Re: [Haskell-beginners] AMQP and nested exception handlers

On Tue, 12 May 2015 08:15:28 +0300 Kostiantyn Rybnikov wrote:

Hi.

I would suggest to separate data-extraction from request stage and data-sending one. Create a data-structure that will represent a thing you will want to send into RabbitMQ, and build it before you send anything. Then catch exceptions in IO-based layer to handle exception case.

This way you won't need any evaluation tricks and will get all exceptions during that phase.

In my case, the HTTP request is immediately parsed and a data structure representing the request is created. The request is forwarded to the AMQP code which does the following: publishMsg chan "" "the-queue" newMsg { msgDeliveryMode = Just Persistent , msgReplyTo = Just cbQueue , msgBody = encode req } where "encode req" uses aeson to transform the data structure in to JSON. I tried to make the values which comprise the request data structure strict (by prefixing them with `!'), but it does not seem to help.

Cheers. 12 трав. 2015 05:11 "Alex" пише:

...

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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

-- Alex