run to completion - to do so would introduce a possible deadlock.
However, I agree that sometimes you really want to be able to do this,
so perhaps we need another form of 'block' which doesn't allow *any*
exceptions to be delivered, for those times when you know that the time
spent waiting in an interruptible operation is going to be bounded.
Cheers,
Simon
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