Asynchronous Exceptions in Practice · Simon Marlow

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January 24, 2017Asynchronous exceptions are a controversial feature of Haskell. You can throw an exception to another thread, at any time; all you need is its ThreadId:throwTo :: Exception e => ThreadId -> e -> IO ()The other thread will receive the exception immediately, whatever it is doing. So you have to be ready for an asynchronous exception to fire at any point in your code. Isn’t that a scary thought?It’s an old idea - in fact, when we originally added asynchronous exceptions to Haskell (and wrote a paper about it), it was shortly after Java had removed the equivalent feature, because it was impossible to program with.So how do we get away with it in Haskell? I wrote a little about the rationale in my book. Basically it comes down to this: if we want to be able to interrupt purely functional code, asynchronous exceptions are the only way, because polling would be a side-effect. Therefore the remaining problem is how to make asynchronous exceptions safe for the impure parts of…