Hi: I am writing a small application which receives HTTP requests, translates them to JSON, and queues the requests using RabbitMQ. I am using exceptions to handle extreme cases, such as when a client's HTTP request lacks a critical header: lookupHeader :: RequestHeaders -> HeaderName -> Text lookupHeader hs h = decodeUtf8 $ fromMaybe notFound (lookup h hs) where notFound = throw $ MissingHeader $ decodeUtf8 $ original h The problem I am running in to is that the header isn't actually looked up until I call the AMQP library's publishMsg function. If I purposely do not supply a critical header, I get the following error printed to the screen: ConnectionClosedException "ConnectionClosedException \"UNEXPECTED_FRAME - expected content header for class 60, got non content header frame instead\"" If I add the following line just above the publishMsg function (to force evaluation), my exception handler is called successfully: print $ encode req As a result, I suspect that this is due to the fact that the "throw MissingHeader" is getting captured by the AMQP library. What's the best way to deal with this situation? -- Alex