[Haskell-beginners] AMQP and nested exception handlers

[Alexey Shmalko]

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 <alex323 at gmail.com> wrote:

> On Tue, 12 May 2015 05:24:32 +0000
> Alexey Shmalko <rasen.dubi at gmail.com> wrote:
>
> > 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.
>
> > 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
> >
> > On Tue, May 12, 2015 at 5:11 AM Alex <alex323 at gmail.com> wrote:
> >
> > > 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 at haskell.org
> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > >
>
>
>
> --
> Alex
>
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