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

Seems, that you're putting it in the wrong place. Please, publish your code to lpaste [1]. Hmm.... Maybe you need evaluate `encode req`? On Tue, May 12, 2015 at 9:10 AM Alex wrote:

On Tue, 12 May 2015 05:24:32 +0000 Alexey Shmalko wrote:

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Your issue is related to Lazy evaluation. Your function (lookupHeader) isn't actually evaluated before you call publishMsg or print. You need to put seq or deepseq to overcome this, or put your exception handler in place when your function is actually evaluated. Unfortunately, you haven't provided the calling code, so I have no specific suggestion. Just read some info on Lazy vs. Strict.

Per your suggestion, I tried out deepseq, and it doesn't seem to be working. Below is the code which sends the message to RMQ. As you've noted, `req' is not fully evaluated at this point:

publishMsg chan "" "the-queue" newMsg { msgDeliveryMode = Just Persistent , msgReplyTo = Just cbQueue , msgBody = encode req }

If I put this line above publishMsg, the problem still exists:

_ <- return $!! req

This is counter intuitive, because I would have thought that running deepseq on req would fully evaluate it to NF, but that doesn't seem to be the case.

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Parallel and Concurrent Programming in Haskell Chapter 2 [1] has a good description (just skip "The Eval Monad, rpar, and rseq" section and continue with deepseq). Here is a good chapter on exception [2], if you need one.

[1]: http://chimera.labs.oreilly.com/books/1230000000929/ch02.html [2]:

http://chimera.labs.oreilly.com/books/1230000000929/ch08.html#sec_exceptions

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On Tue, May 12, 2015 at 5:11 AM Alex wrote:

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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