Parallel and Concurrent Programming in Haskell

ions like timeout are certainly difficult to get right, but fortunately they only have to be written once. We find that in practice dealing with asynchronous exceptions is fairly straightforward, following a few simple rules: • Use bracket when acquiring resources that need to be released again. • Rather than takeMVar and putMVar, use modifyMVar_ (and friends) which have built-in asynchronous exception safety. • If state handling starts getting complicated with multiple layers of exception handlers, then there are two approaches to simplifying things: – Switching to polling mode with mask can help manage complexity. The GHC I/O library, for example, runs entirely inside mask. Note that inside mask it is important to remember that asynchronous exceptions can still arise out of interruptible operations; the documentation contains a list of operations that are guaranteed not to be interruptible. Note: a different semantics was originally described in Marlow et al. [3].