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Editorial Team
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Asked: 2026-05-26T04:33:17+00:00

I got an exception (used Prelude.head on an empty list) that made all the

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I got an exception (used Prelude.head on an empty list) that made all the http-request after that return a 502/505 exception (and the happstack app prints “HTTP request failed with: send resource vanished (Broken pipe)” in stdout).

My question is this: What is the best practice for controlling exceptions in Happstack? Should I use something else than simpleHTTP or simply use Control.Exception.catch on the controller function?

It currently looks similar to the example in the Crash Course:

main :: IO ()
main = do
  hSetEncoding stdout utf8
  bracket (startSystemState (Proxy :: Proxy AppState)) createCheckpointAndShutdown $ 
         \_control ->
           simpleHTTP nullConf { port = 1729 } $ do
             _ <- liftIO today
             decodeBody policy
             controller
    where
      createCheckpointAndShutdown control = do 
          createCheckpoint control
          shutdownSystem control
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1 Answer

  1. Editorial Team

    What do you mean by, “the server went down”?

    If you are handling a Request and do head [], that should only kill the thread that is handling that particular request. The server itself should continue running and processing other requests.

    If you have found a way to kill the whole server, that is a bug and bug report / test case would be highly appreciated.

    If only the current thread is being killed, and you want to catch that exception, then I think you need to use MonadPeelIO,

    http://hackage.haskell.org/packages/archive/monad-peel/0.1/doc/html/Control-Monad-IO-Peel.html

    Someone has submitted a patch for that, but it has not been reviewed/merged yet.

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